$\alpha = \frac{Energy\ absorbed\ by\ a\ body}{Total\ Irradition\ on\ the\ surface}$ Spectral, directional absorptance is equal to spectral directional emittance in thermodynamic equilibrium (according to Kirchoff's law) Spectral, hemispherical absorptance is equal to spectral hemispherical emittance if the incoming radiation is diffuse or the surface is diffuse. Total hemispherical absorptance is equal to total hemispherical emittance only if emittance absorptance is independent of wavelength and surface is diffuse. This is called a grey surface Or if the irradiation is independent of wavelength (which never happens) Absorptance is not purely a surface property like [[Emittance]]. [[Solar Absorptance]]