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.
Mathematical definitions
Hemispherical absorptance
Hemispherical absorptance of a surface, denoted is defined as
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 and respectively, are defined as
A_\nu &= \mathrm{ \frac{\Phi_{e,\nu}^a}{\Phi_{e,\nu}^i} }, \\
A_\lambda &= \mathrm{ \frac{\Phi_{e,\lambda}^a}{\Phi_{e,\lambda}^i} },
\end{align}
where
-
is the Radiant flux absorbed by that surface;
-
is the spectral radiant flux in frequency received by that surface;
-
is the Radiant flux 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 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
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{align}
where
-
is the Radiance absorbed by that surface;
-
is the spectral radiance received by that surface;
-
is the Radiance absorbed by that surface;
-
is the spectral radiance in wavelength received by that surface.
Other radiometric coefficients
