In the study of
heat transfer
Heat transfer is a discipline of thermal engineering that concerns the generation, use, conversion, and exchange of thermal energy (heat) between physical systems. Heat transfer is classified into various mechanisms, such as thermal conduction, ...
, absorptance of the surface of a material is its effectiveness in absorbing
radiant energy
In physics, and in particular as measured by radiometry, radiant energy is the energy of electromagnetic radiation, electromagnetic and gravitational radiation. As energy, its SI unit is the joule (J). The quantity of radiant energy may be calcul ...
. It is the ratio of the absorbed to the incident
radiant power.
Mathematical definitions
Hemispherical absorptance
Hemispherical absorptance of a surface, denoted is defined as
:
where
* is the
radiant flux
In radiometry, radiant flux or radiant power is the radiant energy emitted, reflected, transmitted, or received per unit time, and spectral flux or spectral power is the radiant flux per unit frequency or wavelength, depending on whether the ...
''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 and respectively, are defined as
:
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 , is defined as
:
where
* is the
radiance
In radiometry, radiance is the radiant flux emitted, reflected, transmitted or received by a given surface, per unit solid angle per unit projected area. Radiance is used to characterize diffuse emission and reflection of electromagnetic radiati ...
''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 and respectively, are defined as
:
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
References
{{reflist
Physical quantities
Radiometry