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.
[1] Hemispherical absorptance of a surface, denoted A is defined as[2] where Spectral hemispherical absorptance in frequency and spectral hemispherical absorptance in wavelength of a surface, denoted Aν and Aλ respectively, are defined as[2] where Directional absorptance of a surface, denoted AΩ, is defined as[2] where Spectral directional absorptance in frequency and spectral directional absorptance in wavelength of a surface, denoted Aν,Ω and Aλ,Ω respectively, are defined as[2] where