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{{Short description|Semiconductor and solid-state light source}}
{{redirect|LED}}
{{About|the electronic device|specific use in lighting|LED lamp}}
{{Redirect2|LED|Led|other uses|LED (disambiguation)}}
{{Infobox electronic component
| name = Light-emitting diode
| image = File:RBG-LED.jpg|A white led (Gray Filament)
| caption = Blue, green, and red LEDs in 5 mm diffused cases. [[#Types|There are many different variants]] of LEDs.
| working_principle = [[Electroluminescence]]
| invented = {{ubl|[[H. J. Round]] (1907)<ref>{{cite web|url=http://www.myledpassion.com/History/hj-round.htm|title=HJ Round was a pioneer in the development of the LED|website=www.myledpassion.com|access-date=April 11, 2017|archive-date=October 28, 2020|archive-url=https://web.archive.org/web/20201028074225/http://www.myledpassion.com/History/hj-round.htm|url-status=dead}}</ref>|[[Oleg Losev]] (1927)<ref name="100-YEAR HISTORY">{{cite news| url=http://holly.orc.soton.ac.uk/fileadmin/downloads/100_years_of_optoelectronics__2_.pdf| title=The life and times of the LED — a 100-year history| date=April 2007| agency=The Optoelectronics Research Centre, University of Southampton| access-date=September 4, 2012| url-status=dead| archive-url=https://web.archive.org/web/20120915034646/http://holly.orc.soton.ac.uk/fileadmin/downloads/100_years_of_optoelectronics__2_.pdf| archive-date=September 15, 2012| df=mdy-all}}</ref>|[[James R. Biard]] (1961)<ref>[http://www.freepatentsonline.com/3293513.pdf US Patent 3293513], "Semiconductor Radiant Diode", James R. Biard and Gary Pittman, Filed on Aug. 8th, 1962, Issued on Dec. 20th, 1966.</ref>|[[Nick Holonyak]] (1962)<ref name="LEMELSON-MIT">{{cite news|url=http://web.mit.edu/invent/n-pressreleases/n-press-04LMP.html |title=Inventor of Long-Lasting, Low-Heat Light Source Awarded $500,000 Lemelson-MIT Prize for Invention |date=April 21, 2004 |agency=Massachusetts Institute of Technology |access-date=December 21, 2011 |location=Washington, D.C. |url-status=dead |archive-url=https://web.archive.org/web/20111009111042/http://web.mit.edu/invent/n-pressreleases/n-press-04LMP.html |archive-date=October 9, 2011 }}</ref>}}
| first_produced = {{start date and age|1962|10|}}
| symbol = [[File:IEEE 315-1975 (1993) 8.5.4.2.svg]]
| pins = [[Anode]] and [[cathode]]
}}
[[File:LED, 5mm, green (en).svg|thumb|Parts of a conventional LED. The flat bottom surfaces of the anvil and post embedded inside the epoxy act as anchors, to prevent the conductors from being forcefully pulled out via mechanical strain or vibration.]]
[[File:Surface mount LED close up image.png|thumb|Close-up image of a [[SMD LED|surface-mount LED]]]]
[[File:LED Operation.ogg|thumb|Close-up of an LED with the voltage being increased and decreased to show a detailed view of its operation]]
[[File:Br20 1.jpg|thumb|alt=Modern LED [[Green retrofit|retrofit]] with E27 screw in base|A bulb-shaped modern retrofit [[LED lamp]] with aluminum [[heat sink]], a light [[Diffuser (optics)|diffusing]] dome and [[Edison screw|E27 screw]] base, using a built-in power supply working on [[Mains electricity|mains voltage]]]]


A '''light-emitting diode''' ('''LED''') is a [[semiconductor]] [[Electronics|device]] that [[Light#Light sources|emits light]] when [[Electric current|current]] flows through it. [[Electron]]s in the semiconductor recombine with [[electron hole]]s, releasing energy in the form of [[photon]]s. The color of the light (corresponding to the energy of the photons) is determined by the energy required for electrons to cross the [[band gap]] of the semiconductor.<ref>{{cite web |url=http://faculty.sites.uci.edu/chem1l/files/2013/11/RDGLED.pdf |work=[[University of California, Irvine]] |access-date=12 January 2019 |title=Light Emitting Diodes |last=Edwards |first=Kimberly D. |page=2 |archive-date=February 14, 2019 |archive-url=https://web.archive.org/web/20190214175634/http://faculty.sites.uci.edu/chem1l/files/2013/11/RDGLED.pdf |url-status=dead }}</ref> White light is obtained by using multiple semiconductors or a layer of light-emitting [[phosphor]] on the semiconductor device.<ref>{{cite web |url=https://www.lrc.rpi.edu/programs/nlpip/lightinganswers/led/whitelight.asp |title=How is white light made with LEDs? |work=[[Rensselaer Polytechnic Institute]] |author=Lighting Research Center |access-date=12 January 2019 |archive-date=May 2, 2021 |archive-url=https://web.archive.org/web/20210502084248/https://www.lrc.rpi.edu/programs/nlpip/lightinganswers/led/whiteLight.asp |url-status=dead }}</ref>
[[Image:RBG-LED.jpg|thumb|right|250px|Blue, green, and red LEDs; these can be combined to produce most perceptible colors, including white.
<br><br>
Infrared and ultraviolet (UVA) LEDs are also available.]]


Appearing as practical electronic components in 1962, the earliest LEDs emitted low-intensity [[infrared]] (IR) light.<ref name=FirstPracticalLED>{{cite web |author1=Okon, Thomas M. |author2=Biard, James R. |title=The First Practical LED |url=http://edisontechcenter.org/lighting/LED/TheFirstPracticalLED.pdf |website=EdisonTechCenter.org |publisher=[[Edison Tech Center]] |date=2015 |access-date=2016-02-02}}</ref> Infrared LEDs are used in [[Remote control|remote-control]] circuits, such as those used with a wide variety of consumer electronics. The first visible-light LEDs were of low intensity and limited to red.
[[Image:LED symbol.svg|right|thumb|250px|LED schematic symbol]]


Early LEDs were often used as indicator lamps, replacing small [[Incandescent light bulb|incandescent bulbs]], and in [[seven-segment display]]s. Later developments produced LEDs available in [[Visible spectrum|visible]], [[ultraviolet]] (UV), and infrared wavelengths with high, low, or intermediate light output, for instance, white LEDs suitable for room and outdoor lighting. LEDs have also given rise to new types of displays and sensors, while their high switching rates are useful in advanced communications technology with applications as diverse as [[Navigation light|aviation lighting]], [[Christmas lights|fairy lights]], [[LED strip light|strip lights]], [[Automotive lighting#Light-emitting diodes (LED)|automotive headlamps]], advertising, [[Lighting|general lighting]], [[Traffic light|traffic signals]], camera flashes, [[LED wallpaper|lighted wallpaper]], [[Grow light|horticultural grow lights]], and medical devices.<ref name="Aguilar">{{Cite book|pmid=18002450|doi= 10.1109/IEMBS.2007.4352784|year= 2007|last1= Peláez|first1= E. A|volume= 2007|pages= 2296–9|last2= Villegas|first2= E. R|title= 2007 29th Annual International Conference of the IEEE Engineering in Medicine and Biology Society|chapter= LED power reduction trade-offs for ambulatory pulse oximetry|isbn= 978-1-4244-0787-3|s2cid= 34626885 |issn = 1557-170X}}</ref>
[[Image:Al Sheedakim and Panel.JPG|thumb|right|250px|[[LED display]]s allow for smaller sets of interchangeable LEDs to be one large display.]]


LEDs have many advantages over incandescent light sources, including lower power consumption, a longer lifetime, improved physical robustness, smaller sizes, and faster switching. In exchange for these generally favorable attributes, disadvantages of LEDs include electrical limitations to low voltage and generally to DC (not AC) power, the inability to provide steady illumination from a pulsing DC or an AC electrical supply source, and a lesser maximum operating temperature and storage temperature.
A '''light-emitting-diode''' ('''LED''') ({{pronEng|ˌɛliːˈdiː}}),<ref>{{cite web
| url = http://en.wiktionary.org/wiki/LED
| title = LED
| accessdate = 2008-01-04
}}</ref> is a [[semiconductor diode]] that emits light when an [[electric current]] is applied in the forward direction of the device, as in the simple [[LED circuit]]. The effect is a form of [[electroluminescence]] where [[coherence (physics)|incoherent]] and narrow-[[spectrum]] light is emitted from the [[p-n junction]].


LEDs are [[transducer]]s of electricity into light. They operate in reverse of [[photodiode]]s, which convert light into electricity.
LEDs are widely used as indicator lights on electronic devices and increasingly in higher power applications such as flashlights and area [[lighting]].
An LED is usually a small area (less than 1 mm<sup>2</sup>) light source, often with optics added to the chip to shape its radiation pattern and assist in reflection.<ref>{{cite paper
|url=http://www.opticsexpress.org/viewmedia.cfm?id=149957&seq=0
|title='''Modeling the radiation pattern of LEDs'''
|work=...
|author=...
|publisher=Optics Express
|year=2008
|accessdate=2008-01-25}}
</ref><ref>{{cite paper
|url=http://planck.reduaz.mx/~imoreno/Publicaciones/IODC2006.pdf
|format=PDF|title=LED Intensity Distribution
|work=International Optical Design Conference
|author=I. Moreno
|publisher=International Optical Design, Technical Digest
|year=2006 |accessdate=2007-08-13}}
</ref>
The [[color]] of the emitted light depends on the composition and condition of the semiconducting material used, and can be [[infrared]], [[visible spectrum|visible]], or [[ultraviolet]]. Besides lighting, interesting applications include using [[UV]]-LEDs for sterilization of water and disinfection of devices,<ref name=water sterilization">[http://www.springerlink.com/content/668j066286443722/ Development of a new water sterilization device with a 365 nm UV-LED, Medical and Biological Engineering and Computing, Volume 45, Number 12 / December, 2007]</ref> and as a [[grow light]] to enhance [[photosynthesis]] in plants.<ref>{{cite paper
|title=Light-emitting diodes as a light source for photosynthesis research
|author=Tennessen, D.J. and Singsaas, E.L. and Sharkey, T.D.
|journal=Photosynthesis Research
|publisher=Springer
|year=1994
|pages=85-92
|accessdate=2008-07-24}}
</ref>


==History==
==History==
{{Main|History of LEDs}}
The first known report of a light-emitting solid-state diode was made in 1907 by the British experimenter [[H. J. Round]] of [[Marconi Labs]] when he noticed [[electroluminescence]] produced from a crystal of [[silicon carbide]] while using a [[cat's-whisker detector]].<ref>{{cite journal
|author=H. J. Round
|year=1907
|title=A Note on Carborundum
|journal=Electrical World
|volume=19
|pages=309}}
</ref> Russian [[Oleg Vladimirovich Losev]] independently created the first LED in the mid 1920s; his research, though distributed in Russian, German and British scientific journals, was ignored,<ref name="Zheludev_100yearhistory">{{cite journal
|author=Zheludev, N.
|year=2007
|title=The life and times of the LED — a 100-year history
|journal=Nature Photonics
|volume=1
|issue=4
|pages=189–192
|url=http://www.nanophotonics.org.uk/niz/publications/zheludev-2007-ltl.pdf
|format=PDF
|doi = 10.1038/nphoton.2007.34 <!--Retrieved from CrossRef by DOI bot-->}}
</ref><ref>{{cite web
|url=http://www.jmargolin.com/history/trans.htm
|author=Margolin J
|title=''The Road to the Transistor''}}
</ref> and no practical use was made of the discovery for several decades. [[Rubin Braunstein]] of the [[Radio Corporation of America]] reported on infrared emission from [[gallium arsenide|gallium arsenide (GaAs)]] and other semiconductor alloys in 1955.<ref>{{cite paper
|author=Braunstein, Rubin
|year=1955.
|title="Radiative Transitions in Semiconductors,"
|journal=Physical Review
|volume=99
|pages=1892-3.
|url=http://prola.aps.org/abstract/PR/v99/i6/p1892_1
|doi=10.1103/PhysRev.99.1892}}
</ref> Braunstein observed infrared emission generated by simple diode structures using [[Gallium antimonide|GaSb]], [[GaAs]], [[Indium phosphide|InP]], and Ge-Si alloys at room temperature and at 77 kelvin. In 1961, experimenters Bob Biard and Gary Pittman working at [[Texas Instruments]],<ref>{{cite web
|url=http://invention.smithsonian.org/centerpieces/quartz/inventors/biard.html
|title=The first LEDs were infrared (invisible)
|work=The Quartz Watch |publisher=The Lemelson Center
|accessdate=2007-08-13}}
</ref> found that gallium arsenide gave off infrared radiation when electric current was applied. Biard and Pittman were able to establish the priority of their work and received the patent for the infrared light-emitting [[diode]].


The first LED was created by Soviet inventor [[Oleg Losev]]<ref>{{Cite journal |last=Lossev |first=O.V. |date=November 1928 |title=CII. Luminous carborundum detector and detection effect and oscillations with crystals |url=http://www.tandfonline.com/doi/abs/10.1080/14786441108564683 |journal=The London, Edinburgh, and Dublin Philosophical Magazine and Journal of Science |language=en |volume=6 |issue=39 |pages=1024–1044 |doi=10.1080/14786441108564683 |issn=1941-5982}}</ref> in 1927, but [[electroluminescence]] was already known for 20 years, and relied on a diode made of [[silicon carbide]].
The first practical visible-spectrum (red) LED was developed in 1962 by [[Nick Holonyak|Nick Holonyak Jr.]], while working at [[General Electric Company]]. He later moved to the [[University of Illinois at Urbana-Champaign]].<ref>{{cite web
|url=http://web.mit.edu/invent/a-winners/a-holonyak.html
|title=Nick Holonyak, Jr. 2004 Lemelson-MIT Prize Winner
|publisher=Lemenson-MIT Program |accessdate=2007-08-13}}
</ref> Holonyak is seen as the "father of the light-emitting diode".<ref name="chisuntimes">{{cite news
| title = U. of I.'s Holonyak out to take some of Edison's luster
| author = Wolinsky, Howard
| date = February 5, 2005
| url = http://findarticles.com/p/articles/mi_qn4155/is_20050202/ai_n9504926
| publisher = ''Chicago Sun-Times''
| accessdate = 2007-07-29 }}</ref>
M. George Craford, a former graduate student of Holonyak's, invented the first yellow LED and 10x brighter red and red-orange LEDs in 1972.<ref>{{cite web
|url=http://www.technology.gov/Medal/2002/bios/Holonyak_Craford_Dupuis.pdf
|title=Brief Biography – Holonyak, Craford, Dupuis
|publisher=Technology Administration
|format=PDF
|accessdate=2007-05-30}}
</ref>


Commercially viable LEDs only became available after [[Texas Instruments]] engineers patented efficient near-infrared emission from a diode based on [[Gallium arsenide|GaAs]] in 1962.
[[Shuji Nakamura]] of [[Nichia Corporation]] of Japan demonstrated the first high-brightness blue LED based on [[Indium gallium nitride|InGaN]] borrowing on critical developments in [[Gallium nitride|GaN]] nucleation on sapphire substrates and the demonstration of p-type doping of GaN which were developed by I. Akasaki and H. Amano in [[Nagoya]]. In 1995, [[Alberto Barbieri]] at the [[Cardiff University]] Laboratory (GB) investigated the efficiency and reliability of high-brightness LEDs demonstrating very high result by using a transparent contact made of [[indium tin oxide]] (ITO) on (AlGaInP/GaAs) LED. The existence of blue LEDs and high efficiency LEDs quickly led to the development of the first white LED, which employed a Y<sub>3</sub>Al<sub>5</sub>O<sub>12</sub>:Ce, or "[[YAG]]", phosphor coating to mix yellow (down-converted) light with blue to produce light that appears white. Nakamura was awarded the 2006 [[Millennium Technology Prize]] for his invention.<ref>{{cite web
|url=http://www.ia.ucsb.edu/pa/display.aspx?pkey=1475
|title=2006 Millennium technolgy prize awarded to UCSB's Shuji Nakamura
|accessdate=2007-05-30}}</ref>


From 1968, commercial LEDs were extremely costly and saw no practical use. Monstanto and Hewlett-Packard led the development of LEDs to the point where, in the 1970s, a unit cost less than five cents.<ref>{{Cite book |url=https://www.worldcat.org/title/ocm47203707 |title=Light-emitting diodes: research, manufacturing, and applications V: 24-25 January 2001, San Jose, USA |date=2001 |publisher=SPIE |isbn=978-0-8194-3956-7 |editor-last=Yao |editor-first=H. Walter |series=SPIE proceedings series |location=Bellingham, Wash |oclc=ocm47203707 |editor-last2=Schubert |editor-first2=E. Fred |editor-last3=United States |editor-last4=AIXTRON, Inc |editor-last5=Society of Photo-optical Instrumentation Engineers}}</ref>
The development of LED technology has caused their efficiency and light output to increase [[exponentially]], with a doubling occurring about every 36 months since the 1960s, in a similar way to [[Moore's law]]. The advances are generally attributed to the parallel development of other semiconductor technologies and advances in optics and material science. This trend is normally called [[Haitz's Law]] after Dr. Roland Haitz.


== Physics of light production and emission ==
=== Practical use ===
{{main|Light-emitting diode physics}}
[[Image:Charger Squad Car Rear Quarter Shot.jpg|thumb|right|Some police vehicle [[lightbar]]s incorporate LEDs.]]
In a light-emitting diode, the recombination of electrons and electron holes in a semiconductor produces light (be it infrared, visible or UV), a process called "[[electroluminescence]]". The wavelength of the light depends on the energy [[band gap]] of the semiconductors used. Since these materials have a high index of refraction, design features of the devices such as special optical coatings and die shape are required to efficiently emit light.<ref name="pears1">{{cite book|last1=Pearsall|first1=Thomas|title=Photonics Essentials, 2nd edition|publisher=McGraw-Hill|date=2010|url=https://www.mheducation.com/highered/product/photonics-essentials-second-edition-pearsall/9780071629355.html|isbn=978-0-07-162935-5|access-date=February 25, 2021|archive-date=August 17, 2021|archive-url=https://web.archive.org/web/20210817005021/https://www.mheducation.com/highered/product/photonics-essentials-second-edition-pearsall/9780071629355.html|url-status=dead}}</ref>
The first commercial LEDs were commonly used as replacements for [[incandescence|incandescent]] indicators, and in [[seven-segment display]]s, first in expensive equipment such as laboratory and electronics test equipment, then later in such appliances as TVs, radios, telephones, calculators, and even watches (see list of [[Led#Indicators_and_signs|signal applications]]). These red LEDs were bright enough only for use as indicators, as the light output was not enough to illuminate an area. Later, other colors became widely available and also appeared in appliances and equipment. As the LED materials technology became more advanced, the light output was increased, while maintaining the efficiency and the reliability to an acceptable level, causing LEDs to become bright enough to be used for illumination, in [[Led#Lighting|various applications]] such as lamps and other lighting fixtures.


Unlike a [[laser]], the light emitted from an LED is neither spectrally [[Coherence (physics)|coherent]] nor even highly [[monochromatic]]. Its [[Spectrum#Electromagnetic spectrum|spectrum]] is sufficiently narrow that it appears to the [[color vision|human eye]] as a pure ([[Colorfulness#Saturation|saturated]]) color.<ref>{{Cite web|title=LED Basics {{!}} Department of Energy|url=https://www.energy.gov/eere/ssl/led-basics|access-date=2018-10-22|website=www.energy.gov}}</ref><ref>{{cite web|author=<!--Staff writer(s); no by-line.-->|date=2013-07-25|title=LED Spectral Distribution|url=https://optiwave.com/resources/applications-resources/optical-system-led-spectral-distribution/|access-date=20 June 2017|website=optiwave.com}}</ref> Also unlike most lasers, its radiation is not [[Coherence (physics)#Spatial coherence|spatially coherent]], so it cannot approach the very high [[Radiance|intensity]] characteristic of [[laser]]s.
Most LEDs were made in the very common 5&nbsp;mm T1³⁄₄ and 3&nbsp;mm T1 packages, but with higher power, it has become increasingly necessary to shed excess heat in order to maintain reliability, so more complex packages adapted for efficient heat dissipation are becoming common. Packages for state-of-the-art [[Led#High_power_LEDs|high power LEDs]] bear little resemblance to early LEDs.


== Single-color LEDs ==
Hewlett Packard (HP) introduced the first commercially available light-emitting diode (LED) in 1068. The technology proved to have major applications for alphanumeric displays and was integrated into HP’s early handheld calculators.
[[File:Blue light emitting diodes over a proto-board.jpg|thumb|upright|[[Blue]] LEDs]]
{{external media | width = 210px | float = right | headerimage= [[File:Herb Maruska original blue LED College of New Jersey Sarnoff Collection.png|210px]] | video1 = [https://vimeo.com/109205062 "The Original Blue LED"], [[Science History Institute]]}}


By [[Light-emitting diode physics#Materials|selection of different semiconductor materials]], single-color LEDs can be made that emit light in a narrow band of wavelengths from near-infrared through the visible spectrum and into the ultraviolet range. The required operating voltages of LEDs increase as the emitted wavelengths become shorter (higher energy, red to blue), because of their increasing semiconductor band gap.
== LED technology ==
{{anchor|blue LED}}
[[Image:LED label.jpg|thumb|300px|right|Parts of an LED]]
[[Image:PnJunction-LED-E.PNG|thumb|300px|right|The inner workings of an LED]]
[[Image:Rectifier vi curve.GIF|thumb|300px|right|I-V diagram for a [[diode]] an LED will begin to emit light when the on-[[voltage]] is exceeded. Typical on voltages are 2-3 [[Volt]]]]
Like a normal [[diode]], the LED consists of a chip of semiconducting material impregnated, or ''[[Doping (semiconductor)|doped]]'', with impurities to create a ''[[p-n junction]]''. As in other diodes, current flows easily from the p-side, or [[anode]], to the n-side, or [[cathode]], but not in the reverse direction. Charge-carriers—[[electron]]s and [[electron hole|holes]]—flow into the junction from [[electrode]]s with different [[voltage]]s. When an electron meets a hole, it falls into a lower [[energy level]], and releases [[energy]] in the form of a [[photon]].


Blue LEDs have an active region consisting of one or more InGaN [[quantum well]]s sandwiched between thicker layers of GaN, called cladding layers. By varying the relative In/Ga fraction in the InGaN quantum wells, the light emission can in theory be varied from violet to amber.
The [[wavelength]] of the light emitted, and therefore its color, depends on the [[band gap]] energy of the materials forming the ''p-n junction''. In [[silicon]] or [[germanium]] diodes, the electrons and holes recombine by a ''non-radiative transition'' which produces no optical emission, because these are [[indirect band gap]] materials. The materials used for the LED have a [[direct band gap]] with energies corresponding to near-infrared, visible or near-ultraviolet light.


[[Aluminium gallium nitride]] (AlGaN) of varying Al/Ga fraction can be used to manufacture the cladding and quantum well layers for [[ultraviolet]] LEDs, but these devices have not yet reached the level of efficiency and technological maturity of InGaN/GaN blue/green devices. If unalloyed GaN is used in this case to form the active quantum well layers, the device emits near-ultraviolet light with a peak wavelength centred around 365&nbsp;nm. Green LEDs manufactured from the InGaN/GaN system are far more efficient and brighter than green LEDs produced with non-nitride material systems, but practical devices still exhibit efficiency too low for high-brightness applications.{{citation needed|date=March 2016}}
LED development began with infrared and red devices made with [[gallium arsenide]]. Advances in [[materials science]] have made possible the production of devices with ever-shorter [[wavelength]]s, producing light in a variety of colors.


With [[Aluminium gallium nitride|AlGaN]] and [[aluminium gallium indium nitride|AlGaInN]], even shorter wavelengths are achievable. Near-UV emitters at wavelengths around 360–395&nbsp;nm are already cheap and often encountered, for example, as [[black light]] lamp replacements for inspection of anti-[[counterfeiting]] UV watermarks in documents and bank notes, and for [[UV curing#LEDs|UV curing]]. Substantially more expensive, shorter-wavelength diodes are commercially available for wavelengths down to 240&nbsp;nm.<ref>{{cite journal |url=http://www.semiconductor-today.com/features/SemiconductorToday%20-%20Going%20deep%20for%20UV%20sterilization%20LEDs.pdf |journal=Semiconductor Today |title=Going Deep for UV Sterilization LEDs |page=82 |volume=5 |issue=3 |author=Cooke, Mike |date=April–May 2010 |url-status=dead |archive-url=https://web.archive.org/web/20130515030549/http://www.semiconductor-today.com/features/SemiconductorToday%20-%20Going%20deep%20for%20UV%20sterilization%20LEDs.pdf |archive-date=May 15, 2013 }}</ref> As the photosensitivity of microorganisms approximately matches the absorption spectrum of [[DNA]], with a peak at about 260&nbsp;nm, UV LED emitting at 250–270&nbsp;nm are expected in prospective disinfection and sterilization devices. Recent research has shown that commercially available UVA LEDs (365&nbsp;nm) are already effective disinfection and sterilization devices.<ref name="water sterilization">{{Cite journal | last1 = Mori | first1 = M. | last2 = Hamamoto | first2 = A. | last3 = Takahashi | first3 = A. | last4 = Nakano | first4 = M. | last5 = Wakikawa | first5 = N. | last6 = Tachibana | first6 = S. | last7 = Ikehara | first7 = T. | last8 = Nakaya | first8 = Y. | last9 = Akutagawa | first9 = M. | doi = 10.1007/s11517-007-0263-1 | last10 = Kinouchi | first10 = Y. | title = Development of a new water sterilization device with a 365 nm UV-LED | journal = Medical & Biological Engineering & Computing | volume = 45 | issue = 12 | pages = 1237–1241 | year = 2007 | pmid = 17978842 | s2cid = 2821545 | doi-access = free }}</ref>
LEDs are usually built on an n-type substrate, with an electrode attached to the p-type layer deposited on its surface. P-type substrates, while less common, occur as well. Many commercial LEDs, especially GaN/InGaN, also use [[sapphire]] substrate.
UV-C wavelengths were obtained in laboratories using [[aluminium nitride]] (210&nbsp;nm),<ref name="aln">{{Cite journal | last1 = Taniyasu | first1 = Y. | last2 = Kasu | first2 = M. | last3 = Makimoto | first3 = T. | doi = 10.1038/nature04760 | title = An aluminium nitride light-emitting diode with a wavelength of 210 nanometres | journal = Nature | volume = 441 | issue = 7091 | pages = 325–328 | year = 2006 | pmid = 16710416 | bibcode = 2006Natur.441..325T| s2cid = 4373542 }}</ref> [[boron nitride]] (215&nbsp;nm)<ref name="BN">{{Cite journal | last1 = Kubota | first1 = Y. | last2 = Watanabe | first2 = K. | last3 = Tsuda | first3 = O. | last4 = Taniguchi | first4 = T. | title = Deep Ultraviolet Light-Emitting Hexagonal Boron Nitride Synthesized at Atmospheric Pressure | doi = 10.1126/science.1144216 | journal = Science | volume = 317 | issue = 5840 | pages = 932–934 | year = 2007 | pmid = 17702939| bibcode = 2007Sci...317..932K | doi-access = free }}</ref><ref name="bn2">{{Cite journal | last1 = Watanabe | first1 = K. | last2 = Taniguchi | first2 = T. | last3 = Kanda | first3 = H. | doi = 10.1038/nmat1134 | title = Direct-bandgap properties and evidence for ultraviolet lasing of hexagonal boron nitride single crystal | journal = Nature Materials | volume = 3 | issue = 6 | pages = 404–409 | year = 2004 | pmid = 15156198 |bibcode = 2004NatMa...3..404W | s2cid = 23563849 }}</ref> and [[diamond]] (235&nbsp;nm).<ref name="dia">{{Cite journal | last1 = Koizumi | first1 = S. | last2 = Watanabe | first2 = K. | last3 = Hasegawa | first3 = M. | last4 = Kanda | first4 = H. | title = Ultraviolet Emission from a Diamond pn Junction | doi = 10.1126/science.1060258 | journal = Science | volume = 292 | issue = 5523 | pages = 1899–1901 | year = 2001 | pmid = 11397942| bibcode = 2001Sci...292.1899K| s2cid = 10675358 }}</ref>


===Light extraction===
== White LEDs ==
The [[refractive index]] of most LED semiconductor materials is quite high, so in almost all cases the light from the LED is coupled into a much lower-index medium. The large index difference makes the [[reflection]] quite substantial (per the [[Fresnel equations|Fresnel coefficients]]). The produced light gets partially reflected back into the semiconductor, where it may be absorbed and turned into additional heat; this is usually one of the dominant causes of LED inefficiency. Often more than half of the emitted light is reflected back at the LED-package and package-air interfaces.


There are two primary ways of producing [[white]] light-emitting diodes. One is to use individual LEDs that emit three [[primary color]]s—red, green and blue—and then mix all the colors to form white light. The other is to use a phosphor material to convert monochromatic light from a blue or UV LED to broad-spectrum white light, similar to a [[fluorescent lamp]]. The yellow phosphor is [[cerium]]-doped [[Yttrium aluminium garnet|YAG]] crystals suspended in the package or coated on the LED. This YAG phosphor causes white LEDs to appear yellow when off, and the space between the crystals allow some blue light to pass through in LEDs with partial phosphor conversion. Alternatively, white LEDs may use other phosphors like manganese(IV)-doped [[potassium fluorosilicate]] (PFS) or other engineered phosphors. PFS assists in red light generation, and is used in conjunction with conventional Ce:YAG phosphor.
The reflection is most commonly reduced by using a dome-shaped (half-sphere) package with the diode in the center so that the outgoing light rays strike the surface [[perpendicularly]], at which angle the reflection is minimized. Substrates that are transparent to the emitted wavelength, and backed by a reflective layer, increase the LED efficiency. The refractive index of the package material should also match the index of the semiconductor, to minimize back-reflection. An [[anti-reflection coating]] may be added as well.


In LEDs with PFS phosphor, some blue light passes through the phosphors, the Ce:YAG phosphor converts blue light to green and red (yellow) light, and the PFS phosphor converts blue light to red light. The color, emission spectrum or color temperature of white phosphor converted and other phosphor converted LEDs can be controlled by changing the concentration of several phosphors that form a phosphor blend used in an LED package.<ref>{{Cite web | url=https://www.ledinside.com/news/2014/11/seeing_red_with_pfs_phosphor |title = Seeing Red with PFS Phosphor}}</ref><ref>{{Cite web | url=https://www.ledsmagazine.com/architectural-lighting/retail-hospitality/article/16696629/ge-lighting-manufactures-pfs-red-phosphor-for-led-display-backlight-applications | title=GE Lighting manufactures PFS red phosphor for LED display backlight applications| date=March 31, 2015}}</ref><ref>{{cite journal | url=https://sid.onlinelibrary.wiley.com/doi/abs/10.1002/sdtp.10406 | doi=10.1002/sdtp.10406 | title=62.4: PFS, K<sub>2</sub>SiF<sub>6</sub>:Mn<sup>4+</sup>: The Red-line Emitting LED Phosphor behind GE's TriGain Technology™ Platform | date=2015 | last1=Murphy | first1=James E. | last2=Garcia-Santamaria | first2=Florencio | last3=Setlur | first3=Anant A. | last4=Sista | first4=Srinivas | journal=Sid Symposium Digest of Technical Papers | volume=46 | pages=927–930 }}</ref><ref>{{Cite journal|doi = 10.1149/2.0251801jss|title = Full Spectrum White LEDs of Any Color Temperature with Color Rendering Index Higher Than 90 Using a Single Broad-Band Phosphor|year = 2018|last1 = Dutta|first1 = Partha S.|last2 = Liotta|first2 = Kathryn M.|journal = ECS Journal of Solid State Science and Technology|volume = 7|pages = R3194–R3198| s2cid=103600941 |doi-access = free}}</ref>
The package may be colored, but this is only for cosmetic reasons or to improve the contrast ratio; the color of the packaging does not substantially affect the color of the light emitted.


The 'whiteness' of the light produced is engineered to suit the human eye. Because of [[Metamerism (color)|metamerism]], it is possible to have quite different spectra that appear white. The appearance of objects illuminated by that light may vary as the spectrum varies. This is the issue of color rendition, quite separate from color temperature. An orange or cyan object could appear with the wrong color and much darker as the LED or phosphor does not emit the wavelength it reflects. The best color rendition LEDs use a mix of phosphors, resulting in less efficiency and better color rendering.{{citation needed|date=October 2020}}
Other strategies for reducing the impact of the interface reflections include designing the LED to reabsorb and reemit the reflected light (called ''photon recycling'') and manipulating the microscopic structure of the surface to reduce the reflectance, by introducing random roughness, creating programmed ''moth eye'' surface patterns. Recently [[photonic crystal]] have also been used to minimize back-reflections.<ref>{{cite paper
|author = Cho, H.K., Jang, J., Choi, J.H., Choi, J., Kim, J., Lee, J.S., Lee, B., Choe, Y.H., Lee, K.D., Kim, S.H. and others
|title = Light extraction enhancement from nano-imprinted photonic crystal GaN-based blue light-emitting diodes,
|journal = Optics Express,
|year = 2006,
|volume = 14,
|pages = 8654&ndash;8660
|number = 19
|publisher = OSA
}}</ref>
In December 2007, scientists at [[Glasgow University]] claimed to have found a way to make LEDs more energy efficient, imprinting billions of holes into LEDs using a process known as [[nanoimprint lithography]].<ref name="BBC_Rahman">{{cite web
| title = New efficient bulb sees the light
| work = A new type of super-efficient household light bulb is being developed which could spell the end of regular bulbs.
| publisher = BBC News
| date = 28 December 2007
| url = http://news.bbc.co.uk/2/hi/uk_news/scotland/glasgow_and_west/7162606.stm
| format = Web
| doi =
| accessdate = 2008-01-01}}</ref>


The first white light-emitting diodes (LEDs) were offered for sale in the autumn of 1996.<ref>{{Cite journal | doi = 10.1002/lpor.201600147 | last1 = Cho | first1 = Jaehee | last2 = Park | first2 = Jun Hyuk | last3 = Kim | first3 = Jong Kyu | last4 = Schubert | first4 = E. Fred | title = White light-emitting diodes: History, progress, and future | journal = Laser & Photonics Reviews | volume = 11 | issue = 2 | pages = 1600147 | year = 2017| bibcode = 2017LPRv...1100147C | s2cid = 53645208 | issn = 1863-8880 | url=https://onlinelibrary.wiley.com/doi/10.1002/lpor.201600147 }}</ref> Nichia made some of the first white LEDs which were based on blue LEDs with Ce:YAG phosphor.<ref>{{cite book | url=https://books.google.com/books?id=GEFKDwAAQBAJ&dq=ce+yag+led&pg=PA36 | isbn=978-0-9863826-6-6 | title=Light-Emitting Diodes (3rd Edition, 2018) | date=February 3, 2018 | publisher=E. Fred Schubert }}</ref> Ce:YAG is often grown using the [[Czochralski method]].<ref>{{cite book | url=https://books.google.com/books?id=aJyCDAAAQBAJ&dq=growing+ce+yag&pg=PA113 | isbn=978-1-119-23600-9 | title=Additive Manufacturing and Strategic Technologies in Advanced Ceramics | date=August 16, 2016 | publisher=John Wiley & Sons }}</ref>
=== Efficiency and operational parameters===


=== RGB systems ===
Typical indicator LEDs are designed to operate with no more than 30–60 [[milliwatt]]s (mW) of electrical power. Around 1999, [[Philips Lumileds Lighting Company|Philips Lumileds]] introduced power LEDs capable of continuous use at one [[watt]] (W). These LEDs used much larger semiconductor die sizes to handle the large power inputs. Also, the semiconductor dies were mounted onto metal slugs to allow for heat removal from the LED die.
[[File:RGB_LED_Spectrum.svg|thumb|Combined spectral curves for blue, yellow-green, and high-brightness red solid-state semiconductor LEDs. [[Full width at half maximum|FWHM]] spectral bandwidth is approximately 24–27 nm for all three colors.]]


[[File:RGB-Led-projection.jpg|thumb|An RGB LED projecting red, green, and blue onto a surface]]
One of the key advantages of LED-based lighting is its high efficiency, as measured by its light output per unit power input. White LEDs quickly matched and overtook the efficiency of standard incandescent lighting systems. In 2002, [[Philips Lumileds Lighting Company|Lumileds]] made five-watt LEDs available with a [[luminous efficiency]] of 18–22 [[lumen (unit)|lumens]] per watt (lm/W). For comparison, a conventional 60–100 W incandescent lightbulb produces around 15 lm/W, and standard fluorescent lights produce up to 100 lm/W. (The [[luminous efficiency]] article discusses these comparisons in more detail.)


Mixing red, green, and blue sources to produce white light needs electronic circuits to control the blending of the colors. Since LEDs have slightly different emission patterns, the color balance may change depending on the angle of view, even if the RGB sources are in a single package, so RGB diodes are seldom used to produce white lighting. Nonetheless, this method has many applications because of the flexibility of mixing different colors,<ref>{{Cite journal | doi = 10.1364/OE.15.003607 | last1 = Moreno | first1 = I. | last2 = Contreras | first2 = U. | title = Color distribution from multicolor LED arrays | journal = Optics Express | volume = 15 | issue = 6 | pages = 3607–3618 | year = 2007 | pmid = 19532605| bibcode = 2007OExpr..15.3607M | s2cid = 35468615 | doi-access = free }}</ref> and in principle, this mechanism also has higher [[quantum efficiency]] in producing white light.<ref>{{Cite web|url=http://spie.org/newsroom/1069-making-white-light-emitting-diodes-without-phosphors?SSO=1|title=Making white-light-emitting diodes without phosphors {{!}} SPIE Homepage: SPIE|last1=Yeh|first1=Dong-Ming|last2=Huang|first2=Chi-Feng|website=spie.org|access-date=2019-04-07|last3=Lu|first3=Chih-Feng|last4=Yang|first4=Chih-Chung}}</ref>
In September 2003, a new type of blue LED was demonstrated by the company [[Cree Inc.|Cree, Inc.]] to provide 24 mW at 20 [[milliampere]]s (mA). This produced a commercially packaged white light giving 65 lm/W at 20 mA, becoming the brightest white LED commercially available at the time, and more than four times as efficient as standard incandescents. In 2006 they demonstrated a prototype with a record white LED luminous efficiency of 131 lm/W at 20 mA. Also, [[Seoul Semiconductor]] has plans for 135 lm/W by 2007 and 145 lm/W by 2008, which would be approaching an order of magnitude improvement over standard incandescents and better even than standard fluorescents.<ref>{{cite news |url=http://www.engadget.com/2006/12/12/seoul-semiconductor-squeezes-240-lumens-into-brightest-led/ |title=Seoul Semiconductor squeezes 240 lumens into "brightest" LED |publisher=engadget |date=December 12, 2006 |accessdate=2007-08-13}}</ref> [[Nichia Corporation]] has developed a white light LED with luminous efficiency of 150 lm/W at a forward current of 20 mA.<ref>{{cite news |url=http://techon.nikkeibp.co.jp/english/NEWS_EN/20061221/125713/ |title=Nichia Unveils White LED with 150 lm/W Luminous Efficiency |publisher=Tech-On! |date=December 21, 2006 |accessdate=2007-08-13}}</ref>


There are several types of multicolor white LEDs: [[:wiktionary:dichromatic|di-]], [[trichromatic|tri-]], and [[tetrachromatic]] white LEDs. Several key factors that play among these different methods include color stability, [[color rendering index|color rendering]] capability, and luminous efficacy. Often, higher efficiency means lower color rendering, presenting a trade-off between the luminous efficacy and color rendering. For example, the dichromatic white LEDs have the best luminous efficacy (120 lm/W), but the lowest color rendering capability. Although tetrachromatic white LEDs have excellent color rendering capability, they often have poor luminous efficacy. Trichromatic white LEDs are in between, having both good luminous efficacy (>70 lm/W) and fair color rendering capability.<ref>{{cite book |last1=Cabrera |first1=Rowan |title=Electronic Devices and Circuits |date=2019 |publisher=EDTECH |isbn=978-1839473838}}</ref>
It should be noted that high-power (≥ 1 W) LEDs are necessary for practical general lighting applications. Typical operating currents for these devices begin at 350 mA. The highest efficiency high-power white LED is claimed<ref>Datasheet is not sufficient to confirm the claim, comparing Philips LXHL-LW6C and OSRAM LUW W5AM-LXLY-6P7R </ref> by Philips Lumileds Lighting Co. with a luminous efficiency of 115 lm/W (350 mA).


One of the challenges is the development of more efficient green LEDs. The theoretical maximum for green LEDs is 683 lumens per watt but as of 2010 few green LEDs exceed even 100 lumens per watt. The blue and red LEDs approach their theoretical limits.{{citation needed|date=October 2020}}
Cree issued a press release on November 19th, 2008 about a laboratory prototype LED achieving 161 lumens/watt at 350 mA (Over 10 times more efficient than incandescent lightbulbs). Output was 173 lumens. Power works out to 1.075 watts. Voltage drop works out to 3.07 volts. Correlated color temperature was reported to be 4689 K.<ref> http://www.cree.com/press/press_detail.asp?i=1227101620851</ref>


Multicolor LEDs offer a means to form light of different colors. Most [[color#Perception|perceivable colors]] can be formed by mixing different amounts of three primary colors. This allows precise dynamic color control. Their emission power [[exponential decay|decays exponentially]] with rising temperature,<ref>{{Cite journal |last1=Schubert |first1=E. Fred |last2=Kim |first2=Jong Kyu |journal=Science |volume=308 |issue=5726 |doi=10.1126/science.1108712 |pmid=15919985 |pages=1274–1278 |year=2005 |title=Solid-State Light Sources Getting Smart |bibcode=2005Sci...308.1274S |s2cid=6354382 |url=https://www.ecse.rpi.edu/~schubert/Reprints/2005%20Schubert%20and%20Kim%20(Science)%20Solid-state%20light%20sources%20getting%20smart.pdf|archive-url=https://web.archive.org/web/20160205165109/https://www.ecse.rpi.edu/~schubert/Reprints/2005%20Schubert%20and%20Kim%20(Science)%20Solid-state%20light%20sources%20getting%20smart.pdf |archive-date=February 5, 2016 }}</ref>
===Electrical polarity===
resulting in a substantial change in color stability. Such problems inhibit industrial use. Multicolor LEDs without phosphors cannot provide good color rendering because each LED is a narrowband source. LEDs without phosphor, while a poorer solution for general lighting, are the best solution for displays, either backlight of LCD, or direct LED based pixels.
[[Image:+- of LED.svg|thumb|right]]


Dimming a multicolor LED source to match the characteristics of incandescent lamps is difficult because manufacturing variations, age, and temperature change the actual color value output. To emulate the appearance of dimming incandescent lamps may require a feedback system with color sensor to actively monitor and control the color.<ref>{{cite journal | title = Sensors and Feedback Control of Multicolor LED Systems | format = PDF | first1 = Thomas | last1 = Nimz | first2 = Fredrik | last2 = Hailer | first3 = Kevin | last3 = Jensen | journal = Led Professional Review: Trends & Technologie for Future Lighting Solutions | publisher = LED Professional | date = November 2012 | issue = 34 | issn = 1993-890X | pages = 2–5 | url = http://www.mazet.de/en/english-documents/english/featured-articles/sensors-and-feedback-control-of-multi-color-led-systems-1/download#.UX7VXYIcUZI | archive-url = https://web.archive.org/web/20140429162806/http://www.mazet.de/en/english-documents/english/featured-articles/sensors-and-feedback-control-of-multi-color-led-systems-1/download#.UX7VXYIcUZI | url-status = dead | archive-date = 2014-04-29 }}</ref>
Unlike [[incandescent light bulb]]s, which illuminate regardless of the electrical [[Polarity (physics)|polarity]], LEDs will only light with correct electrical polarity. When the voltage across the ''p-n junction'' is in the correct direction, a significant current flows and the device is said to be ''forward-biased''. If the voltage is of the wrong polarity, the device is said to be ''reverse biased'', very little current flows, and no light is emitted. LEDs can be operated on an [[alternating current]] voltage, but they will only light with positive voltage, causing the LED to turn on and off at the frequency of the AC supply.


=== Phosphor-based LEDs ===
Most LEDs have low [[breakdown voltage|reverse breakdown voltage]] ratings, so they will also be damaged by an applied reverse voltage above this threshold. If it is desired to drive the LED directly from an AC supply of more than the reverse breakdown voltage then it may be protected by placing a diode (or another LED) in [[Antiparallel (electronics)|inverse parallel]].
[[File:White LED.png|thumb|upright=1.6|Spectrum of a white LED showing blue light directly emitted by the GaN-based LED (peak at about 465 nm) and the more broadband [[Stokes shift|Stokes-shifted]] light emitted by the Ce<sup>3+</sup>:YAG phosphor, which emits at roughly 500–700 nm]]


This method involves [[coating]] LEDs of one color (mostly blue LEDs made of [[InGaN]]) with [[phosphor]]s of different colors to form white light; the resultant LEDs are called phosphor-based or phosphor-converted white LEDs (pcLEDs).<ref>{{cite book|title=Fifth International Conference on Solid State Lighting|author1=Tanabe, S. |author2=Fujita, S. |author3=Yoshihara, S. |author4=Sakamoto, A. |author5=Yamamoto, S.|chapter=YAG glass-ceramic phosphor for white LED (II): Luminescence characteristics |journal=Proceedings of SPIE|chapter-url=http://lib.semi.ac.cn:8080/tsh/dzzy/wsqk/SPIE/vol5941/594112.pdf|archive-url=https://web.archive.org/web/20110511182527/http://lib.semi.ac.cn:8080/tsh/dzzy/wsqk/SPIE/vol5941/594112.pdf|archive-date=2011-05-11|volume=5941|doi=10.1117/12.614681|page=594112|year=2005|bibcode=2005SPIE.5941..193T |s2cid=38290951 |editor1-last=Ferguson |editor1-first=Ian T |editor2-last=Carrano |editor2-first=John C |editor3-last=Taguchi |editor3-first=Tsunemasa |editor4-last=Ashdown |editor4-first=Ian E }}</ref> A fraction of the blue light undergoes the Stokes shift, which transforms it from shorter wavelengths to longer. Depending on the original LED's color, various color phosphors are used. Using several phosphor layers of distinct colors broadens the emitted spectrum, effectively raising the [[Color Rendering Index|color rendering index]] (CRI).<ref>{{Cite journal|title=Color rendering and luminous efficacy of white LED spectra|author=Ohno, Y.|journal=Proc. SPIE|doi=10.1117/12.565757|url=http://lib.semi.ac.cn:8080/tsh/dzzy/wsqk/SPIE/vol5530/5530-88.pdf|archive-url=https://web.archive.org/web/20110511182632/http://lib.semi.ac.cn:8080/tsh/dzzy/wsqk/SPIE/vol5530/5530-88.pdf|archive-date=2011-05-11|volume=5530|page=89|year=2004|series=Fourth International Conference on Solid State Lighting|bibcode=2004SPIE.5530...88O|s2cid=122777225|editor1-last=Ferguson|editor1-first=Ian T|editor2-last=Narendran|editor2-first=Nadarajah|editor3-last=Denbaars|editor3-first=Steven P|editor4-last=Carrano|editor4-first=John C}}</ref>
The manufacturer will normally advise how to determine the polarity of the LED in the product datasheet. However, these methods may also be used:
{|class="wikitable"
|-
|'''sign:'''
|'''+'''
|'''-'''
|-
|terminal:
|anode (A)
|cathode (K)
|-
|leads:
|long
|short
|-
|exterior:
|round
|flat
|-
|interior:
|small
|large
|-
|wiring:
|red
|black
|-
|*marking:
|none
|stripe
|-
|*pin:
|1
|2
|-
|*[[Printed circuit board|PCB]]:
|round
|square
|-
|*Die placement:
|connector
|cup
|}
(*)Less reliable methods of determining polarity


Phosphor-based LEDs have efficiency losses due to heat loss from the [[Stokes shift]] and also other phosphor-related issues. Their luminous efficacies compared to normal LEDs depend on the spectral distribution of the resultant light output and the original wavelength of the LED itself. For example, the luminous efficacy of a typical YAG yellow phosphor based white LED ranges from 3 to 5 times the luminous efficacy of the original blue LED because of the human eye's greater sensitivity to yellow than to blue (as modeled in the [[luminosity function]]).
It is strongly recommended to apply a safe voltage and observe the illumination as a test regardless of what method is used to determine the polarity.


Due to the simplicity of manufacturing, the phosphor method is still the most popular method for making high-intensity white LEDs. The design and production of a light source or light fixture using a monochrome emitter with phosphor conversion is simpler and cheaper than a complex [[#RGB systems|RGB]] system, and the majority of high-intensity white LEDs presently on the market are manufactured using phosphor light conversion.{{citation needed|date=October 2020}}
=== Failure modes ===
[[File:1 watt 9 volt SMD LED.jpg|thumb|1 watt 9 volt three chips SMD phosphor based white LED]]
The most common way for LEDs (and [[diode laser]]s) to fail is the gradual lowering of light output and loss of efficiency. Sudden failures, however rare, can occur as well. Early red LEDs were notable for their short lifetime.
Among the challenges being faced to improve the efficiency of LED-based white light sources is the development of more efficient phosphors. As of 2010, the most efficient yellow phosphor is still the YAG phosphor, with less than 10% Stokes shift loss. Losses attributable to internal optical losses due to re-absorption in the LED chip and in the LED packaging itself account typically for another 10% to 30% of efficiency loss. Currently, in the area of phosphor LED development, much effort is being spent on optimizing these devices to higher light output and higher operation temperatures. For instance, the efficiency can be raised by adapting better package design or by using a more suitable type of phosphor. Conformal coating process is frequently used to address the issue of varying phosphor thickness.{{citation needed|date=October 2020}}


Some phosphor-based white LEDs encapsulate InGaN blue LEDs inside phosphor-coated epoxy. Alternatively, the LED might be paired with a remote phosphor, a preformed polycarbonate piece coated with the phosphor material. Remote phosphors provide more diffuse light, which is desirable for many applications. Remote phosphor designs are also more tolerant of variations in the LED emissions spectrum. A common yellow phosphor material is [[cerium]]-[[Doping (Semiconductors)|doped]] [[yttrium aluminium garnet]] (Ce<sup>3+</sup>:YAG).{{citation needed|date=October 2020}}
*'''[[Nucleation]] and growth of [[dislocation]]s''' is a known mechanism for degradation of the active region, where the radiative recombination occurs. This requires a presence of an existing defect in the crystal and is accelerated by heat, high current density, and emitted light. [[Gallium arsenide]] and [[aluminium gallium arsenide]] are more susceptible to this mechanism than [[gallium arsenide phosphide]] and [[indium phosphide]]. Due to different properties of the active regions, [[gallium nitride]] and [[indium gallium nitride]] are virtually insensitive to this kind of defect.


White LEDs can also be made by [[coating]] near-ultraviolet (NUV) LEDs with a mixture of high-efficiency [[europium]]-based phosphors that emit red and blue, plus copper and aluminium-doped zinc sulfide (ZnS:Cu, Al) that emits green. This is a method analogous to the way [[fluorescent lamp]]s work. This method is less efficient than blue LEDs with YAG:Ce phosphor, as the Stokes shift is larger, so more energy is converted to heat, but yields light with better spectral characteristics, which render color better. Due to the higher radiative output of the ultraviolet LEDs than of the blue ones, both methods offer comparable brightness. A concern is that UV light may leak from a malfunctioning light source and cause harm to human eyes or skin.{{citation needed|date=August 2023}}
*'''[[Electromigration]]''' caused by high current density can move atoms out of the active regions, leading to emergence of dislocations and point defects, acting as nonradiative recombination centers and producing heat instead of light.


A new style of wafers composed of gallium-nitride-on-silicon (GaN-on-Si) is being used to produce white LEDs using 200-mm silicon wafers. This avoids the typical costly [[sapphire]] [[Substrate (materials science)|substrate]] in relatively small 100- or 150-mm wafer sizes.<ref name="electronicdesign.com">[http://electronicdesign.com/europe-news/next-generation-gan-si-white-leds-suppress-costs Next-Generation GaN-on-Si White LEDs Suppress Costs], Electronic Design, 19 November 2013</ref> The sapphire apparatus must be coupled with a mirror-like collector to reflect light that would otherwise be wasted. It was predicted that since 2020, 40% of all GaN LEDs are made with GaN-on-Si. Manufacturing large sapphire material is difficult, while large silicon material is cheaper and more abundant. LED companies shifting from using sapphire to silicon should be a minimal investment.<ref>[http://www.isuppli.com/Semiconductor-Value-Chain/News/Pages/GaN-on-Silicon-LEDs-Forecast-to-Increase-Market-Share-to-40Percent-by-2020.aspx GaN-on-Silicon LEDs Forecast to Increase Market Share to 40 Percent by 2020], iSuppli, 4 December 2013</ref>
*'''[[Ionizing radiation]]''' can lead to the creation of defects as well, which leads to issues with [[radiation hardening]] of circuits containing LEDs (e.g., in [[optoisolator]]s)


=== Mixed white LEDs ===
* '''Differentiated phosphor degeneration.''' The different phosphors used in white LEDs tend to degrade with heat and age, but at different rates causing changes in the produced light color, for example, purple and pink LEDs often use an organic phosphor formulation which may degrade after just a few hours of operation causing a major shift in output color.<ref>{{cite web
[[File:Led Lights Panel.jpg|thumb|Tunable white LED array in a floodlight]]
| url = http://ledmuseum.candlepower.us/ledpink.htm
There are RGBW LEDs that combine RGB units with a phosphor white LED on the market. Doing so retains the extremely tunable color of RGB LED, but allows color rendering and efficiency to be optimized when a color close to white is selected.<ref>{{cite web |title=All You Want to Know about RGBW LED Light |url=https://www.agcled.com/blog/all-you-want-to-know-about-rgbw-led-light.html |website=AGC Lighting}}</ref>
| title = Candlepower Pink LED Reviews
| accessdate = 2008-09-19
}}</ref>


Some phosphor white LED units are "tunable white", blending two extremes of color temperatures (commonly 2700K and 6500K) to produce intermediate values. This feature allows users to change the lighting to suit the current use of a multifunction room.<ref>{{cite web |title=Tunable White Application Note |url=https://support.enlightedinc.com/hc/en-us/articles/360031886233-Tunable-White-Application-Note |website=enlightedinc.com}}</ref> As illustrated by a straight line on the chromaticity diagram, simple two-white blends will have a pink bias, becoming most severe in the middle. A small amount of green light, provided by another LED, could correct the problem.<ref>{{Cite web|url=https://leducation.org/green-light-can-maximize/|title=2021 How Green Light Can Maximize the Quality of Tunable White – LEDucation}}</ref> Some products are RGBWW, i.e. RGBW with tunable white.<ref name=EG.COL.T>{{cite web |title=Understanding LED Color-Tunable Products |url=https://www.energy.gov/eere/ssl/understanding-led-color-tunable-products |website=Energy.gov |language=en}}</ref>
* '''Metal diffusion''' caused by high electrical currents or voltages at elevated temperatures can move metal atoms from the electrodes into the active region. Some materials, notably [[indium tin oxide]] and [[silver]], are subject to electromigration which causes leakage current and non radiative recombination along the chip edges. In some cases, especially with GaN/InGaN diodes, a [[barrier metal]] layer is used to hinder the electromigration effects.


A final class of white LED with mixed light is dim-to-warm. These are ordinary 2700K white LED bulbs with a small red LED that turns on when the bulb is dimmed. Doing so makes the color warmer, emulating an incandescent light bulb.<ref name=EG.COL.T/>
*'''[[Short circuits]]''' Mechanical stresses, high currents, and corrosive environment can lead to formation of [[whisker (metallurgy)|whiskers]], causing short circuits.


=== Other white LEDs ===
*'''[[Thermal runaway]]''' Nonhomogenities in the substrate, causing localized loss of [[thermal conductivity]], can cause thermal runaway where heat causes damage which causes more heat etc. Most common ones are voids caused by incomplete [[soldering]], or by electromigration effects and [[Kirkendall voiding]].
Another method used to produce experimental white light LEDs used no phosphors at all and was based on [[epitaxy|homoepitaxially]] grown [[zinc selenide]] (ZnSe) on a ZnSe substrate that simultaneously emitted blue light from its active region and yellow light from the substrate.<ref>{{cite web| title = Joint venture to make ZnSe white LEDs| date = December 6, 2002| author = Whitaker, Tim| url = http://optics.org/cws/article/research/16534| access-date = January 3, 2009}}</ref>


==Organic light-emitting diodes (OLEDs)==
*'''[[Current crowding]]''', non-homogenous distribution of the current density over the junction. This may lead to creation of localized [[hot spot]]s, which poses risk of [[thermal runaway]].
{{Main|OLED}}


In an organic light-emitting diode ([[OLED]]), the [[Electroluminescence|electroluminescent]] material composing the emissive layer of the diode is an [[organic compound]]. The organic material is electrically conductive due to the [[Delocalized electron|delocalization]] of [[Pi bond|pi electrons]] caused by [[Conjugated system|conjugation]] over all or part of the molecule, and the material therefore functions as an [[organic semiconductor]].<ref>{{Cite journal |last1 = Burroughes |first1 = J. H. |last2 = Bradley | first2 = D. D. C. |last3 = Brown |first3 = A. R. |last4 = Marks |first4 = R. N. |last5 = MacKay |first5 = K. |last6 = Friend |first6 = R. H. |last7 = Burns |first7 = P. L. |last8 = Holmes |first8 = A. B. |doi = 10.1038/347539a0 |title = Light-emitting diodes based on conjugated polymers |journal = Nature |volume = 347 |issue = 6293 |pages = 539–541 |year = 1990 |bibcode=1990Natur.347..539B|s2cid = 43158308 }}</ref> The organic materials can be small organic [[molecule]]s in a [[crystal]]line [[phase (matter)|phase]], or [[polymer]]s.<ref name="OLEDSolidState">{{Cite conference |last1=Kho |first1=Mu-Jeong |last2=Javed |first2=T. |last3=Mark |first3=R. |last4=Maier |first4=E. |last5=David |first5=C |title=Final Report: OLED Solid State Lighting |conference=Kodak European Research |date=March 4, 2008 |location=Cambridge Science Park, Cambridge, UK}}</ref>
*'''Epoxy degradation''' Some materials of the plastic package tend to yellow when subjected to heat, causing partial absorption (and therefore loss of efficiency) of the affected wavelengths.


The potential advantages of OLEDs include thin, low-cost displays with a low driving voltage, wide viewing angle, and high contrast and color [[gamut]].<ref name="bardsley">{{Cite journal |last1 = Bardsley |first1 = J. N. |doi = 10.1109/JSTQE.2004.824077 |title = International OLED Technology Roadmap |journal = IEEE Journal of Selected Topics in Quantum Electronics |volume = 10 | issue = 1 |pages = 3–4 |year = 2004 |bibcode = 2004IJSTQ..10....3B |s2cid = 30084021 |url = https://zenodo.org/record/1232213 }}</ref> Polymer LEDs have the added benefit of printable and [[flexible organic light-emitting diode|flexible]] displays.<ref>{{Cite journal |last1 = Hebner | first1 = T. R. |last2 = Wu |first2 = C. C. |last3 = Marcy |first3 = D. |last4 = Lu |first4 = M. H. |last5 = Sturm |first5 = J. C. |title = Ink-jet printing of doped polymers for organic light emitting devices |doi = 10.1063/1.120807 |journal = Applied Physics Letters |volume = 72 |issue = 5 | page = 519 |year = 1998 |bibcode = 1998ApPhL..72..519H | s2cid = 119648364 }}</ref><ref>{{Cite journal |last1 = Bharathan |first1 = J. |last2 = Yang |first2 = Y. |doi = 10.1063/1.121090 |title = Polymer electroluminescent devices processed by inkjet printing: I. Polymer light-emitting logo |journal = Applied Physics Letters |volume = 72 |issue = 21 |page = 2660 |year = 1998 |bibcode = 1998ApPhL..72.2660B |s2cid = 44128025 }}</ref><ref>{{Cite journal |last1 = Gustafsson |first1 = G. |last2 = Cao |first2 = Y. |last3 = Treacy |first3 = G. M. |last4 = Klavetter |first4 = F. |last5 = Colaneri |first5 = N. |last6 = Heeger |first6 = A. J. |doi = 10.1038/357477a0 |title = Flexible light-emitting diodes made from soluble conducting polymers |journal = Nature |volume = 357 |issue = 6378 |pages = 477–479 |year = 1992 |bibcode=1992Natur.357..477G|s2cid = 4366944 }}</ref> OLEDs have been used to make visual displays for portable electronic devices such as cellphones, digital cameras, lighting and televisions.<ref name="OLEDSolidState" /><ref name="bardsley" />
* '''[[Thermal stress]]''' Sudden failures are most often caused by thermal stresses. When the [[epoxy resin]] package reaches its [[glass transition temperature]], it starts rapidly expanding, causing mechanical stresses on the semiconductor and the [[wire bonding|bonded]] contact, weakening it or even tearing it off. Conversely, very low temperatures can cause cracking of the packaging.


==Types==
*'''[[Electrostatic discharge]]''' (ESD) may cause immediate failure of the semiconductor junction, a permanent shift of its parameters, or latent damage causing increased rate of degradation. LEDs and lasers grown on [[sapphire]] substrate are more susceptible to ESD damage.
[[File:Verschiedene LEDs.jpg|thumb|LEDs are produced in a variety of shapes and sizes. The color of the plastic lens is often the same as the actual color of light emitted, but not always. For instance, purple plastic is often used for infrared LEDs, and most blue devices have colorless housings. Modern high-power LEDs such as those used for lighting and backlighting are generally found in [[surface-mount technology]] (SMT) packages (not shown).|397x397px]]
[[File:LED Rainbow Pack - 5mm PTH 12903-01 new aranged.jpg|thumb|upright|A variety of different diffused 5&nbsp;mm [[through-hole|THT]]-LEDs{{bulleted list|
| Red, 650 – 625nm
| Orange, 600 – 610nm
| Yellow, 587 – 591nm
| Green, 570 – 575nm
| Blue, 465 – 467nm
| Purple, 395 – 400nm}}]]


LEDs are made in different packages for different applications. A single or a few LED junctions may be packed in one miniature device for use as an indicator or pilot lamp. An LED array may include controlling circuits within the same package, which may range from a simple [[resistor]], blinking or color changing control, or an addressable controller for RGB devices. Higher-powered white-emitting devices will be mounted on heat sinks and will be used for illumination. Alphanumeric displays in dot matrix or bar formats are widely available. Special packages permit connection of LEDs to optical fibers for high-speed data communication links.
*'''[[Reverse bias]]''' Although the LED is based on a diode junction and is nominally a rectifier, the reverse-breakdown mode for some types can occur at very low voltages and essentially any excess reverse bias causes immediate degradation, and may lead to vastly accelerated failure. 5V is a typical, "maximum reverse bias voltage" figure for ordinary LEDS, some special types may have lower limits.


===Miniature===
==Colors and materials==
[[File:Single and multicolor surface mount miniature LEDs in most common sizes.jpg|thumb|Image of miniature [[SMD LED|surface mount LED]]s in most common sizes. They can be much smaller than a traditional 5{{nbsp}}mm lamp type LED, shown on the upper left corner.]]
Conventional LEDs are made from a variety of inorganic [[semiconductor materials]], the following table shows the available colors with wavelength range, voltage drop and material:
[[File:Very small 1.6x1.6x0.35 mm RGB Surface Mount LED EAST1616RGBA2.jpg|thumb|Very small (1.6×1.6×0.35{{nbsp}}mm) red, green, and blue surface mount miniature LED package with gold [[wire bonding]] details]]


These are mostly single-die LEDs used as indicators, and they come in various sizes from 1.8&nbsp;mm to 10&nbsp;mm, [[through-hole]] and [[surface mount]] packages.<ref>[http://www.elektor.com/magazines/2008/february/power-to-the-leds.350167.lynkx LED-design]. Elektor.com. Retrieved on March 16, 2012. {{webarchive |url=https://web.archive.org/web/20120831112624/http://www.elektor.com/magazines/2008/february/power-to-the-leds.350167.lynkx |date=August 31, 2012 }}</ref> Typical current ratings range from around 1&nbsp;mA to above 20&nbsp;mA. LED's can be soldered to a flexible PCB strip to form LED tape popularly used for decoration.
{| class=wikitable
!Color
![[Wavelength]] (nm)
![[Voltage]] (V)
!Semi-conductor Material
|-
|[[Infrared]] ||[[Wavelength|λ]] > 760 ||[[Delta (letter)|Δ]]V < 1.9 || [[Gallium arsenide]] (GaAs)<br /> [[Aluminium gallium arsenide]] (AlGaAs)
|-
|[[Red]] ||610 < λ < 760 ||1.63 < Δ[[Voltage|V]] < 2.03 || [[Aluminium gallium arsenide]] (AlGaAs)<br />[[Gallium arsenide phosphide]] (GaAsP)<br \>[[Aluminium gallium indium phosphide]] (AlGaInP) <br /> [[Gallium(III) phosphide]] (GaP)
|-
|[[Orange (colour)|Orange]] ||590 < λ < 610 ||2.03 < ΔV < 2.10 || [[Gallium arsenide phosphide]] (GaAsP)<br \>[[Aluminium gallium indium phosphide]] (AlGaInP) <br />[[Gallium(III) phosphide]] (GaP)
|-
|[[Yellow]] ||570 < λ < 590 ||2.10 < ΔV < 2.18 || [[Gallium arsenide phosphide]] (GaAsP)<br \>[[Aluminium gallium indium phosphide]] (AlGaInP) <br/> [[Gallium(III) phosphide]] (GaP)
|-
|[[Green]] ||500 < λ < 570 ||2.18 < ΔV < 4.0 || [[Indium gallium nitride]] (InGaN) / [[Gallium(III) nitride]] (GaN)<br />[[Gallium(III) phosphide]] (GaP)<br \>[[Aluminium gallium indium phosphide]] (AlGaInP)<br \>[[Aluminium gallium phosphide]] (AlGaP)
|-
|[[Blue]] ||450 < λ < 500 ||2.48 < ΔV < 3.7 || [[Zinc selenide]] (ZnSe)<br />[[Indium gallium nitride]] (InGaN)<br />[[Silicon carbide]] (SiC) as substrate<br \>[[Silicon]] (Si) as substrate — (under development)
|-
|[[Purple]] ||multiple types ||2.48 < ΔV < 3.7 || Dual blue/red LEDs,<br> blue with red phosphor,<br> or white with purple plastic
|-
|[[Violet (color)|Violet]] ||400 < λ < 450 ||2.76 < ΔV < 4.0 || [[Indium gallium nitride]] (InGaN)
|-
|[[Ultraviolet]] ||λ < 400 ||3.1 < ΔV < 4.4 || [[diamond]] (C)<br \>[[Aluminium nitride]] (AlN)<br \> [[Aluminium gallium nitride]] (AlGaN)<br \> [[Aluminium gallium indium nitride]] (AlGaInN) — (down to 210&nbsp;nm<ref>{{cite news |url=http://physicsworld.com/cws/article/news/24926 |title=LEDs move into the ultraviolet |date=May 17, 2006 |publisher=physicsworld.com |accessdate=2007-08-13}}</ref>)
|-
|[[White]] || Broad spectrum ||ΔV = 3.5 || Blue/UV diode with yellow phosphor &nbsp;
|}


Common package shapes include round, with a domed or flat top, rectangular with a flat top (as used in bar-graph displays), and triangular or square with a flat top. The encapsulation may also be clear or tinted to improve contrast and viewing angle. Infrared devices may have a black tint to block visible light while passing infrared radiation, such as the Osram SFH 4546.<ref>{{Cite web |title=OSRAM Radial T1 3/4, SFH 4546 IR LEDs - ams-osram - ams |url=https://ams-osram.com/products/leds/ir-leds/osram-radial-t1-34-sfh-4546 |access-date=2024-09-19 |website=ams-osram |language=en-US}}</ref>
=== Ultraviolet and blue LEDs ===


5&nbsp;V and 12&nbsp;V LEDs are ordinary miniature LEDs that have a series resistor for direct connection to a 5{{nbsp}}V or 12{{nbsp}}V supply.<ref>{{Cite web |title=LED Through Hole 5mm (T-1 3/4) Red Built-in resistor 635 nm 4500 mcd 12V |url=https://vcclite.com/product/lth5mm12vfr4100/ |access-date=2024-09-19 |website=VCC |language=en-US}}</ref>
[[Image:Blue LED and Reflection.JPG|thumb|right|230px|[[Blue]] LEDs.]]


===High-power===
Blue LEDs are based on the wide [[band gap]] semiconductors GaN ([[gallium nitride]]) and [[InGaN]] (indium gallium nitride). They can be added to existing red and green LEDs to produce the impression of [[white]] light, though white LEDs today rarely use this principle.
[[File:2007-07-24 High-power light emitting diodes (Luxeon, Lumiled).jpg|thumb|High-power light-emitting diodes attached to an LED star base ([[Luxeon]], [[Philips Lumileds Lighting Company|Lumileds]])]]
{{See also|Solid-state lighting|LED lamp|Thermal management of high-power LEDs}}


High-power LEDs (HP-LEDs) or high-output LEDs (HO-LEDs) can be driven at currents from hundreds of mA to more than an ampere, compared with the tens of mA for other LEDs. Some can emit over a thousand lumens.<ref>{{cite web |url=http://www.luminus.com/content1044|archive-url=https://web.archive.org/web/20080725033952/http://www.luminus.com/content1044 |archive-date=2008-07-25 |title=Luminus Products |publisher=Luminus Devices |access-date=October 21, 2009}}</ref><ref>{{cite web |url=http://www.luminus.com/stuff/contentmgr/files/0/7c8547b3575bcecc577525b80d210ac7/misc/pds_001314_rev_03__cst_90_w_product_datasheet_illumination.pdf |archive-url=https://web.archive.org/web/20100331100545/http://www.luminus.com/stuff/contentmgr/files/0/7c8547b3575bcecc577525b80d210ac7/misc/pds_001314_rev_03__cst_90_w_product_datasheet_illumination.pdf |archive-date=2010-03-31 |title=Luminus Products CST-90 Series Datasheet |publisher=Luminus Devices |access-date=2009-10-25}}</ref> LED [[Power density|power densities]] up to 300 W/cm<sup>2</sup> have been achieved. Since overheating is destructive, the HP-LEDs must be mounted on a heat sink to allow for heat dissipation. If the heat from an HP-LED is not removed, the device fails in seconds. One HP-LED can often replace an incandescent bulb in a [[flashlight]], or be set in an array to form a powerful [[LED lamp]].
The first blue LEDs were made in 1971 by Jacques Pankove (inventor of the gallium nitride LED) at [[RCA|RCA Laboratories]].<ref>{{cite web
| title = Alumni society honors four leaders in engineering and technology
| work = Berkeley Engineering News
| publisher =
| date = [[2000-09-04]]
| url = http://www.coe.berkeley.edu/EPA/EngNews/00F/EN2F/deaa.html
| format =
| doi =
| accessdate = 2007-01-23 }}</ref> However, these devices had too little light output to be of much practical use. In the late 1980s, key breakthroughs in GaN [[epitaxial]] growth and [[p-type]] doping by [[Isamu Akasaki]] and Hiroshi Amano (Nagoya, Japan)<ref>{{cite web
| title = GaN-based blue light emitting device development by Akasaki and Amano
| work = Takeda Award 2002 Achievement Facts Sheet
| publisher = The Takeda Foundation
| date = [[2002-04-05]]
| url = http://www.takeda-foundation.jp/en/award/takeda/2002/fact/pdf/fact01.pdf
| format = pdf
| doi =
| accessdate = 2007-11-28 }}</ref> ushered in the modern era of GaN-based optoelectronic devices. Building upon this foundation, in 1993 high brightness blue LEDs were demonstrated through the work of [[Shuji Nakamura]] at [[Nichia Corporation]].<ref>{{cite web
| title = United States Patent No. 5,578,839 (Nakamura et al.)
| work =
| publisher = [[United States Patent and Trademark Office]]
| date = filed [[1993-11-17]]
| url = http://patft.uspto.gov/netacgi/nph-Parser?Sect2=PTO1&Sect2=HITOFF&p=1&u=%2Fnetahtml%2FPTO%2Fsearch-bool.html&r=1&f=G&l=50&d=PALL&RefSrch=yes&Query=PN%2F5578839
| format =
| doi =
| accessdate = 2007-01-23 }}</ref>


Some HP-LEDs in this category are the [[Nichia]] 19 series, [[Lumileds]] Rebel Led, Osram Opto Semiconductors Golden Dragon, and Cree X-lamp. As of September 2009, some HP-LEDs manufactured by Cree exceed 105&nbsp;lm/W.<ref name="Xlamp Xp-G Led">{{cite web |url=http://www.cree.com/products/xlamp_xpg.asp |title=Xlamp Xp-G Led |website=Cree.com |publisher=[[Cree, Inc.]] |access-date=2012-03-16 |url-status=dead |archive-url=https://web.archive.org/web/20120313082324/http://www.cree.com/products/xlamp_xpg.asp |archive-date=2012-03-13 }}</ref>
By the late 1990s, blue LEDs had become widely available. They have an active region consisting of one or more InGaN [[quantum well]]s sandwiched between thicker layers of GaN, called cladding layers. By varying the relative InN-GaN fraction in the InGaN quantum wells, the light emission can be varied from violet to amber. AlGaN [[aluminium gallium nitride]] of varying AlN fraction can be used to manufacture the cladding and quantum well layers for ultraviolet LEDs, but these devices have not yet reached the level of efficiency and technological maturity of the InGaN-GaN blue/green devices. If the active quantum well layers are GaN, as opposed to alloyed InGaN or AlGaN, the device will emit near-ultraviolet light with wavelengths around 350–370&nbsp;nm. Green LEDs manufactured from the InGaN-GaN system are far more efficient and brighter than green LEDs produced with non-nitride material systems.


Examples for [[Haitz's law]]—which predicts an exponential rise in light output and efficacy of LEDs over time—are the CREE XP-G series LED, which achieved 105{{nbsp}}lm/W in 2009<ref name="Xlamp Xp-G Led" /> and the Nichia 19 series with a typical efficacy of 140{{nbsp}}lm/W, released in 2010.<ref>[http://www.nichia.co.jp/en/about_nichia/2010/2010_110201.html High Power Point Source White Led NVSx219A] {{Webarchive|url=https://web.archive.org/web/20210729062935/https://www.nichia.co.jp/en/about_nichia/2010/2010_110201.html |date=July 29, 2021 }}. Nichia.co.jp, November 2, 2010.</ref>
With nitrides containing aluminium, most often [[Aluminum gallium nitride|AlGaN]] and [[aluminum gallium indium nitride|AlGaInN]], even shorter wavelengths are achievable. Ultraviolet LEDs in a range of wavelengths are becoming available on the market. Near-UV emitters at wavelengths around 375–395 nm are already cheap and often encountered, for example, as [[black light]] lamp replacements for inspection of anti-[[counterfeiting]] UV watermarks in some documents and paper currencies. Shorter wavelength diodes, while substantially more expensive, are commercially available for wavelengths down to 247 nm.<ref>[http://www.s-et.com/products.htm Sensor Electronic Technology, Inc.: Nitride Products Manufacturer]</ref> As the photosensitivity of microorganisms approximately matches the absorption spectrum of [[DNA]], with a peak at about 260 nm, UV LEDs emitting at 250–270 nm are to be expected in prospective disinfection and sterilization devices. Recent research has shown that commercially available UVA LEDs (365 nm) are already effective disinfection and sterilization devices.<ref name=water sterilization"/>


===AC-driven===
Wavelengths down to 210 nm were obtained in laboratories using [[aluminium nitride]].
LEDs developed by Seoul Semiconductor can operate on AC power without a DC converter. For each half-cycle, part of the LED emits light and part is dark, and this is reversed during the next half-cycle. The efficiency of this type of HP-LED is typically 40{{nbsp}}lm/W.<ref>{{cite web|url=http://www.ledsmagazine.com/news/3/11/14|title=Seoul Semiconductor launches AC LED lighting source Acrich|publisher=LEDS Magazine|access-date=February 17, 2008|date=November 17, 2006|archive-date=October 15, 2007|archive-url=https://web.archive.org/web/20071015021634/http://www.ledsmagazine.com/news/3/11/14|url-status=dead}}</ref> A large number of LED elements in series may be able to operate directly from line voltage. In 2009, Seoul Semiconductor released a high DC voltage LED, named 'Acrich MJT', capable of being driven from AC power with a simple controlling circuit. The low-power dissipation of these LEDs affords them more flexibility than the original AC LED design.<ref name="IDA" />


===Strip===
While not an LED as such, an ordinary NPN bipolar transistor will emit violet light if its emitter-base junction is subjected to non-destructive reverse breakdown. This is easy to demonstrate by filing the top off a metal-can transistor (BC107, 2N2222 or similar) and biasing it well above emitter-base breakdown (≥ 20 V) via a current-limiting resistor.
{{excerpt|LED strip light}}


=== White light LEDs ===
=== Application-specific ===
{{more citations needed|section|date=October 2020}}
[[File:RGB-SMD-LED.jpg|thumb|RGB-SMD-LED]]
[[File:Macro photo of LED matrix.jpg|thumb|upright|Composite image of an {{nowrap|11 × 44}} LED matrix lapel [[name tag]] display using 1608/0603-type SMD LEDs. Top: A little over half of the {{nowrap|21 × 86 mm}} display. Center: Close-up of LEDs in ambient light. Bottom: LEDs in their own red light.]]
; Flashing: Flashing LEDs are used as attention seeking indicators without requiring external electronics. Flashing LEDs resemble standard LEDs but they contain an integrated [[voltage regulator]] and a [[multivibrator]] circuit that causes the LED to flash with a typical period of one second. In diffused lens LEDs, this circuit is visible as a small black dot. Most flashing LEDs emit light of one color, but more sophisticated devices can flash between multiple colors and even fade through a color sequence using RGB color mixing. Flashing SMD LEDs in the 0805 and other size formats have been available since early 2019.
; Flickering: Integrated electronics Simple electronic circuits integrated into the LED package have been around since at least 2011 which produce a random LED intensity pattern reminiscent of a flickering [[candle]].<ref>{{Cite web |last=Oskay |first=Windell |date=2011-06-22 |title=Does this LED sound funny to you? |url=https://www.evilmadscientist.com/2011/does-this-led-sound-funny-to-you/ |url-status=live |archive-url=https://web.archive.org/web/20230924100327/https://www.evilmadscientist.com/2011/does-this-led-sound-funny-to-you/ |archive-date=2023-09-24 |access-date=2024-01-30 |website=Evil Mad Scientist Laboratories |language=en-US}}</ref> [[Reverse engineering]] in 2024 has suggested that some flickering LEDs with automatic sleep and wake modes might be using an integrated [[8-bit computing|8-bit]] [[microcontroller]] for such functionally.<ref>{{Cite web |last=Tim's Blog |date=2024-01-14 |title=Revisiting Candle Flicker-LEDs: Now with integrated Timer |url=https://cpldcpu.wordpress.com/2024/01/14/revisiting-candle-flicker-leds-now-with-integrated-timer/ |url-status=live |archive-url=https://web.archive.org/web/20240129164612/https://cpldcpu.wordpress.com/2024/01/14/revisiting-candle-flicker-leds-now-with-integrated-timer/ |archive-date=2024-01-29 |access-date=2024-01-30 |website=cpldcpu.wordpress.com |language=en}}</ref>
; Bi-color: Bi-color LEDs contain two different LED emitters in one case. There are two types of these. One type consists of two dies connected to the same two leads [[Antiparallel (electronics)|antiparallel]] to each other. Current flow in one direction emits one color, and current in the opposite direction emits the other color. The other type consists of two dies with separate leads for both dies and another lead for common anode or cathode so that they can be controlled independently. The most common bi-color combination is [[RG color space|red/traditional green]]. Others include amber/traditional green, red/pure green, red/blue, and blue/pure green.
; RGB tri-color: Tri-color LEDs contain three different LED emitters in one case. Each emitter is connected to a separate lead so they can be controlled independently. A four-lead arrangement is typical with one common lead (anode or cathode) and an additional lead for each color. Others have only two leads (positive and negative) and have a built-in electronic controller. [[RGB color model|RGB]] LEDs consist of one red, one green, and one blue LED.<ref>{{Cite book |url=https://books.google.com/books?id=qk1hmpEQVxIC&pg=PA349 |title=5th Kuala Lumpur International Conference on Biomedical Engineering 2011: BIOMED 2011, 20–23 June 2011, Kuala Lumpur, Malaysia |last=Ting |first=Hua-Nong |date=2011-06-17|publisher=Springer Science & Business Media |isbn=9783642217296}}</ref> By independently [[pulse-width modulation|adjusting]] each of the three, RGB LEDs are capable of producing a wide color gamut. Unlike dedicated-color LEDs, these do not produce pure wavelengths. Modules may not be optimized for smooth color mixing.
; Decorative-multicolor: Decorative-multicolor LEDs incorporate several emitters of different colors supplied by only two lead-out wires. Colors are switched internally by varying the supply voltage.
; Alphanumeric: Alphanumeric LEDs are available in [[seven-segment display|seven-segment]], [[Starburst display|starburst]], and [[Dot-matrix display|dot-matrix]] format. Seven-segment displays handle all numbers and a limited set of letters. Starburst displays can display all letters. Dot-matrix displays typically use 5×7 pixels per character. Seven-segment LED displays were in widespread use in the 1970s and 1980s, but rising use of [[liquid crystal display]]s, with their lower power needs and greater display flexibility, has reduced the popularity of numeric and alphanumeric LED displays.
; Digital RGB: Digital RGB addressable LEDs contain their own "smart" control electronics. In addition to power and ground, these provide connections for data-in, data-out, clock and sometimes a strobe signal. These are connected in a [[Daisy chain (electrical engineering)|daisy chain]], which allows individual LEDs in a long [[LED strip light]] to be easily controlled by a microcontroller. Data sent to the first LED of the chain can control the brightness and color of each LED independently of the others. They are used where a combination of maximum control and minimum visible electronics are needed such as strings for Christmas and LED matrices. Some even have refresh rates in the kHz range, allowing for basic video applications. These devices are known by their part number ([https://cdn-shop.adafruit.com/datasheets/WS2812.pdf WS2812] being common) or a brand name such as [[Adafruit Industries#NeoPixel|NeoPixel]].
; Filament: An [[LED filament]] consists of multiple LED chips connected in series on a common longitudinal substrate that forms a thin rod reminiscent of a traditional incandescent filament.<ref>{{cite web|url=http://www.ledinside.com/knowledge/2015/2/the_next_generation_of_led_filament_bulbs|title=The Next Generation of LED Filament Bulbs|website=LEDInside.com|publisher=Trendforce|access-date=October 26, 2015}}</ref> These are being used as a low-cost decorative alternative for traditional light bulbs that are being phased out in many countries. The filaments use a rather high voltage, allowing them to work efficiently with mains voltages. Often a simple rectifier and capacitive current limiting are employed to create a low-cost replacement for a traditional light bulb without the complexity of the low voltage, high current converter that single die LEDs need.<ref>Archived at [https://ghostarchive.org/varchive/youtube/20211211/H_XiunR-cAQ Ghostarchive]{{cbignore}} and the [https://web.archive.org/web/20151122213511/https://www.youtube.com/watch?v=H_XiunR-cAQ Wayback Machine]{{cbignore}}: {{cite web|url=https://www.youtube.com/watch?v=H_XiunR-cAQ|title=LED Filaments|website=[[YouTube]]|date=April 5, 2015 |access-date=October 26, 2015}}{{cbignore}}</ref> Usually, they are packaged in bulb similar to the lamps they were designed to replace, and filled with inert gas at slightly lower than ambient pressure to remove heat efficiently and prevent corrosion.
; Chip-on-board arrays: Surface-mounted LEDs are frequently produced in [[chip on board]] (COB) arrays, allowing better heat dissipation than with a single LED of comparable luminous output.<ref>{{cite book|title=Handbook on the Physics and Chemistry of Rare Earths: Including Actinides|url=https://books.google.com/books?id=lO_lCgAAQBAJ&pg=PA89|date=1 August 2016|publisher=Elsevier Science|isbn=978-0-444-63705-5|page=89}}</ref> The LEDs can be arranged around a cylinder, and are called "corn cob lights" because of the rows of yellow LEDs.<ref>{{cite web |title=Corn Lamps: What Are They & Where Can I Use Them? |date=September 1, 2016 |publisher=Shine Retrofits |url=https://www.shineretrofits.com/lighting-center/corn-lamps |access-date=December 30, 2018}}</ref>


== Considerations for use ==
There are two ways of producing high intensity [[white]]-light using LEDs. One is to use individual LEDs that emit three [[primary color]]s<ref>{{cite news
| title = Jan Henrik Wold and Arne Valberg
| publish = Vol. 26, pp. S222
| year = 2001}}</ref> – red, green, and blue, and then mix all the colors to produce [[white]] light. The other is to use a phosphor material to convert monochromatic light from a blue or UV LED to broad-spectrum white light, much in the same way a fluorescent light bulb works.


* Efficiency: LEDs emit more lumens per watt than incandescent light bulbs.<ref>{{cite web|url=http://www1.eere.energy.gov/buildings/ssl/comparing.html|archive-url=https://web.archive.org/web/20090505080533/http://www1.eere.energy.gov/buildings/ssl/comparing.html |archive-date=2009-05-05|title=Solid-State Lighting: Comparing LEDs to Traditional Light Sources|website=eere.energy.gov}}</ref> The efficiency of LED lighting fixtures is not affected by shape and size, unlike fluorescent light bulbs or tubes.
==== RGB Systems ====
* Size: LEDs can be very small (smaller than 2&nbsp;mm<sup>2</sup><ref>{{cite web|url=http://www.dialight.com/Assets/Brochures_And_Catalogs/Indication/MDEI5980603.pdf|archive-url=https://web.archive.org/web/20090205040334/http://www.dialight.com/Assets/Brochures_And_Catalogs/Indication/MDEI5980603.pdf |archive-date=2009-02-05|title=Dialight Micro LED SMD LED "598 SERIES" Datasheet|website=Dialight.com}}
</ref>) and are easily attached to printed circuit boards.


=== Power sources ===
[[Image:Red-YellowGreen-Blue LED spectra.png|thumb|right|350px|Combined spectral curves for blue, yellow-green, and high brightness red solid-state semiconductor LEDs. [[Full width at half maximum|FWHM]] spectral bandwidth is approximately 24–27 nm for all three colors.]]
{{Main|LED power sources}}
[[File:LED circuit.svg|thumb|upright|Simple LED circuit with resistor for current limiting]]


The current in an LED or other diodes rises exponentially with the applied voltage (see [[Shockley diode equation]]), so a small change in voltage can cause a large change in current. Current through the LED must be regulated by an external circuit such as a [[constant current]] source to prevent damage. Since most common power supplies are (nearly) constant-voltage sources, LED fixtures must include a power converter, or at least a current-limiting resistor. In some applications, the internal resistance of small batteries is sufficient to keep current within the LED rating.{{citation needed|date=October 2020}}
[[White_light#Light|White light]] can be produced by mixing differently colored light, the most common method is to use [[rgb|red, green and blue]] (RGB). Hence the method is called multi-colored white LEDs (sometimes referred to as RGB LEDs). Because its mechanism is involved with sophisticated electro-optical design to control the blending and [[diffusion]] of different colors, this approach has rarely been used to mass produce white LEDs in the industry. Nevertheless this method is particularly interesting to many researchers and scientists because of the flexibility of mixing different colors.<ref>{{cite paper |url=http://www.opticsexpress.org/viewmedia.cfm?uri=oe-15-6-3607&seq=0 |title=Color distribution from multicolor LED arrays |work=... |author=... |publisher=Optics Express |year=2007 |accessdate=2008-09-10}} </ref> In principle, this mechanism also has higher quantum efficiency in producing white light.


LEDs are sensitive to voltage. They must be supplied with a voltage above their [[P–n junction#Forward bias|threshold voltage]] and a current below their rating. Current and lifetime change greatly with a small change in applied voltage. They thus require a current-regulated supply (usually just a series resistor for indicator LEDs).<ref>[http://www.ledmuseum.org/ The LED Museum]. Retrieved on March 16, 2012.</ref>
There are several types of multi-colored white LEDs: [[dichromatic|di-]], [[trichromatic|tri-]], and [[tetrachromatic]] white LEDs. Several key factors that play among these different approaches include color [[stability]], [[color rendering index|color rendering]] capability, and [[luminous efficiency]]. Often higher efficiency will mean lower color rendering, presenting a trade off between the luminous efficiency and color rendering. For example, the dichromatic white LEDs have the best luminous efficiency (120 lm/W), but the lowest color rendering capability. Oppositely although [[tetrachromatic]] white LEDs have excellent color rendering capability, they often have poor luminous efficiency. Trichromatic white LEDs are in between, having both good luminous efficiency (>70 lm/W) and fair color rendering capability.


[[LED droop|Efficiency droop]]: The efficiency of LEDs decreases as the [[electric current]] increases. Heating also increases with higher currents, which compromises LED lifetime. These effects put practical limits on the current through an LED in high power applications.<ref name="stevenson">Stevenson, Richard (August 2009), "[https://web.archive.org/web/20090805082614/http://www.spectrum.ieee.org/semiconductors/optoelectronics/the-leds-dark-secret The LED's Dark Secret: Solid-state lighting will not supplant the lightbulb until it can overcome the mysterious malady known as droop]". ''IEEE Spectrum''.</ref>
What multi-color LEDs offer is not merely another solution of producing white light, but is a whole new technique of producing light of different colors. In principle, all [[Colors#Color_perception|perceivable colors]] can be produced by mixing different amounts of three primary colors, and this makes it possible to produce precise dynamic color control as well. As more effort is devoted to investigating this technique, multi-color LEDs should have profound influence on the fundamental method which we use to produce and control light color. However, before this type of LED can truly play a role on the market, several technical problems need to be solved. These certainly include that this type of LED's emission power decays [[exponentially]] with increasing temperature,<ref>{{cite news
| title = E. Fred Schubert and Jong Kyu Kim
| publish = Science 308, 1274
| year = 2005}}</ref>
resulting in a substantial change in color stability. Such problem is not acceptable for industrial usage. Therefore, many new package designs aiming to solve this problem have been proposed, and their results are being reproduced by researchers and scientists.


=== Electrical polarity ===
====Phosphor based LEDs====
{{Main|Electrical polarity of LEDs}}


Unlike a traditional incandescent lamp, an LED will light only when voltage is applied in the forward direction of the diode. No current flows and no light is emitted if voltage is applied in the reverse direction. If the reverse voltage exceeds the [[breakdown voltage]], which is typically about five volts, a large current flows and the LED will be damaged. If the reverse current is sufficiently limited to avoid damage, the reverse-conducting LED is a useful [[Hardware random number generator|noise diode]].{{citation needed|date=October 2020}}
[[Image:White LED.png|thumb|right|350px|Spectrum of a “white” LED clearly showing blue light which is directly emitted by the GaN-based LED (peak at about 465 nm) and the more broadband [[Stokes shift|Stokes-shifted]] light emitted by the Ce<sup>3+</sup>:YAG phosphor which emits at roughly 500–700 nm.]]


By definition, the energy band gap of any diode is higher when reverse-biased than when forward-biased. Because the band gap energy determines the wavelength of the light emitted, the color cannot be the same when reverse-biased. The reverse breakdown voltage is sufficiently high that the emitted wavelength cannot be similar enough to still be visible. Though dual-LED packages exist that contain a different color LED in each direction, it is not expected that any single LED element can emit visible light when reverse-biased.{{citation needed|date=December 2022}}
This method involves [[coating]] an LED of one color (mostly blue LED made of InGaN) with [[phosphor]] of different colors to produce white light, the resultant LEDs are called '''phosphor based white LEDs'''. A fraction of the blue light undergoes the [[Stokes shift]] being transformed from shorter wavelengths to longer. Depending on the color of the original LED, phosphors of different colors can be employed. If several phosphor layers of distinct colors are applied, the emitted spectrum is broadened, effectively increasing the [[Color Rendering Index|color rendering index]] (CRI) value of a given LED.


It is not known if any zener diode could exist that emits light only in reverse-bias mode. Uniquely, this type of LED would conduct when connected backwards.
Phosphor based LEDs have a lower efficiency than normal LEDs due to the heat loss from the Stokes shift and also other phosphor-related degradation issues. However, the phosphor method is still the most popular technique for manufacturing [[Led#High_power LEDs|high intensity]] white LEDs. The design and production of a light source or light fixture using a monochrome emitter with phosphor conversion is simpler and cheaper than a complex [[Led#RGB_Systems|RGB]] system, and the majority of high intensity white LEDs presently on the market are manufactured using phosphor light conversion.


===Appearance===
The greatest barrier to high efficiency is the seemingly unavoidable Stokes energy loss. However, much effort is being spent on optimizing these devices to higher light output and higher operation temperatures. The efficiency can for instance be increased by adapting better package design or by using a more suitable type of phosphor. [[Philips Lumileds Lighting Company|Philips Lumileds]] patented conformal coating process addresses for instance the issue of varying phosphor thickness, giving the white LEDs a more homogeneous white light. With development ongoing the efficiency of phosphor based LEDs is generally increased with every new product announcement.
* Color: LEDs can emit light of an intended color without using any color filters as traditional lighting methods need. This is more efficient and can lower initial costs.
* Cool light: In contrast to most light sources, LEDs radiate very little heat in the form of IR that can cause damage to sensitive objects or fabrics. Wasted energy is dispersed as heat through the base of the LED.
* Color rendition: Most cool-[[#Other white LEDs|white LEDs]] have spectra that differ significantly from a [[black body]] radiator like the sun or an incandescent light. The spike at 460&nbsp;nm and dip at 500&nbsp;nm can make the color of objects [[color vision|appear differently]] under cool-white LED illumination than sunlight or incandescent sources, due to [[metamerism (color)|metamerism]],<ref>{{cite web|url = http://www.jimworthey.com/jimtalk2006feb.html|title = How White Light Works|author = Worthey, James A. |website = LRO Lighting Research Symposium, Light and Color|access-date = October 6, 2007}}</ref> red surfaces being rendered particularly poorly by typical phosphor-based cool-white LEDs. The same is true with green surfaces. The quality of color rendition of an LED is measured by the [[Color rendering index|Color Rendering Index (CRI)]].
* Dimming: LEDs can be [[Dimmer|dimmed]] either by [[pulse-width modulation]] or lowering the forward current.<ref>{{Cite book |last1=Narra |first1=Prathyusha |last2=Zinger |first2=D.S. |title=Conference Record of the 2004 IEEE Industry Applications Conference, 2004. 39th IAS Annual Meeting |chapter=An effective LED dimming approach |year=2004|volume=3 |pages= 1671–1676 |doi=10.1109/IAS.2004.1348695 |isbn=978-0-7803-8486-6 |s2cid=16372401 }}</ref> This pulse-width modulation is why LED lights, particularly headlights on cars, when viewed on camera or by some people, seem to flash or flicker. This is a type of [[stroboscopic effect]].


===Light properties===
Technically the phosphor based white LEDs encapsulate InGaN blue LEDs inside of a phosphor coated epoxy. A common yellow phosphor material is [[cerium]]-[[Doping (Semiconductors)|doped]] [[YAG|yttrium aluminum garnet]] (Ce<sup>3+</sup>:YAG).
* Switch on time: LEDs light up extremely quickly. A typical red indicator LED achieves full brightness in under a [[microsecond]].<ref>{{cite web|url=http://www.avagotech.com/docs/AV02-1555EN|title=Data Sheet&nbsp;— HLMP-1301, T-1 (3 mm) Diffused LED Lamps |publisher=Avago Technologies |access-date=May 30, 2010}}</ref> LEDs used in communications devices can have even faster response times.
* Focus: The solid package of the LED can be designed to [[focus (optics)|focus]] its light. Incandescent and fluorescent sources often require an external reflector to collect light and direct it in a usable manner. For larger LED packages [[total internal reflection]] (TIR) lenses are often used to the same effect. When large quantities of light are needed, many light sources such as LED chips are usually deployed, which are difficult to focus or [[collimate]] on the same target.
* Area light source: Single LEDs do not approximate a [[point source]] of light giving a spherical light distribution, but rather a [[Lambert's cosine law|lambertian]] distribution. So, LEDs are difficult to apply to uses needing a spherical light field. Different fields of light can be manipulated by the application of different optics or "lenses". LEDs cannot provide divergence below a few degrees.<ref>{{Cite book|author=Hecht, E. |title=Optics|url=https://archive.org/details/optics00ehec |url-access=limited |edition=4|page=[https://archive.org/details/optics00ehec/page/n596 591]|publisher=Addison Wesley|year= 2002|isbn=978-0-19-510818-7}}</ref>


===Reliability===
White LEDs can also be made by [[coating]] near [[ultraviolet]] (NUV) emitting LEDs with a mixture of high efficiency [[europium]]-based red and blue emitting phosphors plus green emitting copper and aluminum doped zinc sulfide (ZnS:Cu, Al). This is a method analogous to the way [[fluorescent lamp]]s work. However, the ultraviolet light causes [[photodegradation]] to the [[epoxy resin]] and many other materials used in LED packaging, causing manufacturing challenges and shorter lifetimes. This method is less efficient than the blue LED with YAG:Ce phosphor, as the [[Stokes shift]] is larger and more energy is therefore converted to heat, but yields light with better spectral characteristics, which render color better. Due to the higher radiative output of the ultraviolet LEDs than of the blue ones, both approaches offer comparable brightness. Another concern is that UV light may leak from a malfunctioning light source and cause harm to human eyes or skin.
* Shock resistance: LEDs, being solid-state components, are difficult to damage with external shock, unlike fluorescent and incandescent bulbs, which are fragile.<ref>{{cite web|url=https://www.larsonelectronics.com/a-5-led-light-bars-for-off-road-illumination.aspx|title=LED Light Bars For Off Road Illumination|website=Larson Electronics}}</ref>
* Thermal runaway: Parallel strings of LEDs will not share current evenly due to the manufacturing tolerances in their forward voltage. Running two or more strings from a single current source may result in LED failure as the devices warm up. If forward voltage binning is not possible, a circuit is required to ensure even distribution of current between parallel strands.<ref>{{cite web |url=https://www.ledsmagazine.com/articles/print/volume-6/issue-2/features/led-design-forum-avoiding-thermal-runaway-when-driving-multiple-led-strings-magazine.html |title=LED Design Forum: Avoiding thermal runaway when driving multiple LED strings |work=LEDs Magazine |date=20 April 2009 |access-date=17 January 2019 }}</ref>
* Slow failure: LEDs mainly fail by dimming over time, rather than the abrupt failure of incandescent bulbs.<ref name=eere>{{cite web|url=http://www1.eere.energy.gov/buildings/ssl/lifetime.html |title=Lifetime of White LEDs |access-date=2009-04-10 |url-status=dead |archive-url=https://web.archive.org/web/20090410145015/http://www1.eere.energy.gov/buildings/ssl/lifetime.html |archive-date=April 10, 2009 |df=mdy }}, US Department of Energy</ref>
* Lifetime: LEDs can have a relatively long useful life. One report estimates 35,000 to 50,000 hours of useful life, though time to complete failure may be shorter or longer.<ref>[http://apps1.eere.energy.gov/buildings/publications/pdfs/ssl/lifetime_white_leds_aug16_r1.pdf Lifetime of White LEDs] {{Webarchive|url=https://web.archive.org/web/20160528075610/http://apps1.eere.energy.gov/buildings/publications/pdfs/ssl/lifetime_white_leds_aug16_r1.pdf |date=May 28, 2016 }}. US Department of Energy. (PDF). Retrieved on March 16, 2012.</ref> Fluorescent tubes typically are rated at about 10,000 to 25,000 hours, depending partly on the conditions of use, and incandescent light bulbs at 1,000 to 2,000 hours. Several [[United States Department of Energy|DOE]] demonstrations have shown that reduced maintenance costs from this extended lifetime, rather than energy savings, is the primary factor in determining the payback period for an LED product.<ref>{{cite web|url=http://energy.ltgovernors.com/in-depth-advantages-of-led-lighting.html|title=In depth: Advantages of LED Lighting|website=energy.ltgovernors.com|access-date=July 27, 2012|archive-date=November 14, 2017|archive-url=https://web.archive.org/web/20171114184333/http://energy.ltgovernors.com/in-depth-advantages-of-led-lighting.html|url-status=dead}}</ref>
* Cycling: LEDs are ideal for uses subject to frequent on-off cycling, unlike incandescent and fluorescent lamps that fail faster when cycled often, or [[high-intensity discharge lamp]]s (HID lamps) that require a long time to warm up to full output and to cool down before they can be lighted again if they are being restarted.
* Temperature dependence: LED performance largely depends on the ambient temperature of the operating environment&nbsp;– or thermal management properties. Overdriving an LED in high ambient temperatures may result in overheating the LED package, eventually leading to device failure. An adequate [[heat sink]] is needed to maintain long life. This is especially important in automotive, medical, and military uses where devices must operate over a wide range of temperatures, and require low failure rates.


== Manufacturing ==
The newest method used to produce white light LEDs uses no phosphors at all and is based on [[wiktionary:homoepitaxially|homoepitaxially]] grown [[zinc selenide]] (ZnSe) on a ZnSe substrate which simultaneously emits blue light from its active region and yellow light from the substrate.{{Fact|date=August 2008}}
LED manufacturing involves multiple steps, including epitaxy, chip processing, chip separation, and packaging.<ref>{{Cite journal |last1=Stern |first1=Maike Lorena |last2=Schellenberger |first2=Martin |date=2020-03-31 |title=Fully convolutional networks for chip-wise defect detection employing photoluminescence images |url=http://dx.doi.org/10.1007/s10845-020-01563-4 |journal=Journal of Intelligent Manufacturing |volume=32 |issue=1 |pages=113–126 |doi=10.1007/s10845-020-01563-4 |issn=0956-5515|arxiv=1910.02451 |s2cid=254655125 }}</ref>


In a typical LED manufacturing process, encapsulation is performed after probing, dicing, die transfer from wafer to package, and wire bonding or flip chip mounting,<ref>{{cite journal | doi=10.1007/s10854-019-02393-8 | title=Effects of humidity and phosphor on silicone/Phosphor composite in white light-emitting diode package | date=2019 | last1=Hoque | first1=Md Ashraful | last2=Bradley | first2=Robert Kelley | last3=Fan | first3=Jiajie | last4=Fan | first4=Xuejun | journal=Journal of Materials Science: Materials in Electronics | volume=30 | issue=23 | pages=20471–20478 | doi-access=free }}</ref> perhaps using [[indium tin oxide]], a transparent electrical conductor. In this case, the bond wire(s) are attached to the ITO film that has been deposited in the LEDs.
=== Organic light-emitting diodes (OLEDs) ===


Flip chip circuit on board (COB) is a technique that can be used to manufacture LEDs.<ref>{{Cite web |title=3-Pad LED Flip Chip COB |url=https://www.led-professional.com/resources-1/articles/3-pad-led-flip-chip-cob |access-date=2024-02-15 |website=LED professional - LED Lighting Technology, Application Magazine |language=en}}</ref>
{{main|Organic light-emitting diode}}


== Colors and materials ==
If the emitting layer material of the LED is an [[organic compound]], it is known as an Organic Light Emitting Diode ([[Organic light-emitting diode|OLED]]). To function as a semiconductor, the organic emitting material must have [[conjugated system|conjugated pi bonds]]. The emitting material can be a small organic [[molecule]] in a [[crystal]]line [[phase (matter)|phase]], or a [[polymer]]. Polymer materials can be flexible; such LEDs are known as PLEDs or FLEDs.
Conventional LEDs are made from a variety of inorganic [[semiconductor materials]]. The following table shows the available colors with wavelength range, voltage drop and material:
{| class="wikitable" border="1"
!
!Color
![[Wavelength]] (nm)
!Voltage (V)
!Semiconductor material
|-
| bgcolor="white" |
|[[Infrared]]
|[[Wavelength|''λ'']] > 760
|[[Delta (letter)|Δ]]''V'' < 1.9
|[[Gallium arsenide]] (GaAs)
[[Aluminium gallium arsenide]] (AlGaAs)
|-
| bgcolor="red" |
|[[Red]]
|610 < ''λ'' < 760
|1.63 < Δ''V'' < 2.03
|[[Aluminium gallium arsenide]] (AlGaAs)
[[Gallium arsenide phosphide]] (GaAsP)
[[Aluminium gallium indium phosphide]] (AlGaInP)
[[Gallium(III) phosphide]] (GaP)
|-
| bgcolor="#FF7F00" |
|[[Orange (colour)|Orange]]
|590 < ''λ'' < 610
|2.03 < Δ''V'' < 2.10
|[[Gallium arsenide phosphide]] (GaAsP)
[[Aluminium gallium indium phosphide]] (AlGaInP)
[[Gallium(III) phosphide]] (GaP)
|-
| bgcolor="yellow" |
|[[Yellow]]
|570 < ''λ'' < 590
|2.10 < Δ''V'' < 2.18
|[[Gallium arsenide phosphide]] (GaAsP)
[[Aluminium gallium indium phosphide]] (AlGaInP)
[[Gallium(III) phosphide]] (GaP)
|-
| bgcolor="#00FF00" |
|[[Green]]
|500 < ''λ'' < 570
|1.9<ref>[http://catalog.osram-os.com/media/_en/Graphics/00041987_0.pdf OSRAM: green LED]</ref> < Δ''V'' < 4.0
|[[Indium gallium nitride]] (InGaN) / [[Gallium(III) nitride]] (GaN)
[[Gallium(III) phosphide]] (GaP)
[[Aluminium gallium indium phosphide]] (AlGaInP)
[[Aluminium gallium phosphide]] (AlGaP)
|-
| bgcolor="blue" |
|[[Blue]]
|450 < ''λ'' < 500
|2.48 < Δ''V'' < 3.7
|[[Zinc selenide]] (ZnSe)
[[Indium gallium nitride]] (InGaN)
[[Silicon carbide]] (SiC) as substrate
[[Silicon]] (Si) as substrate — (under development)
|-
| bgcolor="#8B00FF" |
|[[Violet (color)|Violet]]
|400 < ''λ'' < 450
|2.76 < Δ''V'' < 4.0
|[[Indium gallium nitride]] (InGaN)
|-
| bgcolor="#BF00FF" |
|[[Purple]]
|multiple types
|2.48 < Δ''V'' < 3.7
|Dual blue/red LEDs,
blue with red phosphor,
or white with purple plastic
|-
| bgcolor="white" |
|[[Ultraviolet]]
|''λ'' < 400
|3.1 < Δ''V'' < 4.4
|[[Diamond]] (235&nbsp;nm)<ref name="dia2">{{cite journal |last1=Koizumi |first1=S. |last2=Watanabe |first2=K |last3=Hasegawa |first3=M |last4=Kanda |first4=H |year=2001 |title=Ultraviolet Emission from a Diamond pn Junction |journal=Science |volume=292 |issue=5523 |pages=1899–2701 |doi=10.1126/science.1060258 |pmid=11397942|bibcode=2001Sci...292.1899K }}</ref>
[[Boron nitride]] (215&nbsp;nm)<ref name="BN2">{{cite journal |last1=Kubota |first1=Y. |last2=Watanabe |first2=K. |last3=Tsuda |first3=O. |last4=Taniguchi |first4=T. |year=2007 |title=Deep Ultraviolet Light-Emitting Hexagonal Boron Nitride Synthesized at Atmospheric Pressure |journal=Science |volume=317 |issue=5840 |pages=932–934 |doi=10.1126/science.1144216 |pmid=17702939|bibcode=2007Sci...317..932K }}</ref><ref name="bn22">{{cite journal |last1=Watanabe |first1=Kenji |last2=Taniguchi |first2=Takashi |last3=Kanda |first3=Hisao |year=2004 |title=Direct-bandgap properties and evidence for ultraviolet lasing of hexagonal boron nitride single crystal |journal=Nature Materials |volume=3 |issue=6 |pages=404–409 |doi=10.1038/nmat1134 |pmid=15156198|bibcode=2004NatMa...3..404W }}</ref>
[[Aluminium nitride]] (AlN) (210&nbsp;nm)<ref name="aln"/>


[[Aluminium gallium nitride]] (AlGaN)
Compared with regular LEDs, OLEDs are lighter, and polymer LEDs can have the added benefit of being flexible. Some possible future applications of OLEDs could be:
[[Aluminium gallium indium nitride]] (AlGaInN) — (down to 210&nbsp;nm)<ref>{{cite news |date=May 17, 2006 |title=LEDs move into the ultraviolet |url=http://physicsworld.com/cws/article/news/24926 |accessdate=2007-08-13 |publisher=physicsworld.com}}</ref>
|-
| bgcolor="white" |
|White
|Broad spectrum
|2.7 < Δ''V'' < 3.5
|Blue diode with yellow phosphor or violet/UV diode with multi-color phosphor &nbsp;
|}


== Applications ==
* Inexpensive, flexible displays
[[File:LED DaytimeRunningLights.jpg|thumb|[[Daytime running light]] LEDs of an automobile]]
* Light sources
* Wall decorations
* Luminous [[cloth]]


LED uses fall into five major categories:
OLEDs have been used to produce visual displays for portable electronic devices such as cellphones, digital cameras, and MP3 players. Larger displays have been demonstrated, but their life expectancy is still far too short (<1,000 hours) to be practical.


* Visual signals where light goes more or less directly from the source to the human eye, to convey a message or meaning
Today, OLEDs operate at substantially lower efficiency than inorganic (crystalline) LEDs. The best luminous efficiency of an OLED so far is about 68 lm/W {{Fact|date=June 2008}}.
* [[Lighting|Illumination]] where light is reflected from objects to give visual response of these objects
* Measuring and interacting with processes involving no human vision<ref>[[European Photonics Industry Consortium]] (EPIC). This includes use in data communications over [[Optical fiber|fiber optics]] as well as "broadcast" data or signaling.</ref>
* Narrow band light sensors where [[LEDs as light sensors|LEDs operate in a reverse-bias mode]] and respond to incident light, instead of emitting light<ref>
Mims, Forrest M. III. [http://www.instesre.org/papers/Snowmass/MimsSnowmass.htm "An Inexpensive and Accurate Student Sun Photometer with Light-Emitting Diodes as Spectrally Selective Detectors"].</ref><ref>[https://www.cs.drexel.edu/~dbrooks/globe/special_measurements/water_vapor.htm "Water Vapor Measurements with LED Detectors"]. cs.drexel.edu (2002).</ref><ref>Dziekan, Mike (February 6, 2009) [http://www.soamsci.org/tcs/weeklyIssues_2009/2009-02-06/feature1/index.html "Using Light-Emitting Diodes as Sensors"]. soamsci.or. {{webarchive |url=https://web.archive.org/web/20130531090631/http://www.soamsci.org/tcs/weeklyIssues_2009/2009-02-06/feature1/index.html |date=May 31, 2013 }}</ref><ref>{{Cite book|doi=10.1109/CVPR.2008.4587766|chapter=An LED-only BRDF measurement device|title=2008 IEEE Conference on Computer Vision and Pattern Recognition|pages=1–8|year=2008|last1=Ben-Ezra|first1=Moshe|last2=Wang|first2=Jiaping|last3=Wilburn|first3=Bennett|last4=Xiaoyang Li|last5=Le Ma|isbn=978-1-4244-2242-5|citeseerx=10.1.1.165.484|s2cid=206591080}}</ref>
* Indoor cultivation, including cannabis.<ref>Bantis, Filippos, Sonia Smirnakou, Theoharis Ouzounis, Athanasios Koukounaras, Nikolaos Ntagkas, and Kalliopi Radoglou. "[https://www.plantgrower.org/uploads/6/5/5/4/65545169/1-s2.0-s0304423818301420-main.pdf Current status and recent achievements in the field of horticulture with the use of light-emitting diodes (LEDs).]" Scientia horticulturae 235 (2018): 437-451.</ref>


The application of LEDs in horticulture has revolutionized plant cultivation by providing energy-efficient, customizable lighting solutions that optimize plant growth and development.<ref>Miler N., Kulus D., Woźny A., Rymarz D., Hajzer M., Wierzbowski K., Nelke R., Szeffs L., 2019. Application of wide-spectrum light-emitting diodes in micropropagation of popular ornamental plant species: A study on plant quality and cost reduction. In Vitro Cellular and Developmental Biology – Plant 55: 99-108. https://doi.org/10.1007/s11627-018-9939-5</ref> LEDs offer precise control over light spectra, intensity, and photoperiods, enabling growers to tailor lighting conditions to the specific needs of different plant species and growth stages. This technology enhances photosynthesis, improves crop yields, and reduces energy costs compared to traditional lighting systems. Additionally, LEDs generate less heat, allowing closer placement to plants without risking thermal damage, and contribute to sustainable farming practices by lowering carbon footprints and extending growing seasons in controlled environments.<ref>Tymoszuk A., Kulus D., Błażejewska A., Nadolan K., Kulpińska A., Pietrzykowski K., 2023. Application of wide-spectrum light-emitting diodes in the indoor production of cucumber and tomato seedlings. Acta Agrobotanica 76: 762. https://doi.org/10.5586/aa.762</ref> Light spectrum affects growth, metabolite profile, and resistance against fungal phytopathogens of ''[[Tomato|Solanum lycopersicum]]'' seedlings.<ref>Tymoszuk A., Kulus D., Kowalska J., Kulpińska A., Pańka D., Jeske M., Antkowiak M. 2024. Light spectrum affects growth, metabolite profile, and resistance against fungal phytopathogens of Solanum lycopersicum L. seedlings. Journal of Plant Protection Research 64(2). https://doi.org/10.24425/jppr.2024.150247</ref> LEDs can also be used in [[micropropagation]].<ref>Kulus D., Woźny A., 2020. Influence of light conditions on the morphogenetic and biochemical response of selected ornamental plant species under in vitro conditions: A mini-review. BioTechnologia 101(1): 75-83. http://doi.org/10.5114/bta.2020.92930</ref>
===Quantum Dot LEDs (experimental)===


===Indicators and signs===
A new technique developed by Michael Bowers, a graduate student at [[Vanderbilt University]] in Nashville, involves coating a blue LED with [[quantum dots]] that glow white in response to the blue light from the LED. This technique produces a warm, yellowish-white light similar to that produced by [[incandescent bulb]]s.<ref>{{cite news
{{unreferenced section|date=October 2020}}
| title = Accidental Invention Points to End of Light Bulbs
The [[energy conservation|low energy consumption]], low maintenance and small size of LEDs has led to uses as status indicators and displays on a variety of equipment and installations. Large-area [[LED display]]s are used as stadium displays, dynamic decorative displays, and [[dynamic message sign]]s on freeways. Thin, lightweight message displays are used at airports and railway stations, and as [[Destination sign|destination displays]] for trains, buses, trams, and ferries.
| publisher =LiveScience.com
| date = October 21, 2005
| url = http://www.livescience.com/technology/051021_nano_light.html
| accessdate = 2007-01-24 }}</ref>


[[File:Red and green traffic signals, Stamford Road, Singapore - 20111210.jpg|thumb|upright|Red and green LED traffic signals]]
[[Quantum Dots]] are [[semiconductor]] nanocrystals that possess unique optical properties.<ref name=MITqdot2002>[http://web.mit.edu/newsoffice/2002/dot.html Quantum-dot LED may be screen of choice for future electronics] Massachusetts Institute of Technology News Office,December 18, 2002</ref> Their emission color can be tuned from the visible throughout the infrared spectrum. This allows quantum dot LEDs to create almost any color on the [[International Commission on Illumination|CIE]] diagram. This provides more color options and better color rendering white LEDs. Quantum dot LEDs are available in the same package types as traditional [[phosphor]] based LEDs.


One-color light is well suited for [[traffic light]]s and signals, [[exit sign]]s, [[emergency vehicle lighting]], ships' navigation lights, and [[Christmas lighting technology#LEDs|LED-based Christmas lights]]
==Types==
[[Image:Verschiedene LEDs.jpg|thumb|center|750px|LEDs are produced in an array of shapes and sizes. The 5&nbsp;mm cylindrical package (red, fifth from the left) is the most common, estimated at 80% of world production. The color of the plastic lens is often the same as the actual color of light emitted, but not always. For instance, purple plastic is often used for [[infrared]] LEDs, and most blue devices have clear housings. There are also LEDs in [[Surface-mount technology|extremely tiny packages]], such as those found on [[blinky (novelty)|blinkies]] and on cell phone keypads. (not shown).]]
The main types of LEDs are miniature, high power devices and custom designs such as alphanumeric or multi-color.


Because of their long life, fast switching times, and visibility in broad daylight due to their high output and focus, LEDs have been used in automotive brake lights and turn signals. The use in brakes improves safety, due to a great reduction in the time needed to light fully, or faster rise time, about 0.1 second faster{{citation needed|date=April 2016}} than an incandescent bulb. This gives drivers behind more time to react. In a dual intensity circuit (rear markers and brakes) if the LEDs are not pulsed at a fast enough frequency, they can create a [[flicker fusion threshold#Visual phenomena|phantom array]], where ghost images of the LED appear if the eyes quickly scan across the array. White LED headlamps are beginning to appear. Using LEDs has styling advantages because LEDs can form much thinner lights than incandescent lamps with [[parabolic reflector]]s.
===Miniature LEDs===
[[Image:LEDs 8 5 3mm.JPG|thumb|Different sized LEDs. 8 mm, 5 mm and 3 mm, with a wooden match-stick for scale.]]
These are mostly single-die LEDs used as indicators, and they come in various-size packages:
* surface mount
* 2 mm
* 3 mm (T1)
* 5 mm (T1³⁄₄)
* 10 mm
* Other sizes are also available, but less common.


Due to the relative cheapness of low output LEDs, they are also used in many temporary uses such as [[glowstick]]s and throwies. Artists have also used LEDs for [[LED art]].
Common package shapes:
*Round, dome top
*Round, flat top
*Rectangular, flat top (often seen in LED bar-graph displays)
*Triangular or square, flat top


===Lighting===
The encapsulation may also be clear or semi opaque to improve contrast and viewing angle.
{{main|LED lamp}}
With the development of high-efficiency and high-power LEDs, it has become possible to use LEDs in lighting and illumination. To encourage the shift to [[LED lamp]]s and other high-efficiency lighting, in 2008 the [[US Department of Energy]] created the [[L Prize]] competition. The [[Philips]] Lighting North America LED bulb won the first competition on August 3, 2011, after successfully completing 18 months of intensive field, lab, and product testing.<ref>{{usurped|1=[https://web.archive.org/web/20080926010013/http://www.lightingprize.org/ "L-Prize U.S. Department of Energy"]}}, L-Prize Website, August 3, 2011</ref>


Efficient lighting is needed for [[sustainable architecture]]. As of 2011, some LED bulbs provide up to 150&nbsp;lm/W and even inexpensive low-end models typically exceed 50&nbsp;lm/W, so that a 6-watt LED could achieve the same results as a standard 40-watt incandescent bulb. The lower heat output of LEDs also reduces demand on [[air conditioning]] systems. Worldwide, LEDs are rapidly adopted to displace less effective sources such as [[incandescent light bulb|incandescent lamps]] and [[compact fluorescent lamp|CFLs]] and reduce electrical energy consumption and its associated emissions. Solar powered LEDs are used as [[street light]]s and in [[Architectural lighting design|architectural lighting]].
There are three main categories of miniature single die LEDs:
* Low current — typically rated for 2 mA at around 2 V (approximately 4 mW consumption).
* Standard — 20 mA LEDs at around 2 V (approximately 40 mW) for red, orange, yellow & green, and 20 mA at 4–5 V (approximately 100 mW) for blue, violet and white.
* Ultra-high output — 20 mA at approximately 2 V or 4–5 V, designed for viewing in direct sunlight.


The mechanical robustness and long lifetime are used in [[automotive lighting]] on cars, motorcycles, and [[Bicycle lighting#LEDs|bicycle lights]]. [[LED street light]]s are employed on poles and in parking garages. In 2007, the Italian village of [[Torraca]] was the first place to convert its street lighting to LEDs.<ref>[http://www.scientificamerican.com/article.cfm?id=led-there-be-light LED There Be Light], Scientific American, March 18, 2009</ref>
Five- and twelve-volt LEDs are ordinary miniature LEDs that incorporate a suitable series [[resistor]] for direct connection to a 5 V or 12 V supply.


Cabin lighting on recent{{when|date=October 2022}} [[Airbus]] and [[Boeing]] jetliners uses LED lighting. LEDs are also being used in airport and heliport lighting. LED airport fixtures currently include medium-intensity runway lights, runway centerline lights, taxiway centerline and edge lights, guidance signs, and obstruction lighting.
===Flashing LEDs===
Flashing LEDs are used as attention seeking indicators where it is desired to avoid the complexity of external electronics. Flashing LEDs resemble standard LEDs but they contain an integrated [[multivibrator]] circuit inside which causes the LED to flash with a typical period of one second. In diffused lens LEDs this is visible as a small black dot. Most flashing LEDs emit light of a single color, but more sophisticated devices can flash between multiple colors and even fade through a color sequence using RGB color mixing.


LEDs are also used as a light source for [[Digital Light Processing|DLP]] projectors, and to [[backlight]] newer [[Liquid crystal display|LCD]] television (referred to as [[LED-backlit LCD display|LED TV]]), computer monitor (including [[laptop]]) and handheld device LCDs, succeeding older [[CCFL]]-backlit LCDs although being superseded by [[OLED]] screens. RGB LEDs raise the color gamut by as much as 45%. Screens for TV and computer displays can be made thinner using LEDs for backlighting.<ref>{{cite news|url=https://www.nytimes.com/2007/06/24/business/yourmoney/24novel.html |url-access=subscription |newspaper=New York Times|title=In Pursuit of Perfect TV Color, With L.E.D.'s and Lasers|date=June 24, 2007|first=Anne|last=Eisenberg|access-date=April 4, 2010}}</ref>
===High power LEDs===
{{see also|Solid-state lighting|LED lamp}}
[[Image:2007-07-24 High-power light emiting diodes (Luxeon, Lumiled).jpg|thumb|High power LEDs from [[Philips Lumileds Lighting Company]] mounted on a star shaped heat sink]]
High power LEDs (HPLED) can be driven at hundreds of mA (vs. tens of mA for other LEDs), some with more than one [[ampere]] of current, and give out large amounts of light. Since overheating is destructive, the HPLEDs must be highly efficient to minimize excess heat; furthermore, they are often mounted on a heat sink to allow for heat dissipation. If the heat from a HPLED is not removed, the device will burn out in seconds.


LEDs are small, durable and need little power, so they are used in handheld devices such as [[flashlight]]s. LED [[strobe light]]s or [[camera flash]]es operate at a safe, low voltage, instead of the 250+ volts commonly found in [[xenon]] flashlamp-based lighting. This is especially useful in cameras on [[mobile phone]]s, where space is at a premium and bulky voltage-raising circuitry is undesirable.
A single HPLED can often replace an incandescent bulb in a flashlight, or be set in an array to form a powerful [[LED lamp]].


LEDs are used for infrared illumination in [[night vision]] uses including [[security camera]]s. A ring of LEDs around a [[video camera]], aimed forward into a [[retroreflective]] [[Projection screen|background]], allows [[chroma keying]] in [[video production]]s.
LEDs have been developed that can operate on AC power without the need for a DC converter. For each half cycle part of the LED emits light and part is dark, and this is reversed during the next half cycle. The efficiency of HPLEDs is typically 40 lm/W.<ref>{{cite web|url=http://www.ledsmagazine.com/news/3/11/14|title=Seoul Semiconductor launches AC LED lighting source Acriche |publisher=LEDS Magazine |accessdate=2008-02-17}}</ref> As of November 2008 some HPLEDs manufactured by Cree, Inc exceed 95 lm/W
<!-- Calculation: Group M (best available November 2008) 430 lm for four chips at 350 mA each.
Power per chip is 3.2 V (typical) times 350 mA
lm/W = 430/4/3.2/.35 = 95.98 --><ref>{{cite web|url=http://www.cree.com/products/xlamp_mce.asp|title=XLamp® MC-E LED |publisher=Cree, Inc |accessdate=2008-11-11}}</ref> (e.g. the XLamp MC-E LED chip emitting Cool White light) and are being sold in lamps intended to replace incandescent, halogen, and even fluorescent style lights as LEDs become more cost competitive.


[[File:LED for mines.jpg|thumb|LED for miners, to increase visibility inside mines]]
===Multi-color LEDs===
A “bi-color LED” is actually two different LEDs in one case. It consists of two dies connected to the same two leads but in opposite directions. Current flow in one direction produces one color, and current in the opposite direction produces the other color. Alternating the two colors with sufficient frequency causes the appearance of a blended third color. For example, a red/green LED operated in this fashion will color blend to produce a yellow appearance.


[[File:Los Angeles Bridge.jpg|thumb|Los Angeles [[Vincent Thomas Bridge]] illuminated with blue LEDs]]
A “tri-color LED” is also two LEDs in one case, but the two LEDs are connected to separate leads so that the two LEDs can be controlled independently and lit simultaneously. A three-lead arrangement is typical with one commmon lead (anode or cathode).


LEDs are used in [[mining]] operations, as [[cap lamp]]s to provide light for miners. Research has been done to improve LEDs for mining, to reduce glare and to increase illumination, reducing risk of injury to the miners.<ref>{{Cite journal | url = https://www.cdc.gov/niosh/docs/2011-192/ | title = CDC – NIOSH Publications and Products – Impact: NIOSH Light-Emitting Diode (LED) Cap Lamp Improves Illumination and Decreases Injury Risk for Underground Miners | publisher = cdc.gov | access-date=May 3, 2013| doi = 10.26616/NIOSHPUB2011192 | year = 2011 | doi-access = free }}</ref>
RGB LEDs contain red, green and blue emitters, generally using a four-wire connection with one common lead (anode or cathode).


LEDs are increasingly finding uses in medical and educational applications, for example as mood enhancement.<ref>{{cite news |last=Janeway |first=Kimberly |url=https://www.consumerreports.org/cro/news/2014/12/led-lightbulbs-that-promise-to-help-you-sleep/index.htm |title=LED lightbulbs that promise to help you sleep |work=Consumer Reports |date=2014-12-12 |access-date=2018-05-10}}</ref> [[NASA]] has even sponsored research for the use of LEDs to promote health for astronauts.<ref>{{cite press release | url=http://www.sti.nasa.gov/tto/Spinoff2008/hm_3.html | archive-url=https://web.archive.org/web/20081013083802/http://www.sti.nasa.gov/tto/Spinoff2008/hm_3.html | url-status=dead | archive-date=October 13, 2008 | title=LED Device Illuminates New Path to Healing | publisher=nasa.gov | access-date=January 30, 2012}}</ref>
The Taiwanese LED manufacturer Everlight has introduced a 3 watt
RGB package capable of driving each die at 1 watt.


===Data communication and other signalling===
===Alphanumeric LEDs===
{{See also|Li-Fi|fibre optics|Visible light communication|Optical disc}}
[[Image:LED DISP.JPG|thumb|right|200px|Old [[calculator]] LED display.]]
Light can be used to transmit data and analog signals. For example, lighting white LEDs can be used in systems assisting people to navigate in closed spaces while searching necessary rooms or objects.<ref>{{cite journal|url=http://ntv.ifmo.ru/en/article/11192/chastotnye_harakteristiki_sovremennyh_svetodiodnyh_lyuminofornyh_materialov.htm |title=Frequency characteristics of modern LED phosphor materials |author1=Fudin, M. S. |author2=Mynbaev, K. D. |author3=Aifantis, K. E. |author4=Lipsanen H. |author5=Bougrov, V. E. |author6=Romanov, A. E. |journal=Scientific and Technical Journal of Information Technologies, Mechanics and Optics|volume=14|issue=6|year=2014}}</ref>
LED displays are available in [[seven-segment display|seven-segment]] and [[Starburst display|starburst]] format. Seven-segment displays handle all numbers and a limited set of letters. Starburst displays can display all letters.


[[Assistive listening device]]s in many theaters and similar spaces use arrays of infrared LEDs to send sound to listeners' receivers. Light-emitting diodes (as well as semiconductor lasers) are used to send data over many types of [[Optical fiber|fiber optic]] cable, from digital audio over [[TOSLINK]] cables to the very high bandwidth fiber links that form the Internet backbone. For some time, computers were commonly equipped with [[IrDA]] interfaces, which allowed them to send and receive data to nearby machines via infrared.
Seven-segment LED displays were in widespread use in the 1970s and 1980s, but increasing use of [[liquid crystal display]]s, with their lower power consumption and greater display flexibility, has reduced the popularity of numeric and alphanumeric LED displays.


Because LEDs can [[frequency|cycle on and off]] millions of times per second, very high data bandwidth can be achieved.<ref>{{Cite news|first=Hank |last=Green |url=http://www.ecogeek.org/content/view/2194/74/ |title=Transmitting Data Through LED Light Bulbs |publisher=EcoGeek |date=October 9, 2008 |access-date=February 15, 2009 |url-status=dead |archive-url=https://web.archive.org/web/20081212050729/http://www.ecogeek.org/content/view/2194/74/ |archive-date=December 12, 2008 }}</ref> For that reason, [[visible light communication]] (VLC) has been proposed as an alternative to the increasingly competitive radio bandwidth.<ref name=":4">{{Cite book|last1=Dimitrov|first1=Svilen|url=https://www.cambridge.org/core/books/principles-of-led-light-communications/0528063BAA6863F6B6D61F6FF69F37CB|title=Principles of LED Light Communications: Towards Networked Li-Fi|last2=Haas|first2=Harald|date=2015|publisher=Cambridge University Press|isbn=978-1-107-04942-0|location=Cambridge|doi=10.1017/cbo9781107278929}}</ref> VLC operates in the visible part of the electromagnetic spectrum, so data can be transmitted without occupying the frequencies of radio communications.
==Considerations for use==
===Power sources===
{{Citations missing|section|date=August 2008}}


=== Machine vision systems ===
The voltage versus current characteristics of an LED are much like any [[diode]]. Current is approximately an exponential function of voltage, so a small voltage change results in a large change in current. This can result either in a unlit LED or a current above the maximum rating, potentially destroying the LED; as the LED heats, its voltage drop decreases, further increasing current. Consequently, LEDs cannot connect directly to constant-voltage sources. A series resistor is a very simple and common way to stabilize the LED current, but wastes energy in the resistor. A [[constant current]] regulator is commonly used. Low drop-out (LDO) constant current regulators also allow the total LED string voltage to be a higher percentage of the power supply voltage, resulting in improved efficiency and reduced power use. Switching-type converters are used in some LED flashlights, stabilizing light output over a wide range of battery voltages and increasing the useful life of the batteries.
{{main|Machine vision}}
[[Machine vision]] systems often require bright and homogeneous illumination, so features of interest are easier to process. LEDs are often used.


[[Barcode scanner]]s are the most common example of machine vision applications, and many of those scanners use red LEDs instead of lasers. Optical computer mice use LEDs as a light source for the miniature camera within the mouse.
Miniature indicator LEDs are normally driven from low voltage DC via a current limiting [[resistor]]. Currents of 2 mA, 10 mA and 20 mA are common. Sub-mA indicators may be made by driving ultrabright LEDs at very low current. Efficiency tends to reduce at low currents{{Fact|date=August 2008}}, but indicators running on 100 μA are still practical. The cost of ultrabright LEDs is higher than that of 2 mA indicator LEDs.


LEDs are useful for machine vision because they provide a compact, reliable source of light. LED lamps can be turned on and off to suit the needs of the vision system, and the shape of the beam produced can be tailored to match the system's requirements.
Multiple LEDs are normally operated in parallel strings of series LEDs, with the total LED voltage typically adding up to around two-thirds of the supply voltage, with [[resistor]] [[current]] control for each string. In disposable [[button cell|coin cell]] powered keyring type LED lights, the resistance of the cell itself is usually the only current limiting device. The cell should not therefore be replaced with a lower resistance type.


=== Biological detection ===
LEDs can be purchased with built in series resistors. These can save [[printed circuit board]] space and are especially useful when building [[prototype]]s or populating a PCB in a way other than its designers intended. However, the resistor value is set at the time of manufacture, removing one of the key methods of setting the LED's intensity. Alphanumeric LEDs use the same drive strategy as indicator LEDs, the only difference being the larger number of channels, each with its own resistor. Seven-segment and starburst LED arrays are available in both common-anode or common-cathode form. Finally, LEDs can be run from a single cell by use of a constant current switched mode inverter. The extra expense makes this option unpopular.
The discovery of radiative recombination in aluminum gallium nitride (AlGaN) alloys by [[United States Army Research Laboratory|U.S. Army Research Laboratory]] (ARL) led to the conceptualization of UV light-emitting diodes (LEDs) to be incorporated in light-induced [[fluorescence]] sensors used for biological agent detection.<ref>{{Citation |last1=Sampath |first1=A. V. |title=The effects of increasing AlN mole fraction on the performance of AlGaN active regions containing nanometer scale compositionally imhomogeneities |date=2009-12-01 |work=Advanced High Speed Devices |volume=51 |pages=69–76 |series=Selected Topics in Electronics and Systems |publisher=World Scientific |doi=10.1142/9789814287876_0007 |isbn=9789814287869 |last2=Reed |first2=M. L. |last3=Moe |first3=C. |last4=Garrett |first4=G. A. |last5=Readinger |first5=E. D. |last6=Sarney |first6=W. L. |last7=Shen |first7=H. |last8=Wraback |first8=M. |last9=Chua |first9=C.}}</ref><ref name=":1">{{Cite journal |last1=Liao |first1=Yitao |last2=Thomidis |first2=Christos |last3=Kao |first3=Chen-kai |last4=Moustakas |first4=Theodore D. |date=2011-02-21 |title=AlGaN based deep ultraviolet light emitting diodes with high internal quantum efficiency grown by molecular beam epitaxy |journal=Applied Physics Letters |volume=98 |issue=8 |pages=081110 |doi=10.1063/1.3559842 |issn=0003-6951 |bibcode=2011ApPhL..98h1110L |doi-access=free}}</ref><ref name=":2">{{Cite journal |last1=Cabalo |first1=Jerry |last2=DeLucia |first2=Marla |last3=Goad |first3=Aime |last4=Lacis |first4=John |last5=Narayanan |first5=Fiona |last6=Sickenberger |first6=David |date=2008-10-02 |title=Overview of the TAC-BIO detector |journal=Optically Based Biological and Chemical Detection for Defence IV |publisher=International Society for Optics and Photonics |volume=7116 |pages=71160D |doi=10.1117/12.799843 |editor1-last=Carrano |editor1-first=John C. |editor2-last=Zukauskas |editor2-first=Arturas |bibcode=2008SPIE.7116E..0DC |s2cid=108562187}}</ref> In 2004, the [[Edgewood Chemical Biological Center|Edgewood Chemical Biological Center (ECBC)]] initiated the effort to create a biological detector named TAC-BIO. The program capitalized on semiconductor UV optical sources (SUVOS) developed by the [[DARPA|Defense Advanced Research Projects Agency (DARPA)]].<ref name=":2" />


UV-induced fluorescence is one of the most robust techniques used for rapid real-time detection of biological aerosols.<ref name=":2" /> The first UV sensors were lasers lacking in-field-use practicality. In order to address this, DARPA incorporated SUVOS technology to create a low-cost, small, lightweight, low-power device. The TAC-BIO detector's response time was one minute from when it sensed a biological agent. It was also demonstrated that the detector could be operated unattended indoors and outdoors for weeks at a time.<ref name=":2" />
===Lighting LEDs on mains===
LEDs by their very nature, require constant current with low voltage, as opposed to the electrical grid which supplies high voltage with an [[alternating current]].


Aerosolized biological particles fluoresce and scatter light under a UV light beam. Observed fluorescence is dependent on the applied wavelength and the biochemical fluorophores within the biological agent. UV induced fluorescence offers a rapid, accurate, efficient and logistically practical way for biological agent detection. This is because the use of UV fluorescence is reagentless, or a process that does not require an added chemical to produce a reaction, with no consumables, or produces no chemical byproducts.<ref name=":2" />
A CR dropper followed by full-wave rectification is the usual [[electrical ballast]] with series-parallel LED clusters. A single series string minimises dropper losses, while paralleled strings increase reliability. In practice usually three strings or more are used.


Additionally, TAC-BIO can reliably discriminate between threat and non-threat aerosols. It was claimed to be sensitive enough to detect low concentrations, but not so sensitive that it would cause false positives. The particle-counting algorithm used in the device converted raw data into information by counting the photon pulses per unit of time from the fluorescence and scattering detectors, and comparing the value to a set threshold.<ref>{{Cite journal |last1=Poldmae |first1=Aime |last2=Cabalo |first2=Jerry |last3=De Lucia |first3=Marla |last4=Narayanan |first4=Fiona |last5=Strauch III |first5=Lester |last6=Sickenberger |first6=David |date=2006-09-28 |title=Biological aerosol detection with the tactical biological (TAC-BIO) detector |journal=Optically Based Biological and Chemical Detection for Defence III |volume=6398 |pages=63980E |publisher=SPIE |doi=10.1117/12.687944 |s2cid=136864366 |editor1-last=Carrano |editor1-first=John C. |editor2-last=Zukauskas |editor2-first=Arturas}}</ref>
Operation on [[square wave]] and modified [[sine wave]] (MSW) sources, such as many [[inverter (electrical)|inverters]], causes heavily-increased resistor [[dissipation]] in CR droppers, and LED ballasts designed for sine wave use tend to burn on non-sine waveforms. The non-sine waveform also causes high peak LED currents, heavily shortening LED life. An inductor and [[rectifier]] makes a more suitable ballast for such use, and other options are also possible. Dedicated [[integrated circuits]] are available that provide optimal drive for LEDs and maximum overall efficiency.


The original TAC-BIO was introduced in 2010, while the second-generation TAC-BIO GEN&nbsp;II, was designed in 2015 to be more cost-efficient, as plastic parts were used. Its small, light-weight design allows it to be mounted to vehicles, robots, and unmanned aerial vehicles. The second-generation device could also be utilized as an environmental detector to monitor air quality in hospitals, airplanes, or even in households to detect fungus and mold.<ref>{{Cite web |url=https://www.army.mil/article/141363/army_advances_bio_threat_detector |title=Army advances bio-threat detector |website=www.army.mil |date=January 22, 2015 |access-date=2019-10-10}}</ref><ref>{{Cite journal |last1=Kesavan |first1=Jana |last2=Kilper |first2=Gary |last3=Williamson |first3=Mike |last4=Alstadt |first4=Valerie |last5=Dimmock |first5=Anne |last6=Bascom |first6=Rebecca |date=2019-02-01 |title=Laboratory validation and initial field testing of an unobtrusive bioaerosol detector for health care settings |journal=Aerosol and Air Quality Research |volume=19 |issue=2 |pages=331–344 |doi=10.4209/aaqr.2017.10.0371 |issn=1680-8584 |doi-access=free}}</ref>
Multiple LEDs can be connected in [[Series and parallel circuits|series]] with a single current limiting resistor provided the source voltage is greater than the sum of the individual LED threshold voltages. [[Series and parallel circuits|Parallel]] operation is also possible but can be more problematic. Parallel LEDs must have closely matched forward voltages (Vf) in order to have equal branch currents and, therefore, equal light output. Variations in the manufacturing process can make it difficult to obtain satisfactory operation when connecting some types of LEDs in parallel.<ref>{{cite web
|url=http://www.nichia.co.jp/specification/appli/electrical.pdf
|format=PDF
|title=Electrical properties of GaN LEDs & Parallel connections
|accessdate=2007-08-13
|work=Application Note |publisher=Nichia}}</ref>


=== Other applications ===
To increase efficiency (or to allow intensity control without the complexity of a [[Digital-to-analog converter|DAC]]), the power may be applied periodically or intermittently; so long as the flicker rate is greater than the human [[flicker fusion threshold]], the LED will appear to be continuously lit.
[[file:LED Costume by Beo Beyond.jpg|thumb|LED costume for stage performers]]
[[file:Digitally printed LED wallpaper Dolomites.jpg |thumb|LED wallpaper by Meystyle]]
[[file:LED screen behind Tsach Zimroni in Tel Aviv Israel.jpg|thumb|A large LED display behind a [[disc jockey]]]]
[[file:LED Digital Display.jpg|thumb|[[Seven-segment display]] that can display four digits and points]]
[[file:LED panel and plants.jpg|thumb|LED panel light source used in an early experiment on [[potato]] growth during Shuttle mission [[STS-73]] to investigate the potential for growing food on future long duration missions]]


The light from LEDs can be modulated very quickly so they are used extensively in [[optical fiber]] and [[free space optics]] communications. This includes [[remote control]]s, such as for television sets, where infrared LEDs are often used. [[Opto-isolator]]s use an LED combined with a [[photodiode]] or [[phototransistor]] to provide a signal path with electrical isolation between two circuits. This is especially useful in medical equipment where the signals from a low-voltage [[sensor]] circuit (usually battery-powered) in contact with a living organism must be electrically isolated from any possible electrical failure in a recording or monitoring device operating at potentially dangerous voltages. An optoisolator also lets information be transferred between circuits that do not share a common ground potential.
====Christmas lights====
Most LED [[Christmas lights]] (at least in 120-volt [[North America]]) are operated directly from mains electricity, with an in-line resistor (molded inside a small [[cylinder]] the same green or white color as the [[wire]] [[electrical insulation|insulation]]) for each circuit. Older colors are operated in circuits of up to 60 LEDs, while newer or mixed colors are normally in one or two circuits of 25, 30, or 35. An example of [[Halloween]] lights is two different sets of 70 LEDs: the orange set is divded into two circuits with a one-kilo[[ohm]] resistor each, while the purple (blue with red phosphor) set is three circuits with a 1.1kΩ resistor each. Each circuit uses 2.4 watts, and from this it is derived that the LEDs are about 5kΩ in total.


Many sensor systems rely on light as the signal source. LEDs are often ideal as a light source due to the requirements of the sensors. The Nintendo [[Wii]]'s sensor bar uses infrared LEDs. [[Pulse oximeter]]s use them for measuring [[oxygen saturation]]. Some flatbed scanners use arrays of RGB LEDs rather than the typical [[cold-cathode fluorescent lamp]] as the light source. Having independent control of three illuminated colors allows the scanner to calibrate itself for more accurate color balance, and there is no need for warm-up. Further, its sensors only need be monochromatic, since at any one time the page being scanned is only lit by one color of light.
The alternating current can be seen in these sets by spinning one end of the string around. It is then apparent that the LEDs are on less than half of the time, being off when the voltage is negative (reverse-biased) or too low. The slightly-delayed rise and slow [[decay]] of phosphors can also be seen in each flash, depending on their [[phosphorescence]]. While inexpensive, the flickering caused by this method can be annoying to some people. Additionally, the unsmoothed peak voltage of nearly 170 total volts in each cycle shortens the life of the LEDs, though they are still rated for a [[service life]] ([[MTTF]]) of around 25,000 hours (if [[moisture]] does not [[rust]] them first). However, blue and deep-green ones are more prone to failure, especially early in their use.


Since LEDs can also be used as photodiodes, they can be used for both photo emission and detection. This could be used, for example, in a [[touchscreen]] that registers reflected light from a finger or [[stylus]].<ref>{{cite journal |author1=Dietz, P. H. |author2=Yerazunis, W. S. |author3=Leigh, D. L. |title=Very Low-Cost Sensing and Communication Using Bidirectional LEDs |year=2004 |url=http://www.merl.com/publications/TR2003-035/}}</ref> Many materials and biological systems are sensitive to, or dependent on, light. [[Grow lights]] use LEDs to increase [[photosynthesis]] in [[plant]]s,<ref>{{cite journal |author1=Goins, G. D. |author2=Yorio, N. C. |author3=Sanwo, M. M. |author4=Brown, C. S. |title=Photomorphogenesis, photosynthesis, and seed yield of wheat plants grown under red light-emitting diodes (LEDs) with and without supplemental blue lighting |journal=Journal of Experimental Botany |year=1997 |volume=48 |issue=7 |pages=1407–1413 |doi=10.1093/jxb/48.7.1407 |pmid=11541074|doi-access=free }}</ref> and bacteria and viruses can be removed from water and other substances using UV LEDs for [[Sterilization (microbiology)|sterilization]].<ref name="water sterilization" /> LEDs of certain wavelengths have also been used for [[light therapy]] treatment of [[neonatal jaundice]] and [[acne]].<ref>{{cite book |last1=Li |first1=Jinmin |last2=Wang |first2=Junxi |last3=Yi |first3=Xiaoyan |last4=Liu |first4=Zhiqiang |last5=Wei |first5=Tongbo |last6=Yan |first6=Jianchang |last7=Xue |first7=Bin |title=III-Nitrides Light Emitting Diodes: Technology and Applications |date=31 August 2020 |publisher=Springer Nature |isbn=978-981-15-7949-3 |page=248 |url=https://books.google.com/books?id=Smn6DwAAQBAJ&pg=PA248 |language=en}}</ref>
=== Advantages of using LEDs ===


UV LEDs, with spectra range of 220&nbsp;nm to 395&nbsp;nm, have other applications, such as [[water purification|water]]/[[air purification|air]] purification, surface disinfection, glue curing, free-space [[non-line-of-sight communication]], high performance liquid chromatography, UV curing dye printing, [[phototherapy]] (295nm [[Vitamin D]], 308nm [[Excimer lamp]] or laser replacement), medical/ analytical instrumentation, and DNA absorption.<ref name=":1" /><ref>{{Cite book|last1=Gaska|first1=R.|last2=Shur|first2=M. S.|last3=Zhang|first3=J.|title=2006 8th International Conference on Solid-State and Integrated Circuit Technology Proceedings |chapter=Physics and Applications of Deep UV LEDs |date=October 2006|pages=842–844|doi=10.1109/ICSICT.2006.306525|isbn=1-4244-0160-7|s2cid=17258357}}</ref>
*'''Efficiency:''' LEDs produce more light per watt than incandescent bulbs; this is useful in battery powered or energy-saving devices.<ref>{{cite web|url=http://www.netl.doe.gov/ssl/usingLeds/general_illumination_efficiency_comparison.htm|title=Solid-State Lighting: Comparing LEDs to Traditional Light Sources}}</ref>
*'''Colour:''' LEDs can emit light of an intended colour without the use of colour filters that traditional lighting methods require. This is more efficient and can lower initial costs.
*'''Size:''' LEDs can be very small (>2 mm<sup>2</sup>) and are easily populated onto printed circuit boards.
*'''On/Off time:''' LEDs light up very quickly. A typical red indicator LED will achieve full brightness in microseconds.<ref>Philips Lumileds technical datasheet DS23 for the Luxeon Star states "less than 100ns".</ref> LEDs used in communications devices can have even faster response times.
*'''Cycling:''' LEDs are ideal for use in applications that are subject to frequent on-off cycling, unlike fluorescent lamps that burn out more quickly when cycled frequently, or [[HID lamp]]s that require a long time before restarting.
*'''Dimming:''' LEDs can very easily be [[Dimmer|dimmed]] either by [[Pulse-width modulation]] or lowering the forward current.
*'''Cool light:''' In contrast to most light sources, LEDs radiate very little heat in the form of [[Infrared|IR]] that can cause damage to sensitive objects or fabrics. Wasted energy is dispersed as heat through the base of the LED.
*'''Slow failure:''' LEDs mostly fail by dimming over time, rather than the abrupt burn-out of incandescent bulbs.<ref>{{cite web|url=http://www.netl.doe.gov/ssl/usingLeds/general_illumination_life_depreciation.htm|title=Solid-State Lighting: Lumen Depreciation}}</ref>
* '''Lifetime:''' LEDs can have a relatively long useful life. One report estimates 35,000 to 50,000 hours of useful life, though time to complete failure may be longer.<ref>http://www.netl.doe.gov/ssl/PDFs/lifetimeWhiteLEDs_aug16_r1.pdf</ref> Fluorescent tubes typically are rated at about 10,000 to 15,000 hours, depending partly on the conditions of use, and incandescent light bulbs at 1,000–2,000 hours.{{Fact|date=December 2007}}
* '''Shock resistance:''' LEDs, being solid state components, are difficult to damage with external shock, unlike fluorescent and incandescent bulbs which are fragile.
* '''Focus:''' The solid package of the LED can be designed to focus its light. Incandescent and fluorescent sources often require an external reflector to collect light and direct it in a usable manner.
*'''Toxicity:''' LEDs do not contain [[mercury (element)|mercury]], unlike [[fluorescent lamp]]s.


LEDs have also been used as a medium-quality [[voltage reference]] in electronic circuits. The forward voltage drop (about 1.7&nbsp;V for a red LED or 1.2V for an infrared) can be used instead of a [[Zener diode]] in low-voltage regulators. Red LEDs have the flattest I/V curve above the knee. Nitride-based LEDs have a fairly steep I/V curve and are useless for this purpose. Although LED forward voltage is far more current-dependent than a Zener diode, Zener diodes with breakdown voltages below 3&nbsp;V are not widely available.
=== Disadvantages of using LEDs ===


The progressive miniaturization of low-voltage lighting technology, such as LEDs and OLEDs, suitable to incorporate into low-thickness materials has fostered experimentation in combining light sources and wall covering surfaces for interior walls in the form of [[LED wallpaper]].
*'''High price:''' LEDs are currently more expensive, price per lumen, on an initial capital cost basis, than most conventional lighting technologies. The additional expense partially stems from the relatively low lumen output and the drive circuitry and power supplies needed. However, when considering the total cost of ownership (including energy and maintenance costs), LEDs far surpass incandescent or halogen sources and begin to threaten compact fluorescent lamps{{Fact|date=July 2008}}.
*'''Temperature dependence:''' LED performance largely depends on the ambient temperature of the operating environment. Over-driving the LED in high ambient temperatures may result in overheating of the LED package, eventually leading to device failure. Adequate [[heat sink|heat-sinking]] is required to maintain long life. This is especially important when considering automotive, medical, and military applications where the device must operate over a large range of temperatures, and is required to have a low failure rate.
*'''Voltage sensitivity:''' LEDs must be supplied with the voltage above the threshold and a current below the rating. This can involve series resistors or current-regulated power supplies.<ref>[http://www.ledmuseum.org/ The Led Museum]</ref>
*'''Light quality:''' Most [[#White LEDs|white LEDs]] have spectra that differ significantly from a [[black body]] radiator like the sun or an incandescent light. The spike at 460&nbsp;nm and dip at 500&nbsp;nm can cause the color of objects to be [[color vision|perceived differently]] under LED illumination than sunlight or incandescent sources, due to [[metamerism (color)|metamerism]],<ref>{{cite web | url = http://www.jimworthey.com/jimtalk2006feb.html | title = How White Light Works | author = James A. Worthey | work = LRO Lighting Research Symposium, Light and Color | accessdate = 2007-10-06}}</ref> red surfaces being rendered particularly badly by typical phosphor based white LEDs. However, the color rendering properties of common fluorescent lamps are often inferior to what is now available in state-of-art white LEDs.
*'''Area light source:''' LEDs do not approximate a “point source” of light, but rather a [[Lambert's cosine law|lambertian]] distribution. So LEDs is difficult to use in applications needing a spherical light field. LEDs are not capable of providing divergence below a few degrees. This is contrasted with lasers, which can produce beams with divergences of 0.2 degrees or less.<ref>Hecht, E: "Optics", Fourth Edition, page 591. Addison Wesley, 2002.</ref>
*'''Blue Hazard:''' There is increasing concern that [[#Ultraviolet and blue LEDs|blue LEDs]] and [[#White LEDs|white LEDs]] are now capable of exceeding safe limits of the so-called [[blue-light hazard]] as defined in eye safety specifications such as ANSI/IESNA RP-27.1-05: Recommended Practice for Photobiological Safety for Lamp and Lamp Systems.<ref>{{cite news |url=http://texyt.com/bright+blue+leds+annoyance+health+risks |title=Blue LEDs: A health hazard? |date=January 15, 2007 |publisher=texyt.com |accessdate=2007-09-03}}</ref><ref>{{cite web|title=Light Impacts: Science News Online, May 27, 2006|url=http://www.sciencenews.org/articles/20060527/bob9.asp}} 071214 sciencenews.org</ref>
*'''Blue pollution:''' Because [[#White LEDs|white LEDs]] emit much more blue light than conventional outdoor light sources such as [[Sodium vapor lamp|high-pressure sodium lamps]], the strong wavelength dependence of [[Rayleigh scattering]] means that LEDs can cause more [[light pollution]] than other light sources. It is therefore very important that LEDs are fully shielded when used outdoors. Compared to [[Sodium vapor lamp|low-pressure sodium lamps]], which emit at 589.3 nm, the 460 nm emission spike of white and blue LEDs is scattered about 2.7 times more by the Earth's atmosphere. LEDs should not be used for outdoor lighting near astronomical observatories.


== LED applications ==
== Research and development ==
=== Key challenges ===
The many application of LEDs are very diverse but fall into three major categories: Visual signal application where the light goes more or less directly from the LED to the human eye, to convey a message or meaning. [[Lighting|Illumination]] where LED light is reflected from object to give visual response of these objects. Finally LEDs are also used to generate light for measuring and interacting with processes that do not involve the human visual system.
LEDs require optimized efficiency to hinge on ongoing improvements such as phosphor materials and [[quantum dot]]s.<ref name=":0">{{Cite web|url=https://www.energy.gov/eere/ssl/led-rd-challenges|title=LED R&D Challenges|website=Energy.gov|access-date=2019-03-13}}</ref>


The process of down-conversion (the method by which materials convert more-energetic photons to different, less energetic colors) also needs improvement. For example, the red phosphors that are used today are thermally sensitive and need to be improved in that aspect so that they do not color shift and experience efficiency drop-off with temperature. Red phosphors could also benefit from a narrower spectral width to emit more lumens and becoming more efficient at converting photons.<ref>{{Cite web|url=https://www.energy.gov/eere/ssl/downloads/july-2015-postings|title=JULY 2015 POSTINGS|website=Energy.gov|access-date=2019-03-13}}</ref>
==== Indicators and signs ====
[[Image:LED bus destination displays.jpg|thumb|right|200px|LED destination displays on buses, one with a colored route number.]]
[[Image:Outdoor LED screen by Igors Jefimovs.jpg|thumb|left|200px|Outdoor 4 x 3 m large LED screen used as clock.]]
[[Image:LedSignal.JPG|thumb|[[Traffic light]] using LED]]
* Status indicators on a variety of equipment
* [[LED display]]s used as [[stadium]] [[television]] displays, [[electronic billboard]]s and dynamic decorative displays.
* [[Traffic light]]s and signals
* [[Exit sign]]s
* Thin, lightweight message displays at airports and railway stations, and as destination displays for trains, buses, trams, and ferries.
* Red or yellow LEDs are used in indicator and alphanumeric displays in environments where night vision must be retained: aircraft cockpits, submarine and ship bridges, astronomy observatories, and in the field, e.g. night time animal watching and military field use.
* LEDs of all colors, including yellowish white to simulate incandescent lamps, are used for [[model railroading]] applications
* In dot matrix arrangements for displaying messages.
* Because of their long life and fast switching times, LEDs have been used for automotive [[Automotive lighting#Centre High Mount Stop Lamp (CHMSL)|high-mounted brake lights]] and truck and bus brake lights and turn signals for some time, but many high-end vehicles are now starting to use LEDs for their entire rear light clusters. Besides the gain in [[reliability]], this has styling advantages because LEDs are capable of forming much thinner lights than incandescent lamps with [[parabolic reflector]]s. The significant improvement in the time taken to light up (perhaps 0.5s faster than an incandescent bulb) improves safety by giving drivers more time to react. It has been reported that at normal highway speeds this equals one car length increased reaction time for the car behind. White LED headlamps are beginning to make an appearance.
* As a medium quality [[voltage reference]] in electronic circuits. The forward voltage drop (e.g., about 1.7 V for a normal red LED) can be used instead of a [[Zener diode]] in low-voltage regulators. Although LED forward voltage is much more current-dependent than a good Zener, Zener diodes are not available below voltages of about 3 V.
* Glowlights, as a more expensive but longer lasting and reusable alternative to [[glowstick]]s.
* [[Lumalive]], a photonic [[textile]]
* [[Emergency vehicle lighting]]
* LED-based [[Christmas lights]] available in different colors and with low energy consumption.
* LED-modules provide LEDs in a more usable form to people with less knowledge of electronics and soldering: the actual LEDs are contained within in protective and mountable casing, and a lead enables connection to power supply, typically 12 volts. LED modules are available in a wide range of shapes, sizes and colors.


In addition, work remains to be done in the realms of current efficiency droop, color shift, system reliability, light distribution, dimming, thermal management, and power supply performance.<ref name=":0" />
==== Lighting ====
[[Image:LEDFlashlight.jpg|thumb|right|200px|Flashlights and lanterns that utilize white LEDs are becoming increasingly popular because of their durability and longer battery life.]]
[[Image:LED DaytimeRunningLights.jpg|left|thumb|LED [[daytime running light]]s of Audi A4]]
[[Image:Hexagonal White LED module.jpg|thumb|White LED module]]
* Replacement [[light bulb]]s
*[[Flashlight]]s with low energy usage and high durability
* Lanterns
* [[Street light]]s
* Large-scale [[video display]]s
* Architectural lighting
* Light source for [[machine vision]] systems, requiring bright, focused, homogeneous and possibly [[Strobe light|strobed]] illumination.
* [[Vehicle lighting]] on cars, motorcycles and [[Bicycle lighting#LEDs|bicycle lights]]
* [[Backlight]]ing for [[LCD]] televisions and lightweight [[laptop]] displays. Using RGB LEDs increase the color [[gamut]] by as much as 45%.
* Light source for [[DLP]] projectors
* [[Stage light]]s using banks of RGB LEDs to easily change color and decrease heating from traditional stage lighting.
* Medical lighting where IR-radiation and high temperatures are unwanted.
* [[Strobe light]]s or [[camera flash]]es that operate at a safe, low voltage, as opposed to the 250+ volts commonly found in [[xenon]] flashlamp-based lighting. This is particularly applicable to cameras on [[mobile phone]]s, where space is at a premium an bulky voltage-increasing circuitry is undesirable.
* Invisible infrared illumination for [[night vision]], such as many [[security camera]]s.
* A ring of LEDs around a [[video camera]], aimed forward into a [[retroreflective]] [[background]], will allow for [[chroma keying]] in [[video production]]s.


Early suspicions were that the LED droop was caused by elevated temperatures. Scientists showed that temperature was not the root cause of efficiency droop.<ref>[http://www.digikey.com/us/en/techzone/lighting/resources/articles/identifying-the-causes-of-led-efficiency-droop.html Identifying the Causes of LED Efficiency Droop] {{webarchive|url=https://web.archive.org/web/20131213073051/http://www.digikey.com/us/en/techzone/lighting/resources/articles/identifying-the-causes-of-led-efficiency-droop.html |date=13 December 2013}}, By Steven Keeping, Digi-Key Corporation Tech Zone</ref> The mechanism causing efficiency droop was identified in 2007 as [[Carrier generation and recombination#Auger recombination|Auger recombination]], which was taken with mixed reaction.<ref name="stevenson"/> A 2013 study conclusively identified Auger recombination as the cause.<ref>{{cite web|author=Iveland, Justin|title=Cause of LED Efficiency Droop Finally Revealed|url=https://www.sciencedaily.com/releases/2013/04/130423102328.htm|work=Physical Review Letters, 2013|date=23 April 2013|display-authors=etal}}</ref>
==== Smart lighting ====
Light can be used to transmit [[broadband]] data, which is already implemented in [[IrDA]] standards using infrared LEDs. Because LEDs can [[frequency|cycle on and off]] millions of times per second, they can, in effect, become [[wireless router]]s for [[data]] transport.<ref>http://www.ecogeek.org/content/view/2194/74/</ref> [[Laser]]s can also be [[modulation|modulated]] in this manner.


==== Non-visual applications ====
=== Potential technology ===
[[Image:LED panel and plants.jpg|thumb|right|200px|LED panel light source used in an experiment on [[plant]] growth. The findings of such experiments may be used to grow food in space on long duration missions.]]
* [[Grow lights]] using LEDs to increase [[photosynthesis]] in [[plants]]<ref>{{cite journal
|author = Goins, GD and Yorio, NC and Sanwo, MM and Brown, CS
|title = Photomorphogenesis, photosynthesis, and seed yield of wheat plants grown under red light-emitting diodes (LEDs) with and without supplemental blue lighting
|journal = Journal of Experimental Botany
|year = 1997
|volume = 48
|pages = 1407
|number = 7
|keywords = photosynthesis;LED
|publisher = Soc Experiment Biol
|doi = 10.1093/jxb/48.7.1407
}}</ref>
* [[Remote control]]s, such as for TVs and VCRs, often use [[infrared]] LEDs.
* Movement sensors, for example in [[Optical mouse|optical computer mice]]. The Nintendo [[Wii]]'s sensor bar uses [[infrared]] LEDs.
* In [[optical fiber]] and [[Free Space Optics]] communications.
* In [[pulse oximeter]]s for measuring [[oxygen saturation]]
* LED [[Light therapy|phototherapy]] for [[acne]] using blue or red LEDs has been proven to significantly reduce acne over a three-month period.{{Fact|date=February 2007}}
* Some flatbed scanners use arrays of RGB LEDs rather than the typical [[cold-cathode fluorescent lamp]] as the light source. Having independent control of three illuminated colors allows the scanner to calibrate itself for more accurate color balance, and there is no need for warm-up. Furthermore, its sensors only need be monochromatic, since at any one point in time the page being scanned is only lit by a single color of light.
* [[Sterilization]] of water and other substances using [[UV]] light.<ref name=water sterilization"/>
* [[Touchscreen|Touch sensing]]: Since LEDs can also be used as [[photodiode]]s, they can be used for both photo emission and detection. This could be used in for example a touch-sensing screen that register reflected light from a finger or [[stylus]].<ref>{{cite journal
|author=Dietz, Yerazunis, and Leigh
|title=Very Low-Cost Sensing and Communication Using Bidirectional LEDs
|year=2004
|url=http://www.merl.com/publications/TR2003-035/
}}
</ref>
*[[Opto-isolator]]s use an LED combined with a [[photodiode]] or [[phototransistor]] to provide a signal path with electrical isolation between two circuits. This is especially useful in medical equipment where the signals from a low voltage [[sensor]] circuit (usually battery powered) in contact with a living organism must be electrically isolated from any possible electrical failure in a recording or monitoring device operating at potentially dangerous voltages. An optoisolator also allows information to be transferred between circuits not sharing a common ground potential.


A new family of LEDs are based on the semiconductors called [[Perovskite (structure)|perovskites]]. In 2018, less than four years after their discovery, the ability of perovskite LEDs (PLEDs) to produce light from electrons already rivaled those of the best performing [[OLED]]s.<ref>{{Cite journal|last1=Di|first1=Dawei|last2=Romanov|first2=Alexander S.|last3=Yang|first3=Le|last4=Richter|first4=Johannes M.|last5=Rivett|first5=Jasmine P. H.|last6=Jones|first6=Saul|last7=Thomas|first7=Tudor H.|last8=Abdi Jalebi|first8=Mojtaba|last9=Friend|first9=Richard H.|last10=Linnolahti|first10=Mikko|last11=Bochmann|first11=Manfred|date=2017-04-14|title=High-performance light-emitting diodes based on carbene-metal-amides|journal=Science|language=en|volume=356|issue=6334|pages=159–163|doi=10.1126/science.aah4345|pmid=28360136|issn=0036-8075|url=https://ueaeprints.uea.ac.uk/63288/1/Accepted_manuscript.pdf|bibcode=2017Sci...356..159D|arxiv=1606.08868|s2cid=206651900}}</ref> They have a potential for cost-effectiveness as they can be processed from solution, a low-cost and low-tech method, which might allow perovskite-based devices that have large areas to be made with extremely low cost. Their efficiency is superior by eliminating non-radiative losses, in other words, elimination of [[Carrier generation and recombination|recombination]] pathways that do not produce photons; or by solving outcoupling problem (prevalent for thin-film LEDs) or balancing charge carrier injection to increase the [[External quantum efficiency|EQE]] (external quantum efficiency). The most up-to-date PLED devices have broken the performance barrier by shooting the EQE above 20%.<ref name=":3">{{Cite journal|last1=Armin|first1=Ardalan|last2=Meredith|first2=Paul|date=October 2018|title=LED technology breaks performance barrier|journal=Nature|volume=562|issue=7726|pages=197–198|doi=10.1038/d41586-018-06923-y|pmid=30305755|bibcode=2018Natur.562..197M|doi-access=free}}</ref>
=== Light sources for machine vision systems ===
[[Image:WashDownRL4.jpg|thumb|right|200px|Light sources for [[machine vision]] systems.]]


In 2018, Cao et al. and Lin et al. independently published two papers on developing perovskite LEDs with EQE greater than 20%, which made these two papers a mile-stone in PLED development. Their device have similar planar structure, i.e. the active layer (perovskite) is sandwiched between two electrodes. To achieve a high EQE, they not only reduced non-radiative recombination, but also utilized their own, subtly different methods to improve the EQE.<ref name=":3" />
[[Machine vision]] systems often require bright and homogeneous illumination, so features of interest are easier to process.
LEDs are often used to this purpose, and this field of application is likely to remain one of the major application areas until price drops low enough to make signaling and illumination applications more widespread. [[Barcode scanner]]s are the most common example of machine vision, and many inexpensive ones used red LEDs instead of lasers.


In the work of Cao ''et al.'',<ref name="ReferenceA">{{Cite journal|last1=Cao|first1=Yu|last2=Wang|first2=Nana|last3=Tian|first3=He|last4=Guo|first4=Jingshu|last5=Wei|first5=Yingqiang|last6=Chen|first6=Hong|last7=Miao|first7=Yanfeng|last8=Zou|first8=Wei|last9=Pan|first9=Kang|last10=He|first10=Yarong|last11=Cao|first11=Hui|date=October 2018|title=Perovskite light-emitting diodes based on spontaneously formed submicrometre-scale structures|journal=Nature|language=en|volume=562|issue=7726|pages=249–253|doi=10.1038/s41586-018-0576-2|pmid=30305742|issn=1476-4687|bibcode=2018Natur.562..249C|doi-access=free}}</ref> researchers targeted the outcoupling problem, which is that the optical physics of thin-film LEDs causes the majority of light generated by the semiconductor to be trapped in the device.<ref>{{Cite journal|last1=Cho|first1=Sang-Hwan|last2=Song|first2=Young-Woo|last3=Lee|first3=Joon-gu|last4=Kim|first4=Yoon-Chang|last5=Lee|first5=Jong Hyuk|last6=Ha|first6=Jaeheung|last7=Oh|first7=Jong-Suk|last8=Lee|first8=So Young|last9=Lee|first9=Sun Young|last10=Hwang|first10=Kyu Hwan|last11=Zang|first11=Dong-Sik|date=2008-08-18|title=Weak-microcavity organic light-emitting diodes with improved light out-coupling|journal=Optics Express|language=EN|volume=16|issue=17|pages=12632–12639|doi=10.1364/OE.16.012632|pmid=18711500|issn=1094-4087|bibcode=2008OExpr..1612632C|doi-access=free}}</ref> To achieve this goal, they demonstrated that solution-processed perovskites can spontaneously form submicrometre-scale crystal platelets, which can efficiently extract light from the device. These perovskites are formed via the introduction of [[amino acid]] additives into the perovskite [[Precursor (chemistry)|precursor]] solutions. In addition, their method is able to passivate perovskite surface [[Crystallographic defects in diamond|defects]] and reduce nonradiative recombination. Therefore, by improving the light outcoupling and reducing nonradiative losses, Cao and his colleagues successfully achieved PLED with EQE up to 20.7%.<ref name="ReferenceA"/>
LEDs constitute a nearly ideal light source for [[machine vision]] systems for several main reasons:


Lin and his colleague used a different approach to generate high EQE. Instead of modifying the microstructure of perovskite layer, they chose to adopt a new strategy for managing the compositional distribution in the device—an approach that simultaneously provides high [[luminescence]] and balanced charge injection. In other words, they still used flat emissive layer, but tried to optimize the balance of electrons and holes injected into the perovskite, so as to make the most efficient use of the charge carriers. Moreover, in the perovskite layer, the crystals are perfectly enclosed by MABr additive (where MA is CH<sub>3</sub>NH<sub>3</sub>). The MABr shell passivates the nonradiative defects that would otherwise be present perovskite crystals, resulting in reduction of the nonradiative recombination. Therefore, by balancing charge injection and decreasing nonradiative losses, Lin and his colleagues developed PLED with EQE up to 20.3%.<ref>{{Cite journal|last1=Lin|first1=Kebin|last2=Xing|first2=Jun|last3=Quan|first3=Li Na|last4=de Arquer|first4=F. Pelayo García|last5=Gong|first5=Xiwen|last6=Lu|first6=Jianxun|last7=Xie|first7=Liqiang|last8=Zhao|first8=Weijie|last9=Zhang|first9=Di|last10=Yan|first10=Chuanzhong|last11=Li|first11=Wenqiang|date=October 2018|title=Perovskite light-emitting diodes with external quantum efficiency exceeding 20 per cent|journal=Nature|language=en|volume=562|issue=7726|pages=245–248|doi=10.1038/s41586-018-0575-3|pmid=30305741|issn=1476-4687|bibcode=2018Natur.562..245L|hdl=10356/141016|s2cid=52958604|hdl-access=free}}</ref>
* Size of illuminated field is usually comparatively small and Vision systems or [[smart camera]] are quite expensive, so cost of LEDs is usually a minor concern, compared to signaling applications.
* LED elements tend to be small and can be placed with high density over flat or even shaped substrates (PCBs etc) so that bright and homogeneous sources can be designed which direct light from tightly controlled directions on inspected parts.
* LEDs often have or can be used with small, inexpensive lenses and diffusers, helping to achieve high light densities and very good lighting control and homogeneity.
* LEDs can be easily strobed (in the microsecond range and below) and synchronized; their power also has reached high enough levels that sufficiently high intensity can be obtained, allowing well lit images even with very short light pulses: this is often used in order to obtain crisp and sharp “still” images of quickly-moving parts.
* LEDs come in several different colors and wavelengths, easily allowing to use the best color for each application, where different color may provide better visibility of features of interest. Having a precisely known spectrum allows tightly matched filters to be used to separate informative bandwidth or to reduce disturbing effect of ambient light.
* LEDs usually operate at comparatively low working temperatures, simplifying heat management and dissipation, therefore allowing plastic lenses, filters and diffusers to be used. Waterproof units can also easily be designed, allowing for use in harsh or wet environments (food, beverage, oil industries).
* LED sources can be shaped in several main configurations (spot lights for reflective illumination; ring lights for coaxial illumination; back lights for contour illumination; linear assemblies; flat, large format panels; dome sources for diffused, omnidirectional illumination).
* Very compact designs are possible, allowing for small LED illuminators to be integrated within smart cameras and vision sensors.


== See also ==
== Health and safety ==


Certain blue LEDs and cool-white LEDs can exceed safe limits of the so-called [[blue-light hazard]] as defined in eye safety specifications such as "ANSI/IESNA RP-27.1–05: Recommended Practice for Photobiological Safety for Lamp and Lamp Systems".<ref name="BlueLEDAhealthHazard">{{cite news | url=http://texyt.com/bright+blue+leds+annoyance+health+risks | title=Blue LEDs: A health hazard? | publisher=texyt.com | date=January 15, 2007 | access-date=September 3, 2007}}</ref> One study showed no evidence of a risk in normal use at domestic illuminance,<ref>[https://www.radioprotection.org/articles/radiopro/abs/2017/04/radiopro170025/radiopro170025.html Some evidences that white LEDs are toxic for human at domestic radiance?]. Radioprotection (2017-09-12). Retrieved on 2018-07-31.</ref> and that caution is only needed for particular occupational situations or for specific populations.<ref>Point, S. and Barlier-Salsi, A. (2018) [https://web.archive.org/web/20180614144321/http://www.sfrp.asso.fr/medias/sfrp/documents/Divers/Fiche%20lampes%20%C3%A0%20Led%20SFRP%20-%20Anglais%20_%2006-2018%20(2).pdf LEDs lighting and retinal damage], technical information sheets, SFRP</ref> In 2006, the [[International Electrotechnical Commission]] published ''IEC 62471 Photobiological safety of lamps and lamp systems'', replacing the application of early laser-oriented standards for classification of LED sources.<ref>{{cite web |url=https://www.ledsmagazine.com/articles/2012/11/led-based-products-must-meet-photobiological-safety-standards-part-2-magazine.html |title=LED Based Products Must Meet Photobilogical Safety Standards: Part 2 |website=ledsmagazine.com |date=29 November 2011 |access-date=9 January 2022 }}</ref>
{{portalpar|Electronics|Nuvola_apps_ksim.png}}


While LEDs have the advantage over [[fluorescent lamp]]s, in that they do not contain [[mercury (element)|mercury]], they may contain other hazardous metals such as [[lead]] and [[arsenic]].<ref name=Limetal2011>{{Cite journal | last1 = Lim | first1 = S. R. | last2 = Kang | first2 = D. | last3 = Ogunseitan | first3 = O. A. | last4 = Schoenung | first4 = J. M. | title = Potential Environmental Impacts of Light-Emitting Diodes (LEDs): Metallic Resources, Toxicity, and Hazardous Waste Classification | doi = 10.1021/es101052q | journal = Environmental Science & Technology | volume = 45 | issue = 1 | pages = 320–327 | year = 2011 | pmid = 21138290 | bibcode = 2011EnST...45..320L}}</ref>
* [[Nystagmus]] An eye condition in which sufferers have difficulty focusing on LED displays

* [[Photometry (optics)]] Main Photometry/Radiometry article—explains technical terms
In 2016 the [[American Medical Association]] (AMA) issued a statement concerning the possible adverse influence of blueish [[street light]]ing on the [[sleep-wake cycle]] of city-dwellers. Critics in the industry claim exposure levels are not high enough to have a noticeable effect.<ref>{{cite web |url=https://www.ledroadwaylighting.com/fr/nouvelles/612-response-to-the-american-medical-association-statement-on-high-intensity-street-lighting.html |title=Response to the AMA Statement on High Intensity Street Lighting |website=ledroadwaylighting.com |access-date=17 January 2019 |archive-date=January 19, 2019 |archive-url=https://web.archive.org/web/20190119121117/https://www.ledroadwaylighting.com/fr/nouvelles/612-response-to-the-american-medical-association-statement-on-high-intensity-street-lighting.html |url-status=dead }}</ref>
* [[LED lamp]]—[[solid state lighting]] (SSL)

* [[Flashlight]] about the growing use of LED technology in Flashlights
== Environmental issues ==
* [[Blinky (novelty)|Blinkies]]

* [[Throwies]]
* [[Light pollution]]: Because [[#White|white LEDs]] emit more short wavelength light than sources such as high-pressure [[sodium vapor lamp]]s, the increased blue and green sensitivity of [[scotopic vision]] means that white LEDs used in outdoor lighting cause substantially more [[sky glow]].<ref name="IDA">{{Cite book|title=Visibility, Environmental, and Astronomical Issues Associated with Blue-Rich White Outdoor Lighting |publisher=International Dark-Sky Association |date=May 4, 2010 |url=http://www.darksky.org/assets/documents/Reports/IDA-Blue-Rich-Light-White-Paper.pdf |url-status=dead |archive-url=https://web.archive.org/web/20130116003035/http://darksky.org/assets/documents/Reports/IDA-Blue-Rich-Light-White-Paper.pdf |archive-date=January 16, 2013 }}</ref>
* [[LED circuit]]
* Impact on wildlife: LEDs are much more attractive to insects than sodium-vapor lights, so much so that there has been speculative concern about the possibility of disruption to [[food web]]s.<ref>{{cite web |title=LEDs: Good for prizes, bad for insects |url=https://www.science.org/content/article/leds-good-prizes-bad-insects |website=Science |first1=Erik |last1=Stokstad |date=7 October 2014 |access-date=7 October 2014 }}</ref><ref>{{Cite journal|title=LED Lighting Increases the Ecological Impact of Light Pollution Irrespective of Color Temperature |bibcode-access=free |journal=Ecological Applications|volume=24|issue=7|pages=1561–1568|doi=10.1890/14-0468.1|pmid=29210222|year=2014|last1=Pawson|first1=S. M.|last2=Bader|first2=M. K.-F.|bibcode=2014EcoAp..24.1561P |doi-access=free}}</ref> LED lighting near beaches, particularly intense blue and white colors, can disorient turtle hatchlings and make them wander inland instead.<ref>{{Cite web|url=https://news.usc.edu/144389/usc-scientist-database-reduce-effects-of-led-light-on-animals/ |first1=Gary |last1=Polakovic |title=Scientist's new database can help protect wildlife from harmful hues of LED lights|date=2018-06-12|website=USC News|language=en-US|access-date=2019-12-16 |url-status=live |archive-url=https://web.archive.org/web/20200519125811/https://news.usc.edu/144389/usc-scientist-database-reduce-effects-of-led-light-on-animals/ |archive-date= May 19, 2020 }}</ref> The use of "turtle-safe lighting" LEDs that emit only at narrow portions of the visible spectrum is encouraged by conservancy groups in order to reduce harm.<ref>{{Cite web|url=https://conserveturtles.org/information-sea-turtles-threats-artificial-lighting/|title=Information About Sea Turtles: Threats from Artificial Lighting |website=Sea Turtle Conservancy|language=en-US|access-date=2019-12-16}}</ref>
* [[Nixie tube]]
* Use in winter conditions: Since they do not give off much heat in comparison to incandescent lights, LED lights used for traffic control can have snow obscuring them, leading to accidents.<ref>{{cite web|url=https://abcnews.go.com/GMA/ConsumerNews/led-traffic-lights-unusual-potentially-deadly-winter-problem/story?id=9506449|title=Stoplights' Unusual, Potentially Deadly Winter Problem |date=January 8, 2010|publisher=ABC News |url-status=live |archive-url=https://web.archive.org/web/20231212132630/https://abcnews.go.com/GMA/ConsumerNews/led-traffic-lights-unusual-potentially-deadly-winter-problem/story?id=9506449 |archive-date= Dec 12, 2023 }}</ref><ref>{{cite web|url=https://www.cars.com/articles/2009/12/led-traffic-lights-cant-melt-snow-ice/|title=LED Traffic Lights Can't Melt Snow, Ice |website=Cars.com |date= December 17, 2009 |first1=Stephen |last1=Markley |url-status=live |archive-url=https://web.archive.org/web/20190606011840/https://www.cars.com/articles/2009/12/led-traffic-lights-cant-melt-snow-ice/ |archive-date= Jun 6, 2019 }}</ref>
* [[Light Up the World Foundation]]

* [[Lumalive]], a photonic [[textile]]
== See also ==
* [[LEDs as Photodiode Light Sensors]]
{{Portal|Electronics|Energy}}
* [[Organic light-emitting diode]]
* [[LED tattoo]]
* [[High-CRI LED lighting]]
* [[List of light sources]]
* [[MicroLED]]
* [[Superluminescent diode]]
* [[Perovskite light-emitting diode]]


== References ==
== References ==


{{reflist}}
;Cited
{{reflist|2}}


== Further reading ==
;General

<div class="references-small">
* {{Cite book|author1=David L. Heiserman |title=Light -Emitting Diodes|publisher=Electronics World|year=1968|url=https://worldradiohistory.com/Archive-Electronics-World/60s/1968/Electronics-World-1968-01.pdf}}
* [http://www.amazon.com/dp/3540665056/ Shuji Nakamura, Gerhard Fasol, Stephen J Pearton The Blue Laser Diode: The Complete Story, Springer Verlag, 2nd Edition (October 2, 2000)]
* {{Cite book|author1=Shuji Nakamura |author2=Gerhard Fasol |author3=Stephen J Pearton |title=The Blue Laser Diode: The Complete Story|publisher=Springer Verlag|year=2000|isbn=978-3-540-66505-2|url=https://books.google.com/books?id=AHyMBJ_LMykC}}
* {{cite journal | last=Mills |first= Evan | title=The Specter of Fuel-Based Lighting | journal=Science | year=2005 | volume=308 | pages=1263–1264 | url= | doi=10.1126/science.1113090 | pmid=15919979 }}
* Moreno, I., "Spatial distribution of LED radiation," in The International Optical Design Conference, Proc. SPIE vol. 6342, 634216:1-7 (2006).
* {{cite web |last=Salisbury |first=David F. |url=http://exploration.vanderbilt.edu/news/news_quantumdot_led.htm |title=Quantum dots that produce white light could be the light bulb’s successor |work=Exploration—The Online Research Journal of Vanderbilt University |date=October 20, 2005}} (More details regarding the use of quantum dots as a phosphor for white LEDs.)
</div>


== External links ==
== External links ==
{{Commons category multi|Light-emitting diodes|Light-emitting diodes (SMD)}}
{{Wiktionary|light-emitting diode}}
* [https://web.archive.org/web/20121015224322/http://www.dlip.de/?p=99 Building a do-it-yourself LED]
* [http://cdn.sparkfun.com/datasheets/Components/LED/changingLED.pdf Color cycling LED in a single two pin package],
* {{YouTube|4y7p9R2No-4|Educational video on LEDs}}
{{Prone to spam|date=July 2013}}
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{{Artificial light sources}}
{{Commons|LED|Light-emitting diode}}
{{Display technology}}
* [http://www.netl.doe.gov/ssl/faqs.htm Solid State Lighting program at U.S. DOE]
{{Electronic components}}
* [http://www.tyndall.ie/gan Photonics Sources Group, Tyndall National Institute] GaN and other photonics research at the Tyndall National Institute, Ireland.
{{Authority control}}
* [http://ieee.li/pdf/viewgraphs_lighting.pdf Solid State Lighting, Michael Shur - Rensselaer Polytechnic]
* [http://www.intl-lighttech.com/applications/led-lamps Applications notes about Discrete LEDs including basic driver circuits]
* [http://www.lsdiodes.com/tutorial LED Circuitry Tutorial]
* [http://www.led-calculator.com LED Resistor Calculator]


{{Use American English|date=October 2015}}
{{ArtificialLightSources}}
{{Use mdy dates|date=March 2013}}


[[Category:Light-emitting diodes| ]]
{{Display Technology}}
[[Category:LED lamps]]

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[[Category:Optical diodes]]
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[[Category:Semiconductor devices]]
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[[Category:1907 introductions]]
[[Category:20th-century inventions]]

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Latest revision as of 19:53, 22 December 2024

Light-emitting diode
Blue, green, and red LEDs in 5 mm diffused cases. There are many different variants of LEDs.
Working principleElectroluminescence
Inventor
First production October 1962; 62 years ago (1962-10)
Pin namesAnode and cathode
Electronic symbol
Parts of a conventional LED. The flat bottom surfaces of the anvil and post embedded inside the epoxy act as anchors, to prevent the conductors from being forcefully pulled out via mechanical strain or vibration.
Close-up image of a surface-mount LED
Close-up of an LED with the voltage being increased and decreased to show a detailed view of its operation
Modern LED retrofit with E27 screw in base
A bulb-shaped modern retrofit LED lamp with aluminum heat sink, a light diffusing dome and E27 screw base, using a built-in power supply working on mains voltage

A light-emitting diode (LED) is a semiconductor device that emits light when current flows through it. Electrons in the semiconductor recombine with electron holes, releasing energy in the form of photons. The color of the light (corresponding to the energy of the photons) is determined by the energy required for electrons to cross the band gap of the semiconductor.[5] White light is obtained by using multiple semiconductors or a layer of light-emitting phosphor on the semiconductor device.[6]

Appearing as practical electronic components in 1962, the earliest LEDs emitted low-intensity infrared (IR) light.[7] Infrared LEDs are used in remote-control circuits, such as those used with a wide variety of consumer electronics. The first visible-light LEDs were of low intensity and limited to red.

Early LEDs were often used as indicator lamps, replacing small incandescent bulbs, and in seven-segment displays. Later developments produced LEDs available in visible, ultraviolet (UV), and infrared wavelengths with high, low, or intermediate light output, for instance, white LEDs suitable for room and outdoor lighting. LEDs have also given rise to new types of displays and sensors, while their high switching rates are useful in advanced communications technology with applications as diverse as aviation lighting, fairy lights, strip lights, automotive headlamps, advertising, general lighting, traffic signals, camera flashes, lighted wallpaper, horticultural grow lights, and medical devices.[8]

LEDs have many advantages over incandescent light sources, including lower power consumption, a longer lifetime, improved physical robustness, smaller sizes, and faster switching. In exchange for these generally favorable attributes, disadvantages of LEDs include electrical limitations to low voltage and generally to DC (not AC) power, the inability to provide steady illumination from a pulsing DC or an AC electrical supply source, and a lesser maximum operating temperature and storage temperature.

LEDs are transducers of electricity into light. They operate in reverse of photodiodes, which convert light into electricity.

History

[edit]

The first LED was created by Soviet inventor Oleg Losev[9] in 1927, but electroluminescence was already known for 20 years, and relied on a diode made of silicon carbide.

Commercially viable LEDs only became available after Texas Instruments engineers patented efficient near-infrared emission from a diode based on GaAs in 1962.

From 1968, commercial LEDs were extremely costly and saw no practical use. Monstanto and Hewlett-Packard led the development of LEDs to the point where, in the 1970s, a unit cost less than five cents.[10]

Physics of light production and emission

[edit]

In a light-emitting diode, the recombination of electrons and electron holes in a semiconductor produces light (be it infrared, visible or UV), a process called "electroluminescence". The wavelength of the light depends on the energy band gap of the semiconductors used. Since these materials have a high index of refraction, design features of the devices such as special optical coatings and die shape are required to efficiently emit light.[11]

Unlike a laser, the light emitted from an LED is neither spectrally coherent nor even highly monochromatic. Its spectrum is sufficiently narrow that it appears to the human eye as a pure (saturated) color.[12][13] Also unlike most lasers, its radiation is not spatially coherent, so it cannot approach the very high intensity characteristic of lasers.

Single-color LEDs

[edit]
Blue LEDs
External videos
video icon "The Original Blue LED", Science History Institute

By selection of different semiconductor materials, single-color LEDs can be made that emit light in a narrow band of wavelengths from near-infrared through the visible spectrum and into the ultraviolet range. The required operating voltages of LEDs increase as the emitted wavelengths become shorter (higher energy, red to blue), because of their increasing semiconductor band gap.

Blue LEDs have an active region consisting of one or more InGaN quantum wells sandwiched between thicker layers of GaN, called cladding layers. By varying the relative In/Ga fraction in the InGaN quantum wells, the light emission can in theory be varied from violet to amber.

Aluminium gallium nitride (AlGaN) of varying Al/Ga fraction can be used to manufacture the cladding and quantum well layers for ultraviolet LEDs, but these devices have not yet reached the level of efficiency and technological maturity of InGaN/GaN blue/green devices. If unalloyed GaN is used in this case to form the active quantum well layers, the device emits near-ultraviolet light with a peak wavelength centred around 365 nm. Green LEDs manufactured from the InGaN/GaN system are far more efficient and brighter than green LEDs produced with non-nitride material systems, but practical devices still exhibit efficiency too low for high-brightness applications.[citation needed]

With AlGaN and AlGaInN, even shorter wavelengths are achievable. Near-UV emitters at wavelengths around 360–395 nm are already cheap and often encountered, for example, as black light lamp replacements for inspection of anti-counterfeiting UV watermarks in documents and bank notes, and for UV curing. Substantially more expensive, shorter-wavelength diodes are commercially available for wavelengths down to 240 nm.[14] As the photosensitivity of microorganisms approximately matches the absorption spectrum of DNA, with a peak at about 260 nm, UV LED emitting at 250–270 nm are expected in prospective disinfection and sterilization devices. Recent research has shown that commercially available UVA LEDs (365 nm) are already effective disinfection and sterilization devices.[15] UV-C wavelengths were obtained in laboratories using aluminium nitride (210 nm),[16] boron nitride (215 nm)[17][18] and diamond (235 nm).[19]

White LEDs

[edit]

There are two primary ways of producing white light-emitting diodes. One is to use individual LEDs that emit three primary colors—red, green and blue—and then mix all the colors to form white light. The other is to use a phosphor material to convert monochromatic light from a blue or UV LED to broad-spectrum white light, similar to a fluorescent lamp. The yellow phosphor is cerium-doped YAG crystals suspended in the package or coated on the LED. This YAG phosphor causes white LEDs to appear yellow when off, and the space between the crystals allow some blue light to pass through in LEDs with partial phosphor conversion. Alternatively, white LEDs may use other phosphors like manganese(IV)-doped potassium fluorosilicate (PFS) or other engineered phosphors. PFS assists in red light generation, and is used in conjunction with conventional Ce:YAG phosphor.

In LEDs with PFS phosphor, some blue light passes through the phosphors, the Ce:YAG phosphor converts blue light to green and red (yellow) light, and the PFS phosphor converts blue light to red light. The color, emission spectrum or color temperature of white phosphor converted and other phosphor converted LEDs can be controlled by changing the concentration of several phosphors that form a phosphor blend used in an LED package.[20][21][22][23]

The 'whiteness' of the light produced is engineered to suit the human eye. Because of metamerism, it is possible to have quite different spectra that appear white. The appearance of objects illuminated by that light may vary as the spectrum varies. This is the issue of color rendition, quite separate from color temperature. An orange or cyan object could appear with the wrong color and much darker as the LED or phosphor does not emit the wavelength it reflects. The best color rendition LEDs use a mix of phosphors, resulting in less efficiency and better color rendering.[citation needed]

The first white light-emitting diodes (LEDs) were offered for sale in the autumn of 1996.[24] Nichia made some of the first white LEDs which were based on blue LEDs with Ce:YAG phosphor.[25] Ce:YAG is often grown using the Czochralski method.[26]

RGB systems

[edit]
Combined spectral curves for blue, yellow-green, and high-brightness red solid-state semiconductor LEDs. FWHM spectral bandwidth is approximately 24–27 nm for all three colors.
An RGB LED projecting red, green, and blue onto a surface

Mixing red, green, and blue sources to produce white light needs electronic circuits to control the blending of the colors. Since LEDs have slightly different emission patterns, the color balance may change depending on the angle of view, even if the RGB sources are in a single package, so RGB diodes are seldom used to produce white lighting. Nonetheless, this method has many applications because of the flexibility of mixing different colors,[27] and in principle, this mechanism also has higher quantum efficiency in producing white light.[28]

There are several types of multicolor white LEDs: di-, tri-, and tetrachromatic white LEDs. Several key factors that play among these different methods include color stability, color rendering capability, and luminous efficacy. Often, higher efficiency means lower color rendering, presenting a trade-off between the luminous efficacy and color rendering. For example, the dichromatic white LEDs have the best luminous efficacy (120 lm/W), but the lowest color rendering capability. Although tetrachromatic white LEDs have excellent color rendering capability, they often have poor luminous efficacy. Trichromatic white LEDs are in between, having both good luminous efficacy (>70 lm/W) and fair color rendering capability.[29]

One of the challenges is the development of more efficient green LEDs. The theoretical maximum for green LEDs is 683 lumens per watt but as of 2010 few green LEDs exceed even 100 lumens per watt. The blue and red LEDs approach their theoretical limits.[citation needed]

Multicolor LEDs offer a means to form light of different colors. Most perceivable colors can be formed by mixing different amounts of three primary colors. This allows precise dynamic color control. Their emission power decays exponentially with rising temperature,[30] resulting in a substantial change in color stability. Such problems inhibit industrial use. Multicolor LEDs without phosphors cannot provide good color rendering because each LED is a narrowband source. LEDs without phosphor, while a poorer solution for general lighting, are the best solution for displays, either backlight of LCD, or direct LED based pixels.

Dimming a multicolor LED source to match the characteristics of incandescent lamps is difficult because manufacturing variations, age, and temperature change the actual color value output. To emulate the appearance of dimming incandescent lamps may require a feedback system with color sensor to actively monitor and control the color.[31]

Phosphor-based LEDs

[edit]
Spectrum of a white LED showing blue light directly emitted by the GaN-based LED (peak at about 465 nm) and the more broadband Stokes-shifted light emitted by the Ce3+:YAG phosphor, which emits at roughly 500–700 nm

This method involves coating LEDs of one color (mostly blue LEDs made of InGaN) with phosphors of different colors to form white light; the resultant LEDs are called phosphor-based or phosphor-converted white LEDs (pcLEDs).[32] A fraction of the blue light undergoes the Stokes shift, which transforms it from shorter wavelengths to longer. Depending on the original LED's color, various color phosphors are used. Using several phosphor layers of distinct colors broadens the emitted spectrum, effectively raising the color rendering index (CRI).[33]

Phosphor-based LEDs have efficiency losses due to heat loss from the Stokes shift and also other phosphor-related issues. Their luminous efficacies compared to normal LEDs depend on the spectral distribution of the resultant light output and the original wavelength of the LED itself. For example, the luminous efficacy of a typical YAG yellow phosphor based white LED ranges from 3 to 5 times the luminous efficacy of the original blue LED because of the human eye's greater sensitivity to yellow than to blue (as modeled in the luminosity function).

Due to the simplicity of manufacturing, the phosphor method is still the most popular method for making high-intensity white LEDs. The design and production of a light source or light fixture using a monochrome emitter with phosphor conversion is simpler and cheaper than a complex RGB system, and the majority of high-intensity white LEDs presently on the market are manufactured using phosphor light conversion.[citation needed]

1 watt 9 volt three chips SMD phosphor based white LED

Among the challenges being faced to improve the efficiency of LED-based white light sources is the development of more efficient phosphors. As of 2010, the most efficient yellow phosphor is still the YAG phosphor, with less than 10% Stokes shift loss. Losses attributable to internal optical losses due to re-absorption in the LED chip and in the LED packaging itself account typically for another 10% to 30% of efficiency loss. Currently, in the area of phosphor LED development, much effort is being spent on optimizing these devices to higher light output and higher operation temperatures. For instance, the efficiency can be raised by adapting better package design or by using a more suitable type of phosphor. Conformal coating process is frequently used to address the issue of varying phosphor thickness.[citation needed]

Some phosphor-based white LEDs encapsulate InGaN blue LEDs inside phosphor-coated epoxy. Alternatively, the LED might be paired with a remote phosphor, a preformed polycarbonate piece coated with the phosphor material. Remote phosphors provide more diffuse light, which is desirable for many applications. Remote phosphor designs are also more tolerant of variations in the LED emissions spectrum. A common yellow phosphor material is cerium-doped yttrium aluminium garnet (Ce3+:YAG).[citation needed]

White LEDs can also be made by coating near-ultraviolet (NUV) LEDs with a mixture of high-efficiency europium-based phosphors that emit red and blue, plus copper and aluminium-doped zinc sulfide (ZnS:Cu, Al) that emits green. This is a method analogous to the way fluorescent lamps work. This method is less efficient than blue LEDs with YAG:Ce phosphor, as the Stokes shift is larger, so more energy is converted to heat, but yields light with better spectral characteristics, which render color better. Due to the higher radiative output of the ultraviolet LEDs than of the blue ones, both methods offer comparable brightness. A concern is that UV light may leak from a malfunctioning light source and cause harm to human eyes or skin.[citation needed]

A new style of wafers composed of gallium-nitride-on-silicon (GaN-on-Si) is being used to produce white LEDs using 200-mm silicon wafers. This avoids the typical costly sapphire substrate in relatively small 100- or 150-mm wafer sizes.[34] The sapphire apparatus must be coupled with a mirror-like collector to reflect light that would otherwise be wasted. It was predicted that since 2020, 40% of all GaN LEDs are made with GaN-on-Si. Manufacturing large sapphire material is difficult, while large silicon material is cheaper and more abundant. LED companies shifting from using sapphire to silicon should be a minimal investment.[35]

Mixed white LEDs

[edit]
Tunable white LED array in a floodlight

There are RGBW LEDs that combine RGB units with a phosphor white LED on the market. Doing so retains the extremely tunable color of RGB LED, but allows color rendering and efficiency to be optimized when a color close to white is selected.[36]

Some phosphor white LED units are "tunable white", blending two extremes of color temperatures (commonly 2700K and 6500K) to produce intermediate values. This feature allows users to change the lighting to suit the current use of a multifunction room.[37] As illustrated by a straight line on the chromaticity diagram, simple two-white blends will have a pink bias, becoming most severe in the middle. A small amount of green light, provided by another LED, could correct the problem.[38] Some products are RGBWW, i.e. RGBW with tunable white.[39]

A final class of white LED with mixed light is dim-to-warm. These are ordinary 2700K white LED bulbs with a small red LED that turns on when the bulb is dimmed. Doing so makes the color warmer, emulating an incandescent light bulb.[39]

Other white LEDs

[edit]

Another method used to produce experimental white light LEDs used no phosphors at all and was based on homoepitaxially grown zinc selenide (ZnSe) on a ZnSe substrate that simultaneously emitted blue light from its active region and yellow light from the substrate.[40]

Organic light-emitting diodes (OLEDs)

[edit]

In an organic light-emitting diode (OLED), the electroluminescent material composing the emissive layer of the diode is an organic compound. The organic material is electrically conductive due to the delocalization of pi electrons caused by conjugation over all or part of the molecule, and the material therefore functions as an organic semiconductor.[41] The organic materials can be small organic molecules in a crystalline phase, or polymers.[42]

The potential advantages of OLEDs include thin, low-cost displays with a low driving voltage, wide viewing angle, and high contrast and color gamut.[43] Polymer LEDs have the added benefit of printable and flexible displays.[44][45][46] OLEDs have been used to make visual displays for portable electronic devices such as cellphones, digital cameras, lighting and televisions.[42][43]

Types

[edit]
LEDs are produced in a variety of shapes and sizes. The color of the plastic lens is often the same as the actual color of light emitted, but not always. For instance, purple plastic is often used for infrared LEDs, and most blue devices have colorless housings. Modern high-power LEDs such as those used for lighting and backlighting are generally found in surface-mount technology (SMT) packages (not shown).
A variety of different diffused 5 mm THT-LEDs
  • Red, 650 – 625nm
  • Orange, 600 – 610nm
  • Yellow, 587 – 591nm
  • Green, 570 – 575nm
  • Blue, 465 – 467nm
  • Purple, 395 – 400nm

LEDs are made in different packages for different applications. A single or a few LED junctions may be packed in one miniature device for use as an indicator or pilot lamp. An LED array may include controlling circuits within the same package, which may range from a simple resistor, blinking or color changing control, or an addressable controller for RGB devices. Higher-powered white-emitting devices will be mounted on heat sinks and will be used for illumination. Alphanumeric displays in dot matrix or bar formats are widely available. Special packages permit connection of LEDs to optical fibers for high-speed data communication links.

Miniature

[edit]
Image of miniature surface mount LEDs in most common sizes. They can be much smaller than a traditional 5 mm lamp type LED, shown on the upper left corner.
Very small (1.6×1.6×0.35 mm) red, green, and blue surface mount miniature LED package with gold wire bonding details

These are mostly single-die LEDs used as indicators, and they come in various sizes from 1.8 mm to 10 mm, through-hole and surface mount packages.[47] Typical current ratings range from around 1 mA to above 20 mA. LED's can be soldered to a flexible PCB strip to form LED tape popularly used for decoration.

Common package shapes include round, with a domed or flat top, rectangular with a flat top (as used in bar-graph displays), and triangular or square with a flat top. The encapsulation may also be clear or tinted to improve contrast and viewing angle. Infrared devices may have a black tint to block visible light while passing infrared radiation, such as the Osram SFH 4546.[48]

5 V and 12 V LEDs are ordinary miniature LEDs that have a series resistor for direct connection to a 5 V or 12 V supply.[49]

High-power

[edit]
High-power light-emitting diodes attached to an LED star base (Luxeon, Lumileds)

High-power LEDs (HP-LEDs) or high-output LEDs (HO-LEDs) can be driven at currents from hundreds of mA to more than an ampere, compared with the tens of mA for other LEDs. Some can emit over a thousand lumens.[50][51] LED power densities up to 300 W/cm2 have been achieved. Since overheating is destructive, the HP-LEDs must be mounted on a heat sink to allow for heat dissipation. If the heat from an HP-LED is not removed, the device fails in seconds. One HP-LED can often replace an incandescent bulb in a flashlight, or be set in an array to form a powerful LED lamp.

Some HP-LEDs in this category are the Nichia 19 series, Lumileds Rebel Led, Osram Opto Semiconductors Golden Dragon, and Cree X-lamp. As of September 2009, some HP-LEDs manufactured by Cree exceed 105 lm/W.[52]

Examples for Haitz's law—which predicts an exponential rise in light output and efficacy of LEDs over time—are the CREE XP-G series LED, which achieved 105 lm/W in 2009[52] and the Nichia 19 series with a typical efficacy of 140 lm/W, released in 2010.[53]

AC-driven

[edit]

LEDs developed by Seoul Semiconductor can operate on AC power without a DC converter. For each half-cycle, part of the LED emits light and part is dark, and this is reversed during the next half-cycle. The efficiency of this type of HP-LED is typically 40 lm/W.[54] A large number of LED elements in series may be able to operate directly from line voltage. In 2009, Seoul Semiconductor released a high DC voltage LED, named 'Acrich MJT', capable of being driven from AC power with a simple controlling circuit. The low-power dissipation of these LEDs affords them more flexibility than the original AC LED design.[55]

Strip

[edit]
Several LED spots being reflected as continuous lighting strip

An LED strip, tape, or ribbon light is a flexible circuit board populated by surface-mount light-emitting diodes (SMD LEDs) and other components that usually comes with an adhesive backing. Traditionally, strip lights had been used solely in accent lighting, backlighting, task lighting, and decorative lighting applications, such as cove lighting.

LED strip lights originated in the early 2000s. Since then, increased luminous efficacy and higher-power SMDs have allowed them to be used in applications such as high brightness task lighting, fluorescent and halogen lighting fixture replacements, indirect lighting applications, ultraviolet inspection during manufacturing processes, set and costume design, and growing plants.

Application-specific

[edit]
RGB-SMD-LED
Composite image of an 11 × 44 LED matrix lapel name tag display using 1608/0603-type SMD LEDs. Top: A little over half of the 21 × 86 mm display. Center: Close-up of LEDs in ambient light. Bottom: LEDs in their own red light.
Flashing
Flashing LEDs are used as attention seeking indicators without requiring external electronics. Flashing LEDs resemble standard LEDs but they contain an integrated voltage regulator and a multivibrator circuit that causes the LED to flash with a typical period of one second. In diffused lens LEDs, this circuit is visible as a small black dot. Most flashing LEDs emit light of one color, but more sophisticated devices can flash between multiple colors and even fade through a color sequence using RGB color mixing. Flashing SMD LEDs in the 0805 and other size formats have been available since early 2019.
Flickering
Integrated electronics Simple electronic circuits integrated into the LED package have been around since at least 2011 which produce a random LED intensity pattern reminiscent of a flickering candle.[56] Reverse engineering in 2024 has suggested that some flickering LEDs with automatic sleep and wake modes might be using an integrated 8-bit microcontroller for such functionally.[57]
Bi-color
Bi-color LEDs contain two different LED emitters in one case. There are two types of these. One type consists of two dies connected to the same two leads antiparallel to each other. Current flow in one direction emits one color, and current in the opposite direction emits the other color. The other type consists of two dies with separate leads for both dies and another lead for common anode or cathode so that they can be controlled independently. The most common bi-color combination is red/traditional green. Others include amber/traditional green, red/pure green, red/blue, and blue/pure green.
RGB tri-color
Tri-color LEDs contain three different LED emitters in one case. Each emitter is connected to a separate lead so they can be controlled independently. A four-lead arrangement is typical with one common lead (anode or cathode) and an additional lead for each color. Others have only two leads (positive and negative) and have a built-in electronic controller. RGB LEDs consist of one red, one green, and one blue LED.[58] By independently adjusting each of the three, RGB LEDs are capable of producing a wide color gamut. Unlike dedicated-color LEDs, these do not produce pure wavelengths. Modules may not be optimized for smooth color mixing.
Decorative-multicolor
Decorative-multicolor LEDs incorporate several emitters of different colors supplied by only two lead-out wires. Colors are switched internally by varying the supply voltage.
Alphanumeric
Alphanumeric LEDs are available in seven-segment, starburst, and dot-matrix format. Seven-segment displays handle all numbers and a limited set of letters. Starburst displays can display all letters. Dot-matrix displays typically use 5×7 pixels per character. Seven-segment LED displays were in widespread use in the 1970s and 1980s, but rising use of liquid crystal displays, with their lower power needs and greater display flexibility, has reduced the popularity of numeric and alphanumeric LED displays.
Digital RGB
Digital RGB addressable LEDs contain their own "smart" control electronics. In addition to power and ground, these provide connections for data-in, data-out, clock and sometimes a strobe signal. These are connected in a daisy chain, which allows individual LEDs in a long LED strip light to be easily controlled by a microcontroller. Data sent to the first LED of the chain can control the brightness and color of each LED independently of the others. They are used where a combination of maximum control and minimum visible electronics are needed such as strings for Christmas and LED matrices. Some even have refresh rates in the kHz range, allowing for basic video applications. These devices are known by their part number (WS2812 being common) or a brand name such as NeoPixel.
Filament
An LED filament consists of multiple LED chips connected in series on a common longitudinal substrate that forms a thin rod reminiscent of a traditional incandescent filament.[59] These are being used as a low-cost decorative alternative for traditional light bulbs that are being phased out in many countries. The filaments use a rather high voltage, allowing them to work efficiently with mains voltages. Often a simple rectifier and capacitive current limiting are employed to create a low-cost replacement for a traditional light bulb without the complexity of the low voltage, high current converter that single die LEDs need.[60] Usually, they are packaged in bulb similar to the lamps they were designed to replace, and filled with inert gas at slightly lower than ambient pressure to remove heat efficiently and prevent corrosion.
Chip-on-board arrays
Surface-mounted LEDs are frequently produced in chip on board (COB) arrays, allowing better heat dissipation than with a single LED of comparable luminous output.[61] The LEDs can be arranged around a cylinder, and are called "corn cob lights" because of the rows of yellow LEDs.[62]

Considerations for use

[edit]
  • Efficiency: LEDs emit more lumens per watt than incandescent light bulbs.[63] The efficiency of LED lighting fixtures is not affected by shape and size, unlike fluorescent light bulbs or tubes.
  • Size: LEDs can be very small (smaller than 2 mm2[64]) and are easily attached to printed circuit boards.

Power sources

[edit]
Simple LED circuit with resistor for current limiting

The current in an LED or other diodes rises exponentially with the applied voltage (see Shockley diode equation), so a small change in voltage can cause a large change in current. Current through the LED must be regulated by an external circuit such as a constant current source to prevent damage. Since most common power supplies are (nearly) constant-voltage sources, LED fixtures must include a power converter, or at least a current-limiting resistor. In some applications, the internal resistance of small batteries is sufficient to keep current within the LED rating.[citation needed]

LEDs are sensitive to voltage. They must be supplied with a voltage above their threshold voltage and a current below their rating. Current and lifetime change greatly with a small change in applied voltage. They thus require a current-regulated supply (usually just a series resistor for indicator LEDs).[65]

Efficiency droop: The efficiency of LEDs decreases as the electric current increases. Heating also increases with higher currents, which compromises LED lifetime. These effects put practical limits on the current through an LED in high power applications.[66]

Electrical polarity

[edit]

Unlike a traditional incandescent lamp, an LED will light only when voltage is applied in the forward direction of the diode. No current flows and no light is emitted if voltage is applied in the reverse direction. If the reverse voltage exceeds the breakdown voltage, which is typically about five volts, a large current flows and the LED will be damaged. If the reverse current is sufficiently limited to avoid damage, the reverse-conducting LED is a useful noise diode.[citation needed]

By definition, the energy band gap of any diode is higher when reverse-biased than when forward-biased. Because the band gap energy determines the wavelength of the light emitted, the color cannot be the same when reverse-biased. The reverse breakdown voltage is sufficiently high that the emitted wavelength cannot be similar enough to still be visible. Though dual-LED packages exist that contain a different color LED in each direction, it is not expected that any single LED element can emit visible light when reverse-biased.[citation needed]

It is not known if any zener diode could exist that emits light only in reverse-bias mode. Uniquely, this type of LED would conduct when connected backwards.

Appearance

[edit]
  • Color: LEDs can emit light of an intended color without using any color filters as traditional lighting methods need. This is more efficient and can lower initial costs.
  • Cool light: In contrast to most light sources, LEDs radiate very little heat in the form of IR that can cause damage to sensitive objects or fabrics. Wasted energy is dispersed as heat through the base of the LED.
  • Color rendition: Most cool-white LEDs have spectra that differ significantly from a black body radiator like the sun or an incandescent light. The spike at 460 nm and dip at 500 nm can make the color of objects appear differently under cool-white LED illumination than sunlight or incandescent sources, due to metamerism,[67] red surfaces being rendered particularly poorly by typical phosphor-based cool-white LEDs. The same is true with green surfaces. The quality of color rendition of an LED is measured by the Color Rendering Index (CRI).
  • Dimming: LEDs can be dimmed either by pulse-width modulation or lowering the forward current.[68] This pulse-width modulation is why LED lights, particularly headlights on cars, when viewed on camera or by some people, seem to flash or flicker. This is a type of stroboscopic effect.

Light properties

[edit]
  • Switch on time: LEDs light up extremely quickly. A typical red indicator LED achieves full brightness in under a microsecond.[69] LEDs used in communications devices can have even faster response times.
  • Focus: The solid package of the LED can be designed to focus its light. Incandescent and fluorescent sources often require an external reflector to collect light and direct it in a usable manner. For larger LED packages total internal reflection (TIR) lenses are often used to the same effect. When large quantities of light are needed, many light sources such as LED chips are usually deployed, which are difficult to focus or collimate on the same target.
  • Area light source: Single LEDs do not approximate a point source of light giving a spherical light distribution, but rather a lambertian distribution. So, LEDs are difficult to apply to uses needing a spherical light field. Different fields of light can be manipulated by the application of different optics or "lenses". LEDs cannot provide divergence below a few degrees.[70]

Reliability

[edit]
  • Shock resistance: LEDs, being solid-state components, are difficult to damage with external shock, unlike fluorescent and incandescent bulbs, which are fragile.[71]
  • Thermal runaway: Parallel strings of LEDs will not share current evenly due to the manufacturing tolerances in their forward voltage. Running two or more strings from a single current source may result in LED failure as the devices warm up. If forward voltage binning is not possible, a circuit is required to ensure even distribution of current between parallel strands.[72]
  • Slow failure: LEDs mainly fail by dimming over time, rather than the abrupt failure of incandescent bulbs.[73]
  • Lifetime: LEDs can have a relatively long useful life. One report estimates 35,000 to 50,000 hours of useful life, though time to complete failure may be shorter or longer.[74] Fluorescent tubes typically are rated at about 10,000 to 25,000 hours, depending partly on the conditions of use, and incandescent light bulbs at 1,000 to 2,000 hours. Several DOE demonstrations have shown that reduced maintenance costs from this extended lifetime, rather than energy savings, is the primary factor in determining the payback period for an LED product.[75]
  • Cycling: LEDs are ideal for uses subject to frequent on-off cycling, unlike incandescent and fluorescent lamps that fail faster when cycled often, or high-intensity discharge lamps (HID lamps) that require a long time to warm up to full output and to cool down before they can be lighted again if they are being restarted.
  • Temperature dependence: LED performance largely depends on the ambient temperature of the operating environment – or thermal management properties. Overdriving an LED in high ambient temperatures may result in overheating the LED package, eventually leading to device failure. An adequate heat sink is needed to maintain long life. This is especially important in automotive, medical, and military uses where devices must operate over a wide range of temperatures, and require low failure rates.

Manufacturing

[edit]

LED manufacturing involves multiple steps, including epitaxy, chip processing, chip separation, and packaging.[76]

In a typical LED manufacturing process, encapsulation is performed after probing, dicing, die transfer from wafer to package, and wire bonding or flip chip mounting,[77] perhaps using indium tin oxide, a transparent electrical conductor. In this case, the bond wire(s) are attached to the ITO film that has been deposited in the LEDs.

Flip chip circuit on board (COB) is a technique that can be used to manufacture LEDs.[78]

Colors and materials

[edit]

Conventional LEDs are made from a variety of inorganic semiconductor materials. The following table shows the available colors with wavelength range, voltage drop and material:

Color Wavelength (nm) Voltage (V) Semiconductor material
Infrared λ > 760 ΔV < 1.9 Gallium arsenide (GaAs)

Aluminium gallium arsenide (AlGaAs)

Red 610 < λ < 760 1.63 < ΔV < 2.03 Aluminium gallium arsenide (AlGaAs)

Gallium arsenide phosphide (GaAsP) Aluminium gallium indium phosphide (AlGaInP) Gallium(III) phosphide (GaP)

Orange 590 < λ < 610 2.03 < ΔV < 2.10 Gallium arsenide phosphide (GaAsP)

Aluminium gallium indium phosphide (AlGaInP) Gallium(III) phosphide (GaP)

Yellow 570 < λ < 590 2.10 < ΔV < 2.18 Gallium arsenide phosphide (GaAsP)

Aluminium gallium indium phosphide (AlGaInP) Gallium(III) phosphide (GaP)

Green 500 < λ < 570 1.9[79] < ΔV < 4.0 Indium gallium nitride (InGaN) / Gallium(III) nitride (GaN)

Gallium(III) phosphide (GaP) Aluminium gallium indium phosphide (AlGaInP) Aluminium gallium phosphide (AlGaP)

Blue 450 < λ < 500 2.48 < ΔV < 3.7 Zinc selenide (ZnSe)

Indium gallium nitride (InGaN) Silicon carbide (SiC) as substrate Silicon (Si) as substrate — (under development)

Violet 400 < λ < 450 2.76 < ΔV < 4.0 Indium gallium nitride (InGaN)
Purple multiple types 2.48 < ΔV < 3.7 Dual blue/red LEDs,

blue with red phosphor, or white with purple plastic

Ultraviolet λ < 400 3.1 < ΔV < 4.4 Diamond (235 nm)[80]

Boron nitride (215 nm)[81][82] Aluminium nitride (AlN) (210 nm)[16]

Aluminium gallium nitride (AlGaN) Aluminium gallium indium nitride (AlGaInN) — (down to 210 nm)[83]

White Broad spectrum 2.7 < ΔV < 3.5 Blue diode with yellow phosphor or violet/UV diode with multi-color phosphor  

Applications

[edit]
Daytime running light LEDs of an automobile

LED uses fall into five major categories:

  • Visual signals where light goes more or less directly from the source to the human eye, to convey a message or meaning
  • Illumination where light is reflected from objects to give visual response of these objects
  • Measuring and interacting with processes involving no human vision[84]
  • Narrow band light sensors where LEDs operate in a reverse-bias mode and respond to incident light, instead of emitting light[85][86][87][88]
  • Indoor cultivation, including cannabis.[89]

The application of LEDs in horticulture has revolutionized plant cultivation by providing energy-efficient, customizable lighting solutions that optimize plant growth and development.[90] LEDs offer precise control over light spectra, intensity, and photoperiods, enabling growers to tailor lighting conditions to the specific needs of different plant species and growth stages. This technology enhances photosynthesis, improves crop yields, and reduces energy costs compared to traditional lighting systems. Additionally, LEDs generate less heat, allowing closer placement to plants without risking thermal damage, and contribute to sustainable farming practices by lowering carbon footprints and extending growing seasons in controlled environments.[91] Light spectrum affects growth, metabolite profile, and resistance against fungal phytopathogens of Solanum lycopersicum seedlings.[92] LEDs can also be used in micropropagation.[93]

Indicators and signs

[edit]

The low energy consumption, low maintenance and small size of LEDs has led to uses as status indicators and displays on a variety of equipment and installations. Large-area LED displays are used as stadium displays, dynamic decorative displays, and dynamic message signs on freeways. Thin, lightweight message displays are used at airports and railway stations, and as destination displays for trains, buses, trams, and ferries.

Red and green LED traffic signals

One-color light is well suited for traffic lights and signals, exit signs, emergency vehicle lighting, ships' navigation lights, and LED-based Christmas lights

Because of their long life, fast switching times, and visibility in broad daylight due to their high output and focus, LEDs have been used in automotive brake lights and turn signals. The use in brakes improves safety, due to a great reduction in the time needed to light fully, or faster rise time, about 0.1 second faster[citation needed] than an incandescent bulb. This gives drivers behind more time to react. In a dual intensity circuit (rear markers and brakes) if the LEDs are not pulsed at a fast enough frequency, they can create a phantom array, where ghost images of the LED appear if the eyes quickly scan across the array. White LED headlamps are beginning to appear. Using LEDs has styling advantages because LEDs can form much thinner lights than incandescent lamps with parabolic reflectors.

Due to the relative cheapness of low output LEDs, they are also used in many temporary uses such as glowsticks and throwies. Artists have also used LEDs for LED art.

Lighting

[edit]

With the development of high-efficiency and high-power LEDs, it has become possible to use LEDs in lighting and illumination. To encourage the shift to LED lamps and other high-efficiency lighting, in 2008 the US Department of Energy created the L Prize competition. The Philips Lighting North America LED bulb won the first competition on August 3, 2011, after successfully completing 18 months of intensive field, lab, and product testing.[94]

Efficient lighting is needed for sustainable architecture. As of 2011, some LED bulbs provide up to 150 lm/W and even inexpensive low-end models typically exceed 50 lm/W, so that a 6-watt LED could achieve the same results as a standard 40-watt incandescent bulb. The lower heat output of LEDs also reduces demand on air conditioning systems. Worldwide, LEDs are rapidly adopted to displace less effective sources such as incandescent lamps and CFLs and reduce electrical energy consumption and its associated emissions. Solar powered LEDs are used as street lights and in architectural lighting.

The mechanical robustness and long lifetime are used in automotive lighting on cars, motorcycles, and bicycle lights. LED street lights are employed on poles and in parking garages. In 2007, the Italian village of Torraca was the first place to convert its street lighting to LEDs.[95]

Cabin lighting on recent[when?] Airbus and Boeing jetliners uses LED lighting. LEDs are also being used in airport and heliport lighting. LED airport fixtures currently include medium-intensity runway lights, runway centerline lights, taxiway centerline and edge lights, guidance signs, and obstruction lighting.

LEDs are also used as a light source for DLP projectors, and to backlight newer LCD television (referred to as LED TV), computer monitor (including laptop) and handheld device LCDs, succeeding older CCFL-backlit LCDs although being superseded by OLED screens. RGB LEDs raise the color gamut by as much as 45%. Screens for TV and computer displays can be made thinner using LEDs for backlighting.[96]

LEDs are small, durable and need little power, so they are used in handheld devices such as flashlights. LED strobe lights or camera flashes operate at a safe, low voltage, instead of the 250+ volts commonly found in xenon flashlamp-based lighting. This is especially useful in cameras on mobile phones, where space is at a premium and bulky voltage-raising circuitry is undesirable.

LEDs are used for infrared illumination in night vision uses including security cameras. A ring of LEDs around a video camera, aimed forward into a retroreflective background, allows chroma keying in video productions.

LED for miners, to increase visibility inside mines
Los Angeles Vincent Thomas Bridge illuminated with blue LEDs

LEDs are used in mining operations, as cap lamps to provide light for miners. Research has been done to improve LEDs for mining, to reduce glare and to increase illumination, reducing risk of injury to the miners.[97]

LEDs are increasingly finding uses in medical and educational applications, for example as mood enhancement.[98] NASA has even sponsored research for the use of LEDs to promote health for astronauts.[99]

Data communication and other signalling

[edit]

Light can be used to transmit data and analog signals. For example, lighting white LEDs can be used in systems assisting people to navigate in closed spaces while searching necessary rooms or objects.[100]

Assistive listening devices in many theaters and similar spaces use arrays of infrared LEDs to send sound to listeners' receivers. Light-emitting diodes (as well as semiconductor lasers) are used to send data over many types of fiber optic cable, from digital audio over TOSLINK cables to the very high bandwidth fiber links that form the Internet backbone. For some time, computers were commonly equipped with IrDA interfaces, which allowed them to send and receive data to nearby machines via infrared.

Because LEDs can cycle on and off millions of times per second, very high data bandwidth can be achieved.[101] For that reason, visible light communication (VLC) has been proposed as an alternative to the increasingly competitive radio bandwidth.[102] VLC operates in the visible part of the electromagnetic spectrum, so data can be transmitted without occupying the frequencies of radio communications.

Machine vision systems

[edit]

Machine vision systems often require bright and homogeneous illumination, so features of interest are easier to process. LEDs are often used.

Barcode scanners are the most common example of machine vision applications, and many of those scanners use red LEDs instead of lasers. Optical computer mice use LEDs as a light source for the miniature camera within the mouse.

LEDs are useful for machine vision because they provide a compact, reliable source of light. LED lamps can be turned on and off to suit the needs of the vision system, and the shape of the beam produced can be tailored to match the system's requirements.

Biological detection

[edit]

The discovery of radiative recombination in aluminum gallium nitride (AlGaN) alloys by U.S. Army Research Laboratory (ARL) led to the conceptualization of UV light-emitting diodes (LEDs) to be incorporated in light-induced fluorescence sensors used for biological agent detection.[103][104][105] In 2004, the Edgewood Chemical Biological Center (ECBC) initiated the effort to create a biological detector named TAC-BIO. The program capitalized on semiconductor UV optical sources (SUVOS) developed by the Defense Advanced Research Projects Agency (DARPA).[105]

UV-induced fluorescence is one of the most robust techniques used for rapid real-time detection of biological aerosols.[105] The first UV sensors were lasers lacking in-field-use practicality. In order to address this, DARPA incorporated SUVOS technology to create a low-cost, small, lightweight, low-power device. The TAC-BIO detector's response time was one minute from when it sensed a biological agent. It was also demonstrated that the detector could be operated unattended indoors and outdoors for weeks at a time.[105]

Aerosolized biological particles fluoresce and scatter light under a UV light beam. Observed fluorescence is dependent on the applied wavelength and the biochemical fluorophores within the biological agent. UV induced fluorescence offers a rapid, accurate, efficient and logistically practical way for biological agent detection. This is because the use of UV fluorescence is reagentless, or a process that does not require an added chemical to produce a reaction, with no consumables, or produces no chemical byproducts.[105]

Additionally, TAC-BIO can reliably discriminate between threat and non-threat aerosols. It was claimed to be sensitive enough to detect low concentrations, but not so sensitive that it would cause false positives. The particle-counting algorithm used in the device converted raw data into information by counting the photon pulses per unit of time from the fluorescence and scattering detectors, and comparing the value to a set threshold.[106]

The original TAC-BIO was introduced in 2010, while the second-generation TAC-BIO GEN II, was designed in 2015 to be more cost-efficient, as plastic parts were used. Its small, light-weight design allows it to be mounted to vehicles, robots, and unmanned aerial vehicles. The second-generation device could also be utilized as an environmental detector to monitor air quality in hospitals, airplanes, or even in households to detect fungus and mold.[107][108]

Other applications

[edit]
LED costume for stage performers
LED wallpaper by Meystyle
A large LED display behind a disc jockey
Seven-segment display that can display four digits and points
LED panel light source used in an early experiment on potato growth during Shuttle mission STS-73 to investigate the potential for growing food on future long duration missions

The light from LEDs can be modulated very quickly so they are used extensively in optical fiber and free space optics communications. This includes remote controls, such as for television sets, where infrared LEDs are often used. Opto-isolators use an LED combined with a photodiode or phototransistor to provide a signal path with electrical isolation between two circuits. This is especially useful in medical equipment where the signals from a low-voltage sensor circuit (usually battery-powered) in contact with a living organism must be electrically isolated from any possible electrical failure in a recording or monitoring device operating at potentially dangerous voltages. An optoisolator also lets information be transferred between circuits that do not share a common ground potential.

Many sensor systems rely on light as the signal source. LEDs are often ideal as a light source due to the requirements of the sensors. The Nintendo Wii's sensor bar uses infrared LEDs. Pulse oximeters use them for measuring oxygen saturation. Some flatbed scanners use arrays of RGB LEDs rather than the typical cold-cathode fluorescent lamp as the light source. Having independent control of three illuminated colors allows the scanner to calibrate itself for more accurate color balance, and there is no need for warm-up. Further, its sensors only need be monochromatic, since at any one time the page being scanned is only lit by one color of light.

Since LEDs can also be used as photodiodes, they can be used for both photo emission and detection. This could be used, for example, in a touchscreen that registers reflected light from a finger or stylus.[109] Many materials and biological systems are sensitive to, or dependent on, light. Grow lights use LEDs to increase photosynthesis in plants,[110] and bacteria and viruses can be removed from water and other substances using UV LEDs for sterilization.[15] LEDs of certain wavelengths have also been used for light therapy treatment of neonatal jaundice and acne.[111]

UV LEDs, with spectra range of 220 nm to 395 nm, have other applications, such as water/air purification, surface disinfection, glue curing, free-space non-line-of-sight communication, high performance liquid chromatography, UV curing dye printing, phototherapy (295nm Vitamin D, 308nm Excimer lamp or laser replacement), medical/ analytical instrumentation, and DNA absorption.[104][112]

LEDs have also been used as a medium-quality voltage reference in electronic circuits. The forward voltage drop (about 1.7 V for a red LED or 1.2V for an infrared) can be used instead of a Zener diode in low-voltage regulators. Red LEDs have the flattest I/V curve above the knee. Nitride-based LEDs have a fairly steep I/V curve and are useless for this purpose. Although LED forward voltage is far more current-dependent than a Zener diode, Zener diodes with breakdown voltages below 3 V are not widely available.

The progressive miniaturization of low-voltage lighting technology, such as LEDs and OLEDs, suitable to incorporate into low-thickness materials has fostered experimentation in combining light sources and wall covering surfaces for interior walls in the form of LED wallpaper.

Research and development

[edit]

Key challenges

[edit]

LEDs require optimized efficiency to hinge on ongoing improvements such as phosphor materials and quantum dots.[113]

The process of down-conversion (the method by which materials convert more-energetic photons to different, less energetic colors) also needs improvement. For example, the red phosphors that are used today are thermally sensitive and need to be improved in that aspect so that they do not color shift and experience efficiency drop-off with temperature. Red phosphors could also benefit from a narrower spectral width to emit more lumens and becoming more efficient at converting photons.[114]

In addition, work remains to be done in the realms of current efficiency droop, color shift, system reliability, light distribution, dimming, thermal management, and power supply performance.[113]

Early suspicions were that the LED droop was caused by elevated temperatures. Scientists showed that temperature was not the root cause of efficiency droop.[115] The mechanism causing efficiency droop was identified in 2007 as Auger recombination, which was taken with mixed reaction.[66] A 2013 study conclusively identified Auger recombination as the cause.[116]

Potential technology

[edit]

A new family of LEDs are based on the semiconductors called perovskites. In 2018, less than four years after their discovery, the ability of perovskite LEDs (PLEDs) to produce light from electrons already rivaled those of the best performing OLEDs.[117] They have a potential for cost-effectiveness as they can be processed from solution, a low-cost and low-tech method, which might allow perovskite-based devices that have large areas to be made with extremely low cost. Their efficiency is superior by eliminating non-radiative losses, in other words, elimination of recombination pathways that do not produce photons; or by solving outcoupling problem (prevalent for thin-film LEDs) or balancing charge carrier injection to increase the EQE (external quantum efficiency). The most up-to-date PLED devices have broken the performance barrier by shooting the EQE above 20%.[118]

In 2018, Cao et al. and Lin et al. independently published two papers on developing perovskite LEDs with EQE greater than 20%, which made these two papers a mile-stone in PLED development. Their device have similar planar structure, i.e. the active layer (perovskite) is sandwiched between two electrodes. To achieve a high EQE, they not only reduced non-radiative recombination, but also utilized their own, subtly different methods to improve the EQE.[118]

In the work of Cao et al.,[119] researchers targeted the outcoupling problem, which is that the optical physics of thin-film LEDs causes the majority of light generated by the semiconductor to be trapped in the device.[120] To achieve this goal, they demonstrated that solution-processed perovskites can spontaneously form submicrometre-scale crystal platelets, which can efficiently extract light from the device. These perovskites are formed via the introduction of amino acid additives into the perovskite precursor solutions. In addition, their method is able to passivate perovskite surface defects and reduce nonradiative recombination. Therefore, by improving the light outcoupling and reducing nonradiative losses, Cao and his colleagues successfully achieved PLED with EQE up to 20.7%.[119]

Lin and his colleague used a different approach to generate high EQE. Instead of modifying the microstructure of perovskite layer, they chose to adopt a new strategy for managing the compositional distribution in the device—an approach that simultaneously provides high luminescence and balanced charge injection. In other words, they still used flat emissive layer, but tried to optimize the balance of electrons and holes injected into the perovskite, so as to make the most efficient use of the charge carriers. Moreover, in the perovskite layer, the crystals are perfectly enclosed by MABr additive (where MA is CH3NH3). The MABr shell passivates the nonradiative defects that would otherwise be present perovskite crystals, resulting in reduction of the nonradiative recombination. Therefore, by balancing charge injection and decreasing nonradiative losses, Lin and his colleagues developed PLED with EQE up to 20.3%.[121]

Health and safety

[edit]

Certain blue LEDs and cool-white LEDs can exceed safe limits of the so-called blue-light hazard as defined in eye safety specifications such as "ANSI/IESNA RP-27.1–05: Recommended Practice for Photobiological Safety for Lamp and Lamp Systems".[122] One study showed no evidence of a risk in normal use at domestic illuminance,[123] and that caution is only needed for particular occupational situations or for specific populations.[124] In 2006, the International Electrotechnical Commission published IEC 62471 Photobiological safety of lamps and lamp systems, replacing the application of early laser-oriented standards for classification of LED sources.[125]

While LEDs have the advantage over fluorescent lamps, in that they do not contain mercury, they may contain other hazardous metals such as lead and arsenic.[126]

In 2016 the American Medical Association (AMA) issued a statement concerning the possible adverse influence of blueish street lighting on the sleep-wake cycle of city-dwellers. Critics in the industry claim exposure levels are not high enough to have a noticeable effect.[127]

Environmental issues

[edit]
  • Light pollution: Because white LEDs emit more short wavelength light than sources such as high-pressure sodium vapor lamps, the increased blue and green sensitivity of scotopic vision means that white LEDs used in outdoor lighting cause substantially more sky glow.[55]
  • Impact on wildlife: LEDs are much more attractive to insects than sodium-vapor lights, so much so that there has been speculative concern about the possibility of disruption to food webs.[128][129] LED lighting near beaches, particularly intense blue and white colors, can disorient turtle hatchlings and make them wander inland instead.[130] The use of "turtle-safe lighting" LEDs that emit only at narrow portions of the visible spectrum is encouraged by conservancy groups in order to reduce harm.[131]
  • Use in winter conditions: Since they do not give off much heat in comparison to incandescent lights, LED lights used for traffic control can have snow obscuring them, leading to accidents.[132][133]

See also

[edit]

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Further reading

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