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}}

:A = \mathrm{ \frac{\Phi_e^a}{\Phi_e^i} },

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

:\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}

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

:A_\Omega = \frac{L_\mathrm{\mathrm{e},\Omega}^\mathrm{a}}{L_{\mathrm{e},\Omega}^\mathrm{i}},

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

:\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}

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}}

References