View source for Lux

{{short description|SI derived unit of illuminance}}
{{Other uses|Lux (disambiguation)}}
{{Use dmy dates|date=March 2023|cs1-dates=y}}
{{Infobox unit
| bgcolor    =
| name       = lux
| image      = Lux meter.jpg
| caption    = A lux meter for measuring illuminance 
| standard   = [[SI]]
| quantity   = [[illuminance]]
| symbol     = lx
| units1     = [[SI base unit]]s
| inunits1   = [[candela|cd]]⋅[[steradian|sr]]⋅[[metre|m]]<sup>−2</sup>
| units2     = [[United States customary units|US customary units]]
| inunits2   = 0.0929 [[Foot-candle|fc]]
| units3     = [[Centimetre–gram–second system of units|CGS]] units
| inunits3   = {{val|e=-4|u=[[phot|ph]]}}
}}
The '''lux''' (symbol: '''lx''') is the unit of [[illuminance]], or [[luminous flux]] per unit area, in the [[International System of Units]] (SI).<ref name="SIBrochure9thEd">{{citation |title=The International System of Units (SI) |author=International Bureau of Weights and Measures |author-link=New SI |date=20 May 2019  |edition=9th |isbn=978-92-822-2272-0 |url=https://www.bipm.org/utils/common/pdf/si-brochure/SI-Brochure-9.pdf| archive-url = https://web.archive.org/web/20211018184555/https://www.bipm.org/documents/20126/41483022/SI-Brochure-9.pdf/fcf090b2-04e6-88cc-1149-c3e029ad8232 |archive-date=18 October 2021 |url-status=live}}</ref><ref>{{cite book |last1=CIE |title=CIE S 017:2020 ILV: International Lighting Vocabulary, 2nd edition. |date=2020 |publisher=CIE |edition=2 |url=https://cie.co.at/eilvterm/17-21-085}}</ref> It is equal to one [[lumen (unit)|lumen]] per square metre. In [[photometry (optics)|photometry]], this is used as a measure of the [[irradiance]], as perceived by the spectrally unequally responding human eye, of [[light]] that hits or passes through a surface. It is analogous to the [[radiometry|radiometric]] unit [[Irradiance|watt per square metre]], but with the power at each [[wavelength]] weighted according to the [[luminosity function]], a model of [[human]] visual brightness perception, standardized by the CIE and [[International Organization for Standardization|ISO]].<ref>{{cite book | url=https://www.iso.org/standard/83178.html | title=ISO/CIE 23539:2023 CIE TC 2-93 Photometry — The CIE system of physical photometry | publisher=ISO/CIE | date=2023 | language=en | doi=10.25039/IS0.CIE.23539.2023}}</ref> In English, "lux" is used as both the singular and plural form.<ref>[https://www.nist.gov/pml/special-publication-811/nist-guide-si-chapter-9-rules-and-style-conventions-spelling-unit-names NIST Guide to SI Units. Chapter 9 – Rules and Style Conventions for Spelling Unit Names], National Institute of Standards and Technology.</ref>
The word is derived from the [[Latin]] word for "light", [[wikt:lux|lux]].

==Explanation==
===Illuminance===
Illuminance is a measure of how much [[luminous flux]] is spread over a given area.  One can think of luminous flux (with the unit [[lumen (unit)|lumen]]) as a measure of the total "amount" of visible light present, and the illuminance as a measure of the intensity of illumination on a surface. A given amount of light will illuminate a surface more dimly if it is spread over a larger area, so illuminance is inversely proportional to area when the luminous flux is held constant. 

One lux is equal to one lumen per [[square metre]]:
:1&nbsp;lx&nbsp;= 1&nbsp;lm/m<sup>2</sup> = 1&nbsp;[[Candela|cd]]·[[Steradian|sr]]/m<sup>2</sup>.

A flux of 1000&nbsp;lumens, spread uniformly over an area of 1 square metre, lights up that square metre with an illuminance of 1000&nbsp;lux. However, the same 1000&nbsp;lumens spread out over 10 square metres produces a dimmer illuminance of only 100&nbsp;lux.

Achieving an illuminance of 500&nbsp;lx might be possible in a home kitchen with a single [[fluorescent light]] fixture with an output of {{val|12000|u=lumens}}. To light a factory floor with dozens of times the area of the kitchen would require dozens of such fixtures. Thus, lighting a larger area to the same illuminance (lux) requires a greater luminous flux (lumen).

As with other named SI units, [[SI prefixes]] can be used. For example, 1&nbsp;kilolux (klx) is 1000&nbsp;lx.

Here are some examples of the illuminance provided under various conditions:

{| class="wikitable"
! Illuminance (lux) !! style="text-align:left;"|Surfaces illuminated by
|-
|0.0001||Moonless, overcast night sky ([[starlight]])<ref name="radfaq">{{cite web |title=Radiometry and photometry in astronomy |url=http://stjarnhimlen.se/comp/radfaq.html#10 |author-first=Paul |author-last=Schlyter |date=1997–2009}}<br />Starlight illuminance coincides with the human eye's minimum illuminance while moonlight coincides with the human eye's minimum colour vision illuminance (IEE Reviews, 1972, [https://books.google.com/books?id=00dJAQAAIAAJ&q=minimum+illumination+for+human+colour+vision+lx page 1183]).</ref>
|-
|0.002||Moonless clear night sky with [[airglow]]<ref name="radfaq" />
|-
|0.05–0.3||Full moon on a clear night<ref>{{cite journal |author-last1=Kyba |author-first1=Christopher C. M. |author-last2=Mohar |author-first2=Andrej |author-last3=Posch |author-first3=Thomas |title=How bright is moonlight? |journal=Astronomy & Geophysics |date=1 February 2017 |volume=58 |issue=1 |pages=1.31–1.32 |doi=10.1093/astrogeo/atx025 |url=https://gfzpublic.gfz-potsdam.de/pubman/item/item_2022891_4/component/file_2029888/2022891.pdf }}</ref>
|-
|3.4|| Dark limit of [[Twilight#Civil_twilight|civil twilight]] under a clear sky<ref>{{cite web |url=http://www.photonis.com/attachment.php?id_attachment=95 |title=Electro-Optics Handbook |format=pdf |work=photonis.com |page=63 |access-date=2012-04-02}}{{dead link|date=September 2018|fix-attempted=yes}}</ref>
|-
|20–50||Public areas with dark surroundings<ref name="NOAO_CaRLLI">{{cite web |url=https://www.noao.edu/education/QLTkit/ACTIVITY_Documents/Safety/LightLevels_outdoor+indoor.pdf |title=NOAO Common and Recommended Light Levels Indoor |access-date=13 November 2016 |archive-date=6 July 2021 |archive-url=https://web.archive.org/web/20210706034730/https://www.noao.edu/education/QLTkit/ACTIVITY_Documents/Safety/LightLevels_outdoor+indoor.pdf |url-status=dead}}</ref>
|-
|50||Family living room lights (Australia, 1998)<ref name="energyrating">{{Cite book |author-first=Alan |author-last=Pears |publisher=Department of Industry and Science, Commonwealth of Australia |title=Strategic Study of Household Energy and Greenhouse Issues: A report for Environment Australia |date=June 1998 |url=http://www.energyrating.com.au/library/pubs/pears-ago1998.pdf |chapter=Chapter 7: Appliance technologies and scope for emission reduction |page=61 |access-date=2008-06-26 |archive-url=https://web.archive.org/web/20110302110649/http://www.energyrating.gov.au/library/pubs/pears-ago1998.pdf |archive-date=2 March 2011 |url-status= usurped}}</ref>
|-
|80||Office building hallway/[[Toilet (room)|toilet]] lighting<ref>{{Cite book |author=Australian Greenhouse Office |title=Working Energy Resource and Training Kit: Lighting |date=May 2005 |url=http://www.greenhouse.gov.au/lgmodules/wep/lights/index.html |chapter=Chapter 5: Assessing lighting savings |access-date=2007-03-17 |archive-url=https://web.archive.org/web/20070415151053/http://www.greenhouse.gov.au/lgmodules/wep/lights/training/training9.html |archive-date=2007-04-15 |url-status=dead}}</ref><ref>{{cite web |url=http://www.scopecalc.com/ |title=Low-Light Performance Calculator |access-date=27 September 2010 |archive-url=https://web.archive.org/web/20130615074318/http://scopecalc.com/ |archive-date=15 June 2013 |url-status=dead}}</ref>
|-
|100||Very dark overcast day<ref name="radfaq" />
|-
|150||Train station platforms<ref>{{cite web |author-last=Darlington |author-first=Paul |title=London Underground: Keeping the lights on |url=https://www.railengineer.uk/2017/12/05/london-underground-keeping-the-lights-on/ |website=Rail Engineer |access-date=20 December 2017 |date=5 December 2017 |archive-date=16 November 2018 |archive-url=https://web.archive.org/web/20181116020106/https://www.railengineer.uk/2017/12/05/london-underground-keeping-the-lights-on/ |url-status=dead}}</ref>
|-
|320–500|| Office lighting<ref name=energyrating/><ref>{{cite web |url=http://www.resourcesmart.vic.gov.au/documents/lux_meter.pdf |title=How to use a lux meter (Australian recommendation) |publisher=Sustainability Victoria |date=April 2010 |archive-url=https://web.archive.org/web/20110707054658/http://www.resourcesmart.vic.gov.au/documents/lux_meter.pdf |archive-date=7 July 2011}}</ref><ref>{{cite web |url=https://www.osha.gov/pls/oshaweb/owadisp.show_document?p_table=STANDARDS&p_id=10630 |title=Illumination. - 1926.56 |work=Regulations (Standards - 29 CFR) |publisher=Occupational Safety and Health Administration, US Dept. of Labor |archive-url =https://web.archive.org/web/20090508051301/https://www.osha.gov/pls/oshaweb/owadisp.show_document?p_table=STANDARDS&p_id=10630 |archive-date=8 May 2009}}</ref><ref>European law UNI EN 12464</ref>
|-
|400||[[Sunrise]] or [[sunset]] on a clear day.
|-
|1000||Overcast day;<ref name="radfaq" /> typical [[TV studio]] lighting
|-
|10,000–25,000||Full [[daylight]] (not direct sun)<ref name="radfaq" />
|-
|32,000–100,000||Direct [[sunlight]]
|}

The illuminance provided by a light source on a surface perpendicular to the direction to the source is a measure of the strength of that source as perceived from that location. For instance, a star of [[apparent magnitude]] 0 provides 2.08 microlux (μlx) at the Earth's surface.<ref name="Schlyter7">[http://stjarnhimlen.se/comp/radfaq.html#7 Schlyter, Section 7].</ref> A barely perceptible magnitude 6 star provides 8 nanolux (nlx).<ref>[http://stjarnhimlen.se/comp/radfaq.html#14 Schlyter, Section 14].</ref> The unobscured Sun provides an illumination of up to 100&nbsp;kilolux (klx) on the Earth's surface, the exact value depending on time of year and atmospheric conditions. This direct normal illuminance is related to the [[solar illuminance constant]] ''E''<sub>sc</sub>, equal to {{val|128000|u=lux}} (see [[Sunlight]] and [[Solar constant]]).

The illuminance on a surface depends on how the surface is tilted with respect to the source. For example, a pocket flashlight aimed at a wall will produce a given level of illumination if aimed perpendicular to the wall, but if the flashlight is aimed at increasing angles to the perpendicular (maintaining the same distance), the illuminated spot becomes larger and so is less highly illuminated. When a surface is tilted at an angle to a source, the illumination provided on the surface is reduced because the tilted surface subtends a smaller solid angle from the source, and therefore it receives less light. For a point source, the illumination on the tilted surface is reduced by a factor equal to the cosine of the angle between a ray coming from the source and the [[Surface normal|normal]] to the surface.<ref>Jack L. Lindsey, ''Applied Illumination Engineering'', The Fairmont Press, Inc., 1997 {{ISBN|0881732125}} page 218</ref> In practical lighting problems, given information on the way light is emitted from each source and the distance and geometry of the lighted area, a numerical calculation can be made of the illumination on a surface by adding the contributions of every point on every light source.

===Relationship between illuminance and irradiance===
Like all [[photometry (optics)|photometric units]], the lux has a corresponding "[[radiometry|radiometric]]" unit. The difference between any photometric unit and its corresponding radiometric unit is that radiometric units are based on physical power, with all wavelengths being weighted equally, while photometric units take into account the fact that the human eye's image-forming visual system is more sensitive to some wavelengths than others, and accordingly every wavelength is given a different weight. The weighting factor is known as the [[luminosity function]].

The lux is one lumen per square metre (lm/m<sup>2</sup>), and the corresponding radiometric unit, which measures [[irradiance]], is the watt per square metre (W/m<sup>2</sup>). There is no single conversion factor between lux and W/m<sup>2</sup>; there is a different conversion factor for every wavelength, and it is not possible to make a conversion unless one knows the spectral composition of the light.

The peak of the luminosity function is at 555&nbsp;[[nanometre|nm]] (green); the eye's image-forming visual system is more sensitive to light of this wavelength than any other. For monochromatic light ''of this wavelength'', the amount of illuminance for a given amount of irradiance is maximum: 683.002&nbsp;lx per 1&nbsp;W/m<sup>2</sup>; the irradiance needed to make 1&nbsp;lx at this wavelength is about 1.464&nbsp;[[milliwatt|mW]]/m<sup>2</sup>. Other wavelengths of visible light produce fewer lux per watt-per-meter-squared. The luminosity function falls to zero for wavelengths outside the [[visible spectrum]].

For a light source with mixed wavelengths, the number of lumens per watt can be calculated by means of the luminosity function. In order to appear reasonably "white", a light source cannot consist solely of the green light to which the eye's image-forming visual photoreceptors are most sensitive, but must include a generous mixture of red and blue wavelengths, to which they are much less sensitive.

This means that white (or whitish) light sources produce far fewer lumens per watt than the theoretical maximum of 683.002&nbsp;lm/W. The ratio between the actual number of lumens per watt and the theoretical maximum is expressed as a percentage known as the [[luminous efficiency]]. For example, a typical [[incandescent light bulb]] has a luminous efficiency of only about 2%.

In reality, individual eyes vary slightly in their luminosity functions. However, photometric units are precisely defined and precisely measurable. They are based on an agreed-upon standard luminosity function based on measurements of the spectral characteristics of image-forming ''visual photoreception'' in many individual human eyes.

==Use in video-camera specifications==
Specifications for [[video camera]]s such as [[camcorder]]s and [[surveillance camera]]s often include a minimal illuminance level in lux at which the camera will record a satisfactory image.{{citation needed|date=February 2013}} A camera with good low-light capability will have a lower lux rating. [[Still camera]]s do not use such a specification, since longer [[exposure time]]s can generally be used to make pictures at very low illuminance levels, as opposed to the case in video cameras, where a maximal exposure time is generally set by the [[frame rate]].

{{Anchor|Lux versus footcandle}}
==Non-SI units of illuminance==
The corresponding unit in English and American traditional units is the [[foot-candle]]. One foot candle is about 10.764&nbsp;lx. Since one foot-candle is the illuminance cast on a surface by a one-candela source one foot away, a lux could be thought of as a "metre-candle", although this term is discouraged because it does not conform to SI standards for unit names.

One [[phot]]&nbsp;(ph) equals 10 kilolux (10 klx).

{{anchor|Nox}}One nox&nbsp;(nx) equals 1 millilux (1 mlx) at light color 2042&nbsp;K or 2046&nbsp;K (formerly 2360&nbsp;K).<ref name="Lohse-Stille_1948"/><ref name="Westphal_1952"/><ref name="Grimsehl_1988"/><ref name="Keplinger_2021"/>

In [[astronomy]], [[apparent magnitude]] is a measure of the illuminance of a star on the Earth's atmosphere. A star with apparent magnitude 0 is 2.54 microlux outside the earth's atmosphere, and 82% of that (2.08 microlux) under clear skies.<ref name="Schlyter7"/> A magnitude 6 star (just barely visible under good conditions) would be 8.3 nanolux. A standard candle (one candela) a kilometre away would provide an illuminance of 1 microlux—about the same as a magnitude 1 star.

==Legacy Unicode symbol==
[[Unicode]] includes a symbol for "lx": {{unichar|33D3|SQUARE LX}}. It is a legacy code to accommodate old [[code page]]s in some [[Asia]]n languages. Use of this code is not recommended in new documents.

==SI photometry units==
{{SI_light_units}}

==See also==
* [[Exposure value]]

== References ==
{{Reflist|refs=
<ref name="Lohse-Stille_1948">{{cite journal |title=Einführung und Bestimmung des Lichtäquivalents |language=de |trans-title= |author-first1=Bernhard |author-last1=Lohse<!-- *Schöningen 1911-05-22 --> |author-first2=Ulrich |author-link2=:de:Ulrich Stille |author-last2=Stille |editor=Deutsche Physikalische Gesellschaft |editor-link=Deutsche Physikalische Gesellschaft |journal=[[Zeitschrift für Physik]] |issn=0044-3328<!-- 0939-7922 --> |publisher=[[Springer-Verlag]] |publication-place=Berlin / Göttingen / Heidelberg, Germany |location=Braunschweig, Germany |volume=125 |issue=1–3 |date=January 1948 |orig-date=1947-08-19 |doi=10.1007/BF01337623 |pages=133–158 |bibcode=1948ZPhy..125..133L |s2cid=125512557 |url=https://books.google.com/books?id=fxJXDwKAjX8C |access-date=2023-03-19 |url-status= |archive-url= |archive-date=}}</ref>
<ref name="Westphal_1952">{{cite book |title=Physikalisches Wörterbuch |language=de |trans-title= |chapter=Nox, Dunkelleuchtdichte, Skot |author-first=Wilhelm Heinrich |author-last=Westphal |editor-first1=Wilhelm H. |editor-last1=Westphal |author-link=Wilhelm Heinrich Westphal |date=1952 |edition=1 |publisher=[[Springer-Verlag OHG]] |publication-place=Berlin / Göttingen / Heidelberg, Germany |isbn=978-3-662-12707-0 |doi=10.1007/978-3-662-12706-3 |pages=125, 271, 389 |chapter-url=https://books.google.com/books?id=QaCFBwAAQBAJ&pg=RA1-PA125 |access-date=2023-03-16 <!-- |url-status=live --> |quote-pages=125, 271 |quote=Nox, abgek[ürzt] nx, Einheit der Dunkelbeleuchtungsstärke (<!-- > -->Dunkelleuchtdichte), welche für zahlenmäßige Angaben und zum Anschluß der Dunkelbeleuchtungsstärke an die normale <!-- > -->Beleuchtungsstärke 1940 von der {{ill|German Lighting Society|de|Deutsche Lichttechnische Gesellschaft|lt=Deutschen Lichttechnischen Gesellschaft}} geschaffen wurde. Bezüglich der Farbtemperatur der Strahlung und des Anschlusses von Zahlenwerten der Beleuchtungsstärke ''E'' und der Dunkelbeleuchtungsstärke ''{{overline|E}}'' gelten analog die gleichen Festlegungen wie bei der <!-- > -->Dunkelleuchtdichte und dem <!-- > -->Skot (sk). Für eine Strahlung der Farbtemperatur ''T''<sub>1</sub>&nbsp;= 2360&nbsp;K gilt: 1&nbsp;nx&nbsp;= 10<sup>−3</sup>&nbsp;lx (<!-- > -->Lux). Für eine beliebige Strahlung ''bekannter'' spektraler <!-- > -->Strahlungsleistung ''S''<sub>1</sub> lautet die Verknüpfungsbeziehung zwischen in 10<sup>−3</sup>&nbsp;lx gemessenem Zahlenwert {''E''} der Beleuchtungsstärke und in nx gemessenem Zahlenwert {''{{overline|E}}''} der Dunkelbeleuchtungsstärke: {''{{overline|E}}''}<sub>nx</sub>&nbsp;= (2,161&nbsp;± 0,001)&nbsp;· {''E''}<sub>10<sup>−3</sup>&nbsp;lx</sub>&nbsp;· ∫&nbsp;''S''<sub>λ</sub> ''{{overline|V}}''<sub>λ,W</sub> ''dλ''&nbsp;/ ∫&nbsp;''S''<sub>λ</sub> ''V''<sub>λ</sub> ''dλ'', wobei ''V''<sub>λ</sub> die relative spektrale <!-- > -->Hellempfindlichkeit und ''{{overline|V}}''<sub>λ,W</sub> die relative spektrale <!-- > -->Dämmerungsempfindlichkeit des menschlichen Auges nach Weaver{{citeref|Lohse|Stille|1948|A}} bedeuten. [...] Dunkelleuchtdichte. [...] Ist das Auge dunkeladaptiert, d.h. einer <!-- > -->Leuchtdichte von weniger als 0,01&nbsp;[[apostilb|asb]] ausgesetzt, so gilt infolge des <!-- > -->[[Purkinje effect|Purkinje-Phänomen]]s eine von der <!-- > -->spektralen Hellempfindlichkeitskurve abweichende, nach dem kurzwelligen Ende des Spektrums hin verschobene Empfindlichkeitskurve des Auges, die Stäbchenkurve des [[Scotopic vision|Dämmerungssehen]]s. Unter Zugrundelegung dieser Empfindlichkeitskurve hat man 1940 in Deutschland die Dunkelleuchtdichte mit der Einheit <!-- >-->[[Skot (unit)|Skot]] (sk) so festgesetzt, daß bei einem Licht der Farbtemperatur 2360&nbsp;°[[Kelvin (unit)|K]] 1&nbsp;sk&nbsp;= 10<sup>−3</sup>&nbsp;asb gilt. 1948 ist von der [[Internationale Beleuchtungskommission|Internationalen Beleuchtungskommission]] (IBK) die Bezugstemperatur auf 2046&nbsp;K, die Erstarrungstemperatur des [[Platinum|Platin]]s, festgesetzt worden. Die Bezeichnung Skot wurde von der IBK nicht übernommen, dafür soll "[[skotopisches Stilb]]" gesagt werden. Als höchstzulässiger Grenzwert für die Dunkelleuchtdichte ist in Deutschland 10&nbsp;Skot festgesetzt worden, um eine Verwendung der Dunkelleuchtdichte im Gebiet des gemischten [[Photopic vision|Zapfen]]- und [[Scotopic vision|Stäbchensehen]]s zu vermeiden, da in diesem Bereich die photometrischen Maßgrößen wegen der allmählich gleitenden Augenempfindlichkeitskurve ihren Sinn verlieren. |trans-quote=}}</ref>
<ref name="Grimsehl_1988">{{cite book |title=Lehrbuch der Physik: Optik |volume=3 |language=de |trans-title= |chapter=1. Licht: 1.4. Photometrie: 1.4.1. Grundbegriffe |author-first1=Ernst |author-last1=Grimsehl |author-link1=:de:Ernst Grimsehl |author-first2=Walter |author-last2=Schallreuter |author-link2=:de:Walter Schallreuter |editor-first=Heinz |editor-last=Haferkorn |date=1988 |orig-date=1976 |edition=19 |publisher=[[BSB BG Teubner Verlagsgesellschaft]] |publication-place=Leipzig, Germany |isbn=978-3-322-96432-8 |doi=10.1007/978-3-322-96431-1 |id=Order No. 6666211, VLN 294-375/84/88, LSV 1164 |pages=33–38 [37–38] |chapter-url=https://books.google.com/books?id=pkbQBgAAQBAJ&pg=PA37 |access-date=2023-03-16 <!-- |url-status=live --> |quote-pages=37–38 |quote=Dunkelsehen [...] Für das Dunkelsehen, bei dem nur die [[Rod cell|Stäbchen]] angeregt werden, definiert man die Dunkelleuchtdichte mit der Einheit ''[[Skot (unit)|Skot]]'' (sk) und die Dunkelbeleuchtungsstärke mit der Einheit ''Nox'' (nx). Die Umrechnungsfaktoren zwischen den Hell- und Dunkelgrößen hängen von der spektralen Zusammensetzung des Lichtes ab. Sie werden deshalb für die Farbtemperatur 2042&nbsp;K (früher 2360&nbsp;K) festgelegt. Bei dieser ist 1&nbsp;sk&nbsp;= 10<sup>−3</sup>&nbsp;asb und 1&nbsp;nx&nbsp;= 10<sup>−3</sup>&nbsp;lx. |trans-quote=}}</ref>
<ref name="Keplinger_2021">{{cite web |title=1939 bis 1945 – Im Keller glüht das Lumogen |language=de-AT |trans-title= |author-first=Thomas |author-last=Keplinger |website=Worte im Dunkel |location=Vienna, Austria |date=2021-03-29 |url=https://www.worteimdunkel.at/?p=14007 |access-date=2023-03-16 |url-status=live |archive-url=https://web.archive.org/web/20230316194909/https://www.worteimdunkel.at/?p=14007 |archive-date=2023-03-16 |quote=Skot und Nox [...] Interessant ist in diesem Zusammenhang die Einführung neuer Messeinheiten. Die Voraussetzungen der Forschung beziehungsweise die Erfordernisse an die Leuchtfarben unterschieden sich so stark von allen bis dahin erforschten Gebieten, dass die {{ill|German Lighting Society|de|Deutsche Lichttechnische Gesellschaft|lt=Deutsche Lichttechnische Gesellschaft}} 1940 eigene Einheiten ins Leben rief: Die Dunkelleuchtdichte wurde in [[Skot (unit)|Skot]] und die Dunkelbeleuchtungsstärke in Nox gemessen.{{citeref|Westphal|1952|B}} Diese Einheiten grenzten an die bereits bestehenden Größen der [[luminance|Leuchtdichte]] und [[illuminance|Beleuchtungsstärke]] an und dienten der zahlenmäßigen Erfassung geringster Lichtwerte. So entsprach etwa ein Nox 10<sup>−3</sup>&nbsp;Lux. |trans-quote=}}</ref>
}}

==External links==
* [https://web.archive.org/web/20130313095139/http://fp.optics.arizona.edu/Palmer/rpfaq/rpfaq.htm Radiometry and photometry FAQ] Professor Jim Palmer's Radiometry FAQ page ([[University of Arizona]]).

{{SI units}}
{{Portal bar|Physics|Energy}}

[[Category:SI derived units]]
[[Category:Units of illuminance]]