Absorptance

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. This should not be confused with absorbance and absorption coefficient.

Mathematical definitions

Hemispherical absorptance

Hemispherical absorptance of a surface, denoted ''A'', is defined as
:$A = \frac,$
where
*Φ_e^a is the radiant flux ''absorbed'' by that surface;
*Φ_eⁱ 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 ''A''_ν and ''A''_λ respectively, are defined as
:$A_\nu = \frac,$
:$A_\lambda = \frac,$
where
*Φ_e,ν^a is the spectral radiant flux in frequency ''absorbed'' by that surface;
*Φ_e,νⁱ is the spectral radiant flux in frequency received by that surface;
*Φ_e,λ^a is the spectral radiant flux in wavelength ''absorbed'' by that surface;
*Φ_e,λⁱ is the spectral radiant flux in wavelength received by that surface.

Directional absorptance

Directional absorptance of a surface, denoted ''A''_Ω, is defined as
:$A_\Omega = \frac,$
where
*''L''_e,Ω^a is the radiance ''absorbed'' by that surface;
*''L''_e,Ωⁱ 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 ''A''_ν,Ω and ''A''_λ,Ω respectively, are defined as
:$A_ = \frac,$
:$A_ = \frac,$
where
*''L''_e,Ω,ν^a is the spectral radiance in frequency ''absorbed'' by that surface;
*''L''_e,Ω,νⁱ is the spectral radiance received by that surface;
*''L''_e,Ω,λ^a is the spectral radiance in wavelength ''absorbed'' by that surface;
*''L''_e,Ω,λⁱ is the spectral radiance in wavelength received by that surface.

Other radiometric coefficients

References

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Physical quantities
Radiometry