View source for 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]].<ref name=GoldBook>{{GoldBookRef|title=Absorptance|file=A00035|accessdate=2015-03-15}}</ref>

==Mathematical definitions==
===Hemispherical absorptance===
'''Hemispherical absorptance''' of a surface, denoted {{mvar|A}} is defined as<ref name="ISO_9288-1989">{{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=[[International Organization for Standardization|ISO]] catalogue|year=1989|accessdate=2015-03-15}}</ref>
:<math>A = \mathrm{ \frac{\Phi_e^a}{\Phi_e^i} },</math>
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|''A''<sub>ν</sub>}} and {{math|''A''<sub>λ</sub>}} respectively, are defined as<ref name="ISO_9288-1989" />
:<math>\begin{align}
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}</math>
where
*{{tmath|\mathrm{\Phi_{e,\nu}^a} }} is the [[Radiant flux|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 [[Radiant flux|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|''A''<sub>Ω</sub>}}, is defined as<ref name="ISO_9288-1989" />
:<math>A_\Omega = \frac{L_\mathrm{\mathrm{e},\Omega}^\mathrm{a}}{L_{\mathrm{e},\Omega}^\mathrm{i}},</math>
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|''A''<sub>ν,Ω</sub>}} and {{math|''A''<sub>λ,Ω</sub>}} respectively, are defined as<ref name="ISO_9288-1989" />
:<math>\begin{align}
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}</math>
where
*{{tmath|L\mathrm{_{e,\Omega,\nu}^a} }} is the [[Radiance|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 [[Radiance|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}}

==References==
{{reflist}}

[[Category:Physical quantities]]
[[Category:Radiometry]]