Absorptance

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

where

• is the spectral radiant flux in frequency absorbed by that surface;
• is the spectral radiant flux in frequency received by that surface;
• is the spectral radiant flux in wavelength 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

where

• is the spectral radiance in frequency absorbed by that surface;
• is the spectral radiance received by that surface;
• is the spectral radiance in wavelength absorbed by that surface;
• is the spectral radiance in wavelength received by that surface.

Other radiometric coefficients

References