Examine individual changes

'{{Redirect|Radiant heat|the heating method|Radiant heating}} [[Image:Wiens law.svg|300px|thumb|right|This diagram shows how the peak wavelength and total radiated amount vary with temperature. Although this plot shows relatively high temperatures, the same relationships hold true for any temperature down to absolute zero. Visible light is between 380 and 750 nm.]] [[Image:Hot metalwork.jpg|250px|thumb|right|Thermal radiation in visible light can be seen on this hot metalwork. Its emission in the [[infrared]] is invisible to the human eye and the camera the imagePENIS was taken with, but an [[infrared camera]] could show it (See [[Thermography]]).]] '''Thermal radiation''' is [[electromagnetic radiation]] emitted from all matter due to its possessing [[thermal energy]] which is measured by the [[temperature]] of the matter. Examples of thermal radiation are an [[incandescent light bulb]] emitting [[visible-light]], [[infrared]] radiation emitted by a common household [[radiator]] or [[electric heater]], as well as radiation from hot gas in outer space. A person near a raging bonfire feels the radiated energy of the fire, even if the surrounding air is very cold. Thermal radiation is generated when thermal energy is converted to electromagnetic radiation by the movement of the [[Charge (physics)|charge]]s of [[electrons]] and protons in the material. [[Sunlight]] is solar electromagnetic radiation generated by the hot plasma of the [[Sun]], and this thermal radiation heats the [[Earth]] by the reverse process of absorption, generating kinetic, thermal energy in electrons and atomic nuclei. The Earth also emits thermal radiation, but at a much lower intensity and different spectral distribution because it is cooler. The balance between heating by incoming solar radiation and cooling by the Earth's outgoing radiation is the primary process that determines Earth's overall temperature. If a radiation-emitting object meets the physical characteristics of a [[black body]] in [[thermodynamic equilibrium]], the radiation is called blackbody radiation<ref>K. Huang, ''Statistical Mechanics'' (2003), p.278</ref>. The emitted [[frequency]] spectrum of the blackbody radiation is described by a probability distribution depending only on temperature given by [[Planck's law of black-body radiation]]. [[Wien's displacement law]] gives the most likely frequency of the emitted radiation, and the [[Stefan–Boltzmann law]] determines the radiant intensity.<ref>K. Huang, ''Statistical Mechanics'' (2003), p280</ref> In engineering, thermal radiation is considered one of the fundamental methods of [[heat transfer]], although it does not involve the transport of [[heat]]{{Citation needed|date=December 2010}}, which is in [[thermodynamics]] the current of [[thermal energy]] across a system boundary. Rather, in physics, the absorption of thermal radiation is work {{Citation needed|date=December 2010}} performed on a system located in an external electromagnetic field. ==Overview== Thermal radiation is the emission of [[electromagnetic waves]] from all matter that has a [[temperature]] greater than [[absolute zero]].<ref name=blundell>{{cite book |author=S. Blundell, K. Blundell |title=Concepts in Modern Physics |isbn=978–0–19–856769–1 |year=2006 |publisher=Oxford University Press |page=247 }}</ref> It represents a conversion of [[thermal energy]] into [[electromagnetic energy]]. Thermal energy is the collective mean kinetic energy of the random movements of atoms and molecules in matter. Atoms and molecules are composed of charged particles, i.e. [[proton]]s and [[electron]]s and their oscillations result in the electrodynamic generation of coupled electric and magnetic fields, resulting in the emission of [[photon]]s, radiating energy and carrying entropy away from the body through its surface boundary. Electromagnetic radiation, or light, does not require the presence of matter to propagate and travels in the [[vacuum]] of space infinitely far if unobstructed. The characteristics of thermal radiation depends on various properties of the surface it is emanating from, including its temperature, its spectral [[absorptance|absorptivity]] and spectral emissive power, as expressed by [[Kirchhoff's law of thermal radiation|Kirchhoff's law]].<ref name=blundell/> The radiation is not monochromatic, i.e. it does not consist of just a single frequency, but comprises a continuous dispersion of photon energies, its characteristic spectrum. If the radiating body and its surface are in [[thermodynamic equilibrium]] and the surface has perfect absorptivity at all wavelength, it is characterized as a [[black body]]. A black body is also a perfect emitter. The radiation of such perfect emitters is called [[black-body radiation]]. The ratio of any body's emission relative to that of a black body is the body's [[emissivity]], so that a black body has an emissivity of unity. Absorptivity, [[reflectivity]], and emissivity of all bodies are dependent on the wavelength of the radiation. The temperature determines the wavelength distribution of the electromagnetic radiation. For example, fresh snow, which is highly reflective to visible light (reflectivity about 0.90), appears white due to reflecting sunlight with a peak wavelength of about 0.5 micrometres. Its emissivity, however, at a temperature of about -5°C, peak wavelength of about 12 micrometres, is 0.99. The distribution of power that a black body emits with varying frequency is described by [[Planck's law]]. At any given temperature, there is a frequency ''f_max'' at which the power emitted is a maximum. Wien's displacement law, and the fact that the frequency of light is inversely proportional to its wavelength in vacuum, mean that the peak frequency ''f_max'' is proportional to the absolute temperature ''T'' of the black body. The photosphere of the Sun, at a temperature of approximately 6000 K, emits radiation principally in the visible portion of the electromagnetic spectrum. Earth's atmosphere is partly transparent to visible light, and the light reaching the surface is absorbed or reflected. Earth's surface emits the absorbed radiation, approximating the behavior of a black body at 300 K with spectral peak at ''f_max''. At these lower frequencies, the atmosphere is largely opaque and radiation from Earth's surface is absorbed or scattered by the atmosphere. Though some radiation escapes into space, it is absorbed and subsequently re-emitted by atmospheric gases. It is this spectral selectivity of the atmosphere that is responsible for the planetary [[greenhouse effect]], contributing to [[global warming]] and [[climate change]] in general. The common household [[incandescent light bulb]] has a spectrum overlapping the black body spectra of the sun and the earth. A portion of the photons emitted by a tungsten light bulb filament at [[color temperature|3000K]] are in the visible spectrum. However, most of the energy is associated with photons of longer wavelengths; these do not help a person see, but still transfer heat to the environment, as can be deduced empirically by observing a household incandescent light bulb. Whenever EM radiation is emitted and then absorbed, heat is transferred. This principle is used in [[microwave oven]]s, [[laser cutting]], and [[Electrolysis (cosmetology)|RF hair removal]]. Unlike conductive and convective forms of heat transfer, thermal radiation can be concentrated in a tiny spot by using reflecting mirrors. [[Concentrating solar power]] takes advantage of this fact. In many such systems, mirrors are employed to concentrate sunlight into a smaller area. In lieu of mirrors, [[Fresnel lens#Generating_solar_power|Fresnel lenses]] can also be used to concentrate heat flux. Either method can be used to quickly vaporize water into steam using sunlight. For example, the sunlight reflected from mirrors heats the [[PS10 solar power tower]], and during the day it can heat water to 285°C (558.15K) or 545°F. ====Surface effects==== Lighter colors and also whites and metallic substances absorb less illuminating light, and thus heat up less; but otherwise color makes small difference as regards heat transfer between an object at everyday temperatures and its surroundings, since the dominant emitted wavelengths are nowhere near the visible spectrum, but rather in the far infrared. Emissivities at those wavelengths have little to do with visual emissivities (visible colors); in the far infrared, most objects have high emissivities. Thus, except in sunlight, the color of clothing makes little difference as regards warmth; likewise, paint color of houses makes little difference to warmth except when the painted part is sunlit. The main exception to this is shiny metal surfaces, which have low emissivities both in the visible wavelengths and in the far infrared. Such surfaces can be used to reduce heat transfer in both directions; an example of this is the [[multi-layer insulation]] used to insulate spacecraft. [[Low-emissivity]] windows in houses are a more complicated technology, since they must have low emissivity at thermal wavelengths while remaining transparent to visible light. ==Properties== There are four main properties that characterize thermal radiation: *Thermal radiation emitted by a body at any temperature consists of a wide range of frequencies. The frequency distribution is given by [[Planck's law of black-body radiation]] for an idealized emitter. This is shown in the right-hand diagram. *The dominant frequency (or color) range of the emitted radiation shifts to higher frequencies as the temperature of the emitter increases. For example, a ''red hot'' object radiates mainly in the long wavelengths (red and orange) of the visible band. If it is heated further, it also begins to emit discernible amounts of green and blue light, and the spread of frequencies in the entire visible range cause it to appear white to the human eye; it is ''white hot''. However, even at a white-hot temperature of 2000 K, 99% of the energy of the radiation is still in the infrared. This is determined by [[Wien's displacement law]]. In the diagram the peak value for each curve moves to the left as the temperature increases. *The total amount of radiation of all frequencies increases steeply as the temperature rises; it grows as ''T''⁴, where ''T'' is the absolute temperature of the body. An object at the temperature of a kitchen oven, about twice the room temperature on the absolute temperature scale (600 K vs. 300 K) radiates 16 times as much power per unit area. An object at the temperature of the filament in an [[incandescent light bulb]]--roughly 3000 K, or 10 times room temperature—radiates 10,000 times as much energy per unit area. The total radiative intensity of a black body rises as the fourth power of the absolute temperature, as expressed by the [[Stefan–Boltzmann law]]. In the plot, the area under each curve grows rapidly as the temperature increases. *The rate of electromagnetic radiation emitted at a given frequency is proportional to the amount of absorption that it would experience by the source. Thus, a surface that absorbs more red light thermally radiates more red light. This principle applies to all properties of the wave, including [[wavelength]] (color), direction, [[Polarization (waves)|polarization]], and even [[Coherence (physics)|coherence]], so that it is quite possible to have thermal radiation which is polarized, coherent, and directional, though polarized and coherent forms are fairly rare in nature. These properties apply if the distances considered are much larger than the wavelengths contributing to the spectrum (most significant from 8-25 micrometres at 300 K). Indeed, thermal radiation here takes only traveling waves into account. A more sophisticated framework involving electromagnetic theory has to be used for lower distances (near-field thermal radiation). {| class="wikitable" border="1" |- ! °C ! Subjective colour [http://cc.oulu.fi/~kempmp/colours.html] |- | 480 | faint red glow |- | 580 | dark red |- | 730 | bright red, slightly orange |- | 930 | bright orange |- | 1100 | pale yellowish orange |- | 1300 | yellowish white |- | > 1400 | white (yellowish if seen from a distance through atmosphere) |} ==Interchange of energy== [[Image:Radiant heat panel nrc ottawa.jpg|thumb|right|300px|Radiant heat panel for testing precisely quantified energy exposures at [[National Research Council (Canada)|National Research Council]], near [[Ottawa]], [[Ontario]], [[Canada]].]] Thermal radiation is an important concept in [[thermodynamics]] as it is partially responsible for [[heat transfer]] between objects, as warmer [[Physical body|bodies]] radiate more heat than colder ones. Other factors are [[convection]] and [[Heat conduction|conduction]]. The interplay of energy exchange is characterized by the following equation: :<math>\alpha+\rho+\tau=1. \,</math> Here, <math>\alpha \,</math> represents spectral absorption factor, <math>\rho \,</math> spectral reflection factor and <math>\tau \,</math> spectral transmission factor. All these elements depend also on the wavelength <math>\lambda\,</math>. The spectral absorption factor is equal to the [[emissivity]] <math>\epsilon \,</math>; this relation is known as [[Kirchhoff's law of thermal radiation]]. An object is called a black body if, for all frequencies, the following formula applies: :<math>\alpha = \epsilon =1.\,</math> In a practical situation and room-temperature setting, humans lose considerable energy due to thermal radiation. However, the energy lost by emitting [[infrared]] heat is partially regained by absorbing the heat of surrounding objects (the remainder resulting from generated heat through metabolism). Human skin has an emissivity of very close to 1.0 .<ref>{{cite journal |journal=Science |date=24 May 1963 |volume= 140 |issue= 3569 |pages= 870–877 |title= Thermography of the Human Body Infrared-radiant energy provides new concepts and instrumentation for medical diagnosis |author=R. Bowling Barnes |doi=10.1126/science.140.3569.870 }}</ref> Using the formulas below then shows a human being, roughly 2 square meter in area, and about 307 [[kelvin]]s in temperature, continuously radiates about 1000 watts. However, if people are indoors, surrounded by surfaces at 296 K, they receive back about 900 watts from the wall, ceiling, and other surroundings, so the net loss is only about 100 watts. These heat transfer estimates are highly dependent on extrinsic variables, such as wearing clothes (decreasing total thermal "circuit" conductivity, therefore reducing total output heat flux.) Only truly "grey" systems (relative equivalent emissivity/absorptivity and no directional transmissivity dependence in ''all'' control volume bodies considered) can achieve reasonable steady-state heat flux estimates through the Stefan-Boltzmann law. Encountering this "ideally calculable" situation is virtually impossible (although common engineering procedures surrender the dependency of these unknown variables and "assume" this to be the case). Optimistically, these "grey" approximations will get you ''close'' to real solutions, as most divergence from Stefan-Boltzmann solutions is very small (especially in most STP lab controlled environments). If objects appear white (reflective in the [[visual spectrum]]), they are not necessarily equally reflective (and thus non-emissive) in the thermal infrared; e.g., most household radiators are painted white despite the fact that they have to be good thermal radiators. Acrylic and urethane based white paints have 93% blackbody radiation efficiency at room temperature<ref>S. Tanemura, M. Tazawa, P. Jing, T. Miki, K. Yoshimura, K. Igarashi, M. Ohishi, K. Shimono, M. Adachi, Optical Properties and Radiative Cooling Power of White Paints,[http://wire0.ises.org/wire/doclibs/SWC1999.nsf/id/D33990A41EA63969C1256920003D6148/$File/038.pdf] ISES 1999 Solar World Congress</ref> (meaning the term "black body" does not always correspond to the visually perceived color of an object). These materials that do not follow the "black color = high emissivity/absorptivity" caveat will most likely have functional spectral emissivity/absorptivity dependence. Calculation of radiative heat transfer between groups of object, including a 'cavity' or 'surroundings' requires solution of a set of [[simultaneous equations]] using the [[Radiosity (heat transfer)|Radiosity]] method. In these calculations, the geometrical configuration of the problem is distilled to a set of numbers called [[view factor]]s, which give the proportion of radiation leaving any given surface that hits another specific surface. These calculations are important in the fields of [[solar thermal energy]], [[boiler]] and [[furnace]] design and [[Ray tracing (graphics)|raytraced computer graphics]]. ==Radiative power== Thermal radiation power of a black body per unit of [[solid angle]] and per unit [[frequency]] <math>\nu</math> is given by [[Planck's law]] as: :<math>u(\nu,T)=\frac{2 h\nu^3}{c^2}\cdot\frac1{e^{h\nu/k_BT}-1}</math> or :<math>u(\lambda,T)=\frac{\beta}{\lambda^5}\cdot\frac1{e^{hc/k_BT\lambda}-1}</math> where <math>\beta</math> is a constant. This formula mathematically follows from calculation of spectral distribution of energy in [[Quantization (physics)|quantized]] electromagnetic field which is in complete [[thermal equilibrium]] with the radiating object. Integrating the above equation over <math>\nu</math> the power output given by the [[Stefan–Boltzmann law]] is obtained, as: :<math>P = \sigma \cdot A \cdot T^4</math> where the [[constant of proportionality]] <math>\sigma</math> is the [[Stefan–Boltzmann constant]] and <math>A</math> is the radiating surface area. Further, the wavelength <math>\lambda \,</math>, for which the emission intensity is highest, is given by [[Wien's Law]] as: :<math>\lambda_{max} = \frac{b}{T} </math> For surfaces which are not black bodies, one has to consider the (generally frequency dependent) emissivity factor <math>\epsilon(\upsilon)</math>. This factor has to be multiplied with the radiation spectrum formula before integration. If it is taken as a constant, the resulting formula for the power output can be written in a way that contains <math>\epsilon</math> as a factor: :<math>P = \epsilon \cdot \sigma \cdot A \cdot T^4</math> This type of theoretical model, with frequency-independent emissivity lower than that of a perfect black body, is often known as a ''gray body''. For frequency-dependent emissivity, the solution for the integrated power depends on the functional form of the dependence, though in general there is no simple expression for it. Practically speaking, if the emissivity of the body is roughly constant around the peak emission wavelength, the gray body model tends to work fairly well since the weight of the curve around the peak emission tends to dominate the integral. ===Constants=== Definitions of constants used in the above equations: {| class="wikitable" | <math>h \,</math> | [[Planck's constant]] | 6.626 0693(11)×10⁻³⁴ J·s = 4.135 667 43(35)×10⁻¹⁵ eV·s |- | <math>b \,</math> | [[Wien's displacement law|Wien's displacement constant]] | 2.897 7685(51)×10⁻³ m·K |- | <math>k_B \,</math> | [[Boltzmann constant]] | 1.380 6505(24)×10⁻²³ J·K⁻¹ = 8.617 343(15)×10⁻⁵ eV·K⁻¹ |- |<math>\sigma \,</math> |[[Stefan–Boltzmann constant]] | 5.670 400(40)×10⁻⁸ W·m⁻²·K⁻⁴ |- | <math>c \,</math> | [[Speed of light]] | 299,792,458 m·s⁻¹ |} ===Variables=== Definitions of variables, with example values: {| class="wikitable" | <math>T \,</math> | Absolute [[temperature]] | For units used above, must be in [[kelvin]] (''e.g.'' Average surface temperature on Earth = 288 K) |- | <math>A \,</math> | Surface [[area]] | ''A''_cuboid = 2''ab'' + 2''bc'' + 2''ac'';<br> ''A''_cylinder = 2''π·r''(''h'' + ''r'');<br> ''A''_sphere = 4''π·r''² |} ==See also== *[[Black body]] *[[Incandescence]] *[[Thermography]] *[[Infrared photography]] *[[Planck radiation]] *[[Thermal dose unit]] *[[Color temperature]] *[[Sakuma–Hattori equation]] ==References== {{No footnotes|date=October 2007}} {{reflist}} '''Related reading:''' *{{cite book | last = Siegel, John R. Howell | first = Robert | authorlink = | coauthors = Howell. John R. | title = Thermal radiation heat transfer | publisher = Taylor & Francis, Inc. | date = 2001-11 | location = New York | pages = (xix - xxvi ''list of symbols for thermal radiation formulas'') | url = http://books.google.com/?id=O389yQ0-fecC&pg=PA1&dq=Thermal+radiation | doi = | accessdate = 2009-07-23 | isbn = 9781560328391}} ==External links== * [http://www.spectralcalc.com/blackbody_calculator/blackbody.php Black Body Emission Calculator] * [http://sol.sci.uop.edu/~jfalward/heattransfer/heattransfer.html Heat Transfer] * [http://panda.unm.edu/courses/finley/p262/ThermalRad/ThermalRad.html Thermal Radiation] * [http://www.du.edu/~etuttle/weather/atmrad.htm Atmospheric Radiation] * [http://www.hartscientific.com/publications/pdfs/3187781_A_w.pdf Infrared Temperature Calibration 101] {{Radiation}} {{Use dmy dates|date=December 2010}} {{DEFAULTSORT:Thermal Radiation}} [[Category:Electromagnetic radiation]] [[Category:Heat transfer|Radiation]] [[Category:Thermodynamics]] [[ar:إشعاع حراري]] [[ca:Radiació tèrmica]] [[de:Wärmestrahlung]] [[et:Soojuskiirgus]] [[el:Θερμική ακτινοβολία]] [[es:Radiación térmica]] [[eo:Termoradiado]] [[ko:열복사]] [[hr:Toplinsko zračenje]] [[id:Radiasi termal]] [[it:Radiazione termica]] [[ms:Sinaran terma]] [[nl:Warmtestraling]] [[ja:熱放射]] [[nn:Varmestråling]] [[pl:Promieniowanie cieplne]] [[pt:Radiação térmica]] [[ro:Radiaţie termică]] [[ru:Тепловое излучение]] [[sl:Stefan-Boltzmannov zakon]] [[sv:Värmestrålning]] [[zh:熱輻射]]'