Absorptance
In the study of heat transfer, absorptance of the surface of a material is its effectiveness in absorbing radiant energy. It is the ratio of the absorbed to the incident radiant power.[1]
Mathematical definitions
Hemispherical absorptance
Hemispherical absorptance of a surface, denoted A is defined as[2]
where
- is the radiant flux absorbed by that surface;
- is the radiant flux received by that surface.
Spectral hemispherical absorptance
Spectral hemispherical absorptance in frequency and spectral hemispherical absorptance in wavelength of a surface, denoted Aν and Aλ respectively, are defined as[2]
where
- is the spectral radiant flux in frequency absorbed by that surface;
- is the spectral radiant flux in frequency received by that surface;
- is the spectral radiant flux in wavelength absorbed by that surface;
- is the spectral radiant flux in wavelength received by that surface.
Directional absorptance
Directional absorptance of a surface, denoted AΩ, is defined as[2]
where
- is the radiance absorbed by that surface;
- is the radiance received by that surface.
Spectral directional absorptance
Spectral directional absorptance in frequency and spectral directional absorptance in wavelength of a surface, denoted Aν,Ω and Aλ,Ω respectively, are defined as[2]
![{\displaystyle {\begin{aligned}A_{\nu ,\Omega }&={\frac {L\mathrm {_{e,\Omega ,\nu }^{a}} }{L\mathrm {_{e,\Omega ,\nu }^{i}} }},\\[4pt]A_{\lambda ,\Omega }&={\frac {L\mathrm {_{e,\Omega ,\lambda }^{a}} }{L\mathrm {_{e,\Omega ,\lambda }^{i}} }},\end{aligned}}}](https://wikimedia.org/api/rest_v1/media/math/render/svg/fe80669bec46628645300a32529b8b7c45e62047)
where
- is the spectral radiance in frequency absorbed by that surface;
- is the spectral radiance received by that surface;
- is the spectral radiance in wavelength absorbed by that surface;
- is the spectral radiance in wavelength received by that surface.
Other radiometric coefficients
| Quantity | SI units | Notes | |
|---|---|---|---|
| Name | Sym. | ||
| Hemispherical emissivity | ε | — | Radiant exitance of a surface, divided by that of a black body at the same temperature as that surface. |
| Spectral hemispherical emissivity | εν ελ |
— | Spectral exitance of a surface, divided by that of a black body at the same temperature as that surface. |
| Directional emissivity | εΩ | — | Radiance emitted by a surface, divided by that emitted by a black body at the same temperature as that surface. |
| Spectral directional emissivity | εΩ,ν εΩ,λ |
— | Spectral radiance emitted by a surface, divided by that of a black body at the same temperature as that surface. |
| Hemispherical absorptance | A | — | Radiant flux absorbed by a surface, divided by that received by that surface. This should not be confused with "absorbance". |
| Spectral hemispherical absorptance | Aν Aλ |
— | Spectral flux absorbed by a surface, divided by that received by that surface. This should not be confused with "spectral absorbance". |
| Directional absorptance | AΩ | — | Radiance absorbed by a surface, divided by the radiance incident onto that surface. This should not be confused with "absorbance". |
| Spectral directional absorptance | AΩ,ν AΩ,λ |
— | Spectral radiance absorbed by a surface, divided by the spectral radiance incident onto that surface. This should not be confused with "spectral absorbance". |
| Hemispherical reflectance | R | — | Radiant flux reflected by a surface, divided by that received by that surface. |
| Spectral hemispherical reflectance | Rν Rλ |
— | Spectral flux reflected by a surface, divided by that received by that surface. |
| Directional reflectance | RΩ | — | Radiance reflected by a surface, divided by that received by that surface. |
| Spectral directional reflectance | RΩ,ν RΩ,λ |
— | Spectral radiance reflected by a surface, divided by that received by that surface. |
| Hemispherical transmittance | T | — | Radiant flux transmitted by a surface, divided by that received by that surface. |
| Spectral hemispherical transmittance | Tν Tλ |
— | Spectral flux transmitted by a surface, divided by that received by that surface. |
| Directional transmittance | TΩ | — | Radiance transmitted by a surface, divided by that received by that surface. |
| Spectral directional transmittance | TΩ,ν TΩ,λ |
— | Spectral radiance transmitted by a surface, divided by that received by that surface. |
| Hemispherical attenuation coefficient | μ | m−1 | Radiant flux absorbed and scattered by a volume per unit length, divided by that received by that volume. |
| Spectral hemispherical attenuation coefficient | μν μλ |
m−1 | Spectral radiant flux absorbed and scattered by a volume per unit length, divided by that received by that volume. |
| Directional attenuation coefficient | μΩ | m−1 | Radiance absorbed and scattered by a volume per unit length, divided by that received by that volume. |
| Spectral directional attenuation coefficient | μΩ,ν μΩ,λ |
m−1 | Spectral radiance absorbed and scattered by a volume per unit length, divided by that received by that volume. |
References
- ^ IUPAC, Compendium of Chemical Terminology, 2nd ed. (the "Gold Book") (1997). Online corrected version: (2006–) "Absorptance". doi:10.1351/goldbook.A00035
- ^ a b c d "Thermal insulation — Heat transfer by radiation — Physical quantities and definitions". ISO 9288:1989. ISO catalogue. 1989. Retrieved 2015-03-15.