Absorptance#absorptance
{{short description|Ability of a material to absorb radiant energy}}
{{confused|absorbance|absorption coefficient}}
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.{{GoldBookRef|title=Absorptance|file=A00035|accessdate=2015-03-15}}
Mathematical definitions
=Hemispherical absorptance=
Hemispherical absorptance of a surface, denoted {{mvar|A}} is defined as{{cite web|url=http://www.iso.org/iso/home/store/catalogue_tc/catalogue_detail.htm?csnumber=16943|title=Thermal insulation — Heat transfer by radiation — Physical quantities and definitions|work=ISO 9288:1989|publisher=ISO catalogue|year=1989|accessdate=2015-03-15}}
:
where
- {{tmath|\mathrm{\Phi_e^a} }} is the radiant flux absorbed by that surface;
- {{tmath|\mathrm{\Phi_e^i} }} 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 {{math|Aν}} and {{math|Aλ}} 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
- {{tmath|\mathrm{\Phi_{e,\nu}^a} }} is the spectral radiant flux in frequency absorbed by that surface;
- {{tmath|\mathrm{\Phi_{e,\nu}^i} }} is the spectral radiant flux in frequency received by that surface;
- {{tmath|\mathrm{\Phi_{e,\lambda}^a} }} is the spectral radiant flux in wavelength absorbed by that surface;
- {{tmath|\mathrm{\Phi_{e,\lambda}^i} }} is the spectral radiant flux in wavelength received by that surface.
=Directional absorptance=
Directional absorptance of a surface, denoted {{math|AΩ}}, is defined as
:
where
- {{tmath|L\mathrm{_{e,\Omega}^a} }} is the radiance absorbed by that surface;
- {{tmath|L\mathrm{_{e,\Omega}^i} }} 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 {{math|Aν,Ω}} and {{math|Aλ,Ω}} 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
- {{tmath|L\mathrm{_{e,\Omega,\nu}^a} }} is the spectral radiance in frequency absorbed by that surface;
- {{tmath|L\mathrm{_{e,\Omega,\nu}^i} }} is the spectral radiance received by that surface;
- {{tmath|L\mathrm{_{e,\Omega,\lambda}^a} }} is the spectral radiance in wavelength absorbed by that surface;
- {{tmath|L\mathrm{_{e,\Omega,\lambda}^i} }} is the spectral radiance in wavelength received by that surface.
Other radiometric coefficients
{{Radiometry coefficients}}