Thermal radiation

Thermal radiation is the emission of electromagnetic waves from all matter that has a temperature greater than absolute zero.[3] Thermal radiation reflects the conversion of thermal energy into electromagnetic energy. Thermal energy is the kinetic energy of random movements of atoms and molecules in matter. All matter with a nonzero temperature is composed of particles with kinetic energy. These atoms and molecules are composed of charged particles, i.e., protons and electrons. The kinetic interactions among matter particles result in charge acceleration and dipole oscillation. This results in the electrodynamic generation of coupled electric and magnetic fields, resulting in the emission of photons, radiating energy away from the body. Electromagnetic radiation, including visible light, will propagate indefinitely in vacuum.

The characteristics of thermal radiation depend on various properties of the surface from which it is emanating, including its temperature, its spectral emissivity, as expressed by Kirchhoff's law.[3] The radiation is not monochromatic, i.e., it does not consist of only a single frequency, but comprises a continuous spectrum 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 wavelengths, 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 (i.e., one).

Spectral response of two paints and a mirrored surface, in the visible and the infrared. From NASA.

Absorptivity, reflectivity, and emissivity of all bodies are dependent on the wavelength of the radiation. Due to reciprocity, absorptivity and emissivity for any particular wavelength are equal – a good absorber is necessarily a good emitter, and a poor absorber is a poor emitter. The temperature determines the wavelength distribution of the electromagnetic radiation. For example, the white paint in the diagram to the right is highly reflective to visible light (reflectivity about 0.80), and so appears white to the human eye due to reflecting sunlight, which has a peak wavelength of about 0.5 micrometers. However, its emissivity at a temperature of about −5 °C (23 °F), peak wavelength of about 12 micrometers, is 0.95. Thus, to thermal radiation it appears black.

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 is inversely proportional to the wavelength, indicates 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 (human-)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 about 10% of this radiation escapes into space, most is absorbed and then 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 (but also critically contributing to climate stability when the composition and properties of the atmosphere are not changing).

The incandescent light bulb has a spectrum overlapping the black body spectra of the sun and the earth. Some of the photons emitted by a tungsten light bulb filament at 3000 K are in the visible spectrum. 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 an incandescent light bulb. Whenever EM radiation is emitted and then absorbed, heat is transferred. This principle is used in microwave ovens, laser cutting, and RF hair removal.

Unlike conductive and convective forms of heat transfer, thermal radiation can be concentrated in a tiny spot by using reflecting mirrors, which concentrating solar power takes advantage of. Instead of mirrors, Fresnel lenses can also be used to concentrate radiant energy. (In principle, any kind of lens can be used, but only the Fresnel lens design is practical for very large lenses.) 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 Plant, and during the day it can heat water to 285 °C (558 K; 545 °F).

There are 4 main properties that characterize thermal radiation (in the limit of the far field):

• 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 as shown in the diagram at top.
• 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. 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 frequency increases steeply as the temperature rises; it grows as T⁴, 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, a property known as reciprocity. 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, and even 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 far from sources (in terms of wavelength). See section below for more on this qualification.

As for photon statistics thermal light obeys Super-Poissonian statistics.

Radiant heat panel for testing precisely quantified energy exposures at National Research Council, near Ottawa, Ontario, Canada

Thermal radiation is one of the three principal mechanisms of heat transfer. It entails the emission of a spectrum of electromagnetic radiation due to an object's temperature. Other mechanisms are convection and conduction.

Radiation heat transfer is characteristically different from the other two in that it does not require a medium and, in fact it reaches maximum efficiency in a vacuum. Electromagnetic radiation has some proper characteristics depending on the frequency and wavelengths of the radiation. The phenomenon of radiation is not yet fully understood. Two theories have been used to explain radiation; however neither of them is perfectly satisfactory.

First, the earlier theory which originated from the concept of a hypothetical medium referred as ether. Ether supposedly fills all evacuated or non-evacuated spaces. The transmission of light or of radiant heat are allowed by the propagation of electromagnetic waves in the ether.[11] Electromagnetic waves have similar characteristics to television and radio broadcasting waves they only differ in wavelength.[12] All electromagnetic waves travel at the same speed; therefore, shorter wavelengths are associated with high frequencies. Since every body or fluid is submerged in the ether, due to the vibration of the molecules, any body or fluid can potentially initiate an electromagnetic wave. All bodies generate and receive electromagnetic waves at the expense of its stored energy[12]

The second theory of radiation is best known as the quantum theory and was first offered by Max Planck in 1900.[11] According to this theory, energy emitted by a radiator is not continuous but is in the form of quanta. Planck claimed that quantities had different sizes and frequencies of vibration similarly to the wave theory.[13] The energy E is found by the expression E = hν, where h is the Planck's constant and ν is the frequency. Higher frequencies are originated by high temperatures and create an increase of energy in the quantum. While the propagation of electromagnetic waves of all wavelengths is often referred as "radiation," thermal radiation is often constrained to the visible and infrared regions. For engineering purposes, it may be stated that thermal radiation is a form of electromagnetic radiation which varies on the nature of a surface and its temperature.[11] Radiation waves may travel in unusual patterns compared to conduction heat flow. Radiation allows waves to travel from a heated body through a cold nonabsorbing or partially absorbing medium and reach a warmer body again.[11] This is the case of the radiation waves that travel from the sun to the earth.

The interplay of energy exchange by thermal radiation is characterized by the following equation:

${\displaystyle \alpha +\rho +\tau =1.\,}$

Here, ${\displaystyle \alpha \,}$ represents the spectral absorption component, ${\displaystyle \rho \,}$ spectral reflection component and ${\displaystyle \tau \,}$ the spectral transmission component. These elements are a function of the wavelength (${\displaystyle \lambda \,}$) of the electromagnetic radiation. The spectral absorption is equal to the emissivity ${\displaystyle \epsilon \,}$; 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:

${\displaystyle \alpha =\epsilon =1.\,}$

Reflectivity deviates from the other properties in that it is bidirectional in nature. In other words, this property depends on the direction of the incident of radiation as well as the direction of the reflection. Therefore, the reflected rays of a radiation spectrum incident on a real surface in a specified direction forms an irregular shape that is not easily predictable. In practice, surfaces are assumed to reflect in a perfectly specular or diffuse manner. In a specular reflection, the angles of reflection and incidence are equal. In diffuse reflection, radiation is reflected equally in all directions. Reflection from smooth and polished surfaces can be assumed to be specular reflection, whereas reflection from rough surfaces approximates diffuse reflection.[14] In radiation analysis a surface is defined as smooth if the height of the surface roughness is much smaller relative to the wavelength of the incident radiation.

In a practical situation and room-temperature setting, humans lose considerable energy due to thermal radiation in infra-red in addition to that lost by conduction to air (aided by concurrent convection, or other air movement like drafts). The heat energy lost is partially regained by absorbing heat radiation from walls or other surroundings. (Heat gained by conduction would occur for air temperature higher than body temperature.) Otherwise, body temperature is maintained from generated heat through internal metabolism. Human skin has an emissivity of very close to 1.0.[15] Using the formulas below shows a human, having roughly 2 square meter in surface area, and a temperature of about 307 K, continuously radiates approximately 1000 watts. 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, i.e. decreasing total thermal circuit conductivity, therefore reducing total output heat flux. Only truly gray 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 almost impossible (although common engineering procedures surrender the dependency of these unknown variables and "assume" this to be the case). Optimistically, these "gray" approximations will get close to real solutions, as most divergence from Stefan-Boltzmann solutions is very small (especially in most STP lab controlled environments).

Spectral response of two paints and a mirrored surface, in the visible and the infrared. From NASA.

If objects appear white (reflective in the visual spectrum), they are not necessarily equally reflective (and thus non-emissive) in the thermal infrared – see the diagram at the left. Most household radiators are painted white, which is sensible given that they are not hot enough to radiate any significant amount of heat, and are not designed as thermal radiators at all – instead, they are actually convectors, and painting them matt black would make little difference to their efficacy. Acrylic and urethane based white paints have 93% blackbody radiation efficiency at room temperature[16] (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 method. In these calculations, the geometrical configuration of the problem is distilled to a set of numbers called view factors, 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 raytraced computer graphics.

A comparison of a thermal image (top) and an ordinary photograph (bottom). The plastic bag is mostly transparent to long-wavelength infrared, but the man's glasses are opaque.

A selective surface can be used when energy is being extracted from the sun. For instance, when a green house is made, most of the roof and walls are made out of glass. Glass is transparent in the visible (approximately 0.4 μm < λ < 0.8 μm) and near-infrared wavelengths, but opaque to mid- to far-wavelength infrared (approximately λ > 3 μm).[17][18] Therefore, glass lets in radiation in the visible range, allowing us to be able to see through it, but does not let out radiation that is emitted from objects at or close to room temperature. This traps what we feel as heat. This is known as the greenhouse effect and can be observed by getting into a car that has been sitting in the sun. Selective surfaces can also be used on solar collectors. We can find out how much help a selective surface coating is by looking at the equilibrium temperature of a plate that is being heated through solar radiation. If the plate is receiving a solar irradiation of 1350 W/m² (minimum is 1325 W/m² on 4 July and maximum is 1418 W/m² on 3 January) from the sun the temperature of the plate where the radiation leaving is equal to the radiation being received by the plate is 393 K (248 °F). If the plate has a selective surface with an emissivity of 0.9 and a cut off wavelength of 2.0 μm, the equilibrium temperature is approximately 1250 K (1790 °F). The calculations were made neglecting convective heat transfer and neglecting the solar irradiation absorbed in the clouds/atmosphere for simplicity, the theory is still the same for an actual problem.

To reduce the heat transfer from a surface, such as a glass window, a clear reflective film with a low emissivity coating can be placed on the interior of the surface. "Low-emittance (low-E) coatings are microscopically thin, virtually invisible, metal or metallic oxide layers deposited on a window or skylight glazing surface primarily to reduce the U-factor by suppressing radiative heat flow".[19] By adding this coating we are limiting the amount of radiation that leaves the window thus increasing the amount of heat that is retained inside the window.

Since any electromagnetic radiation, including thermal radiation, conveys momentum as well as energy, thermal radiation also induces very small forces on the radiating or absorbing objects. Normally these forces are negligible, but they must be taken into account when considering spacecraft navigation. The Pioneer anomaly, where the motion of the craft slightly deviated from that expected from gravity alone, was eventually tracked down to asymmetric thermal radiation from the spacecraft. Similarly, the orbits of asteroids are perturbed since the asteroid absorbs solar radiation on the side facing the sun, but then re-emits the energy at a different angle as the rotation of the asteroid carries the warm surface out of the sun's view (the YORP effect).

Power emitted by a black body plotted against the temperature according to the Stefan–Boltzmann law.

Thermal radiation power of a black body per unit area of radiating surface per unit of solid angle and per unit frequency ${\displaystyle \nu }$ is given by Planck's law as:

${\displaystyle u(\nu ,T)={\frac {2h\nu ^{3}}{c^{2}}}\cdot {\frac {1}{e^{h\nu /k_{B}T}-1}}}$

or instead of per unit frequency, per unit wavelength as

${\displaystyle u(\lambda ,T)={\frac {2hc^{2}}{\lambda ^{5}}}\cdot {\frac {1}{e^{hc/k_{B}T\lambda }-1}}}$

This formula mathematically follows from calculation of spectral distribution of energy in quantized electromagnetic field which is in complete thermal equilibrium with the radiating object. Plancks law shows that radiative energy increases with temperature, and explains why the peak of an emission spectrum shifts to shorter wavelengths at higher temperatures. It can also be found that energy emitted at shorter wavelengths increases more rapidly with temperature relative to longer wavelengths.[20] The equation is derived as an infinite sum over all possible frequencies in a semi-sphere region. The energy, ${\displaystyle E=h\nu }$, of each photon is multiplied by the number of states available at that frequency, and the probability that each of those states will be occupied.

Integrating the above equation over ${\displaystyle \nu }$ the power output given by the Stefan–Boltzmann law is obtained, as:

${\displaystyle P=\sigma \cdot A\cdot T^{4}}$

where the constant of proportionality ${\displaystyle \sigma }$ is the Stefan–Boltzmann constant and ${\displaystyle A}$ is the radiating surface area.

The wavelength ${\displaystyle \lambda \,}$, for which the emission intensity is highest, is given by Wien's displacement law as:

${\displaystyle \lambda _{\text{max}}={\frac {b}{T}}}$

For surfaces which are not black bodies, one has to consider the (generally frequency dependent) emissivity factor ${\displaystyle \epsilon (\nu )}$. 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 ${\displaystyle \epsilon }$ as a factor:

${\displaystyle P=\epsilon \cdot \sigma \cdot A\cdot T^{4}}$

This type of theoretical model, with frequency-independent emissivity lower than that of a perfect black body, is often known as a grey 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:

${\displaystyle h\,}$	Planck's constant	6.626 069 3(11)×10−34 J·s = 4.135 667 43(35)×10−15 eV·s
${\displaystyle b\,}$	Wien's displacement constant	2.897 768 5(51)×10−3 m·K
${\displaystyle k_{B}\,}$	Boltzmann constant	1.380 650 5(24)×10−23 J·K−1 = 8.617 343 (15)×10−5 eV·K−1
${\displaystyle \sigma \,}$	Stefan–Boltzmann constant	5.670 373 (21)×10−8 W·m−2·K−4
${\displaystyle c\,}$	Speed of light	299 792 458 m·s−1

Variables

Definitions of variables, with example values:

${\displaystyle T}$	Absolute temperature	For units used above, must be in kelvins (e.g. average surface temperature on Earth = 288 K)
${\displaystyle A}$	Surface area	Acuboid = 2ab + 2bc + 2ac; Acylinder = 2π·r(h + r); Asphere = 4π·r2

The net radiative heat transfer from one surface to another is the radiation leaving the first surface for the other minus that arriving from the second surface.

• For black bodies, the rate of energy transfer from surface 1 to surface 2 is:

${\displaystyle {\dot {Q}}_{1\rightarrow 2}=A_{1}E_{b1}F_{1\rightarrow 2}-A_{2}E_{b2}F_{2\rightarrow 1}}$

where ${\displaystyle A}$ is surface area, ${\displaystyle E_{b}}$ is energy flux (the rate of emission per unit surface area) and ${\displaystyle F_{1\rightarrow 2}}$ is the view factor from surface 1 to surface 2. Applying both the reciprocity rule for view factors, ${\displaystyle A_{1}F_{1\rightarrow 2}=A_{2}F_{2\rightarrow 1}}$, and the Stefan–Boltzmann law, ${\displaystyle E_{b}=\sigma T^{4}}$, yields:

${\displaystyle {\dot {Q}}_{1\rightarrow 2}=\sigma A_{1}F_{1\rightarrow 2}(T_{1}^{4}-T_{2}^{4})\!}$

where ${\displaystyle \sigma }$ is the Stefan–Boltzmann constant and ${\displaystyle T}$ is temperature.[17] A negative value for ${\displaystyle {\dot {Q}}}$ indicates that net radiation heat transfer is from surface 2 to surface 1.

• For two grey-body surfaces forming an enclosure, the heat transfer rate is:

${\displaystyle {\dot {Q}}={\dfrac {\sigma (T_{1}^{4}-T_{2}^{4})}{{\dfrac {1-\epsilon _{1}}{A_{1}\epsilon _{1}}}+{\dfrac {1}{A_{1}F_{1\rightarrow 2}}}+{\dfrac {1-\epsilon _{2}}{A_{2}\epsilon _{2}}}}}}$

where ${\displaystyle \epsilon _{1}}$ and ${\displaystyle \epsilon _{2}}$ are the emissivities of the surfaces.[17]

Formulas for radiative heat transfer can be derived for more particular or more elaborate physical arrangements, such as between parallel plates, concentric spheres and the internal surfaces of a cylinder.[17]

• Siegel, John R. Howell, Robert; Howell. John R. (November 2001). Thermal radiation heat transfer. New York: Taylor & Francis, Inc. pp. (xix – xxvi list of symbols for thermal radiation formulas). ISBN 978-1-56032-839-1. Retrieved 23 July 2009.
• E.M. Sparrow and R.D. Cess. Radiation Heat Transfer. Hemisphere Publishing Corporation, 1978.

Thermal Infrared Remote Sensing:

• Kuenzer, C. and S. Dech (2013): Thermal Infrared Remote Sensing: Sensors, Methods, Applications (= Remote Sensing and Digital Image Processing 17). Dordrecht: Springer.

• Black Body Emission Calculator
• Heat transfer
• Atmospheric Radiation
• Infrared Temperature Calibration 101