${\displaystyle \alpha =\frac{{\Phi }_{\text{a}}}{{\Phi }_{\text{m}}}}$

Note 1 to entry: The absorptance is also defined spectrally in terms of wavelength, in which case, "spectral" is added before the quantity name.

Note 2 to entry: Due to energy conservation, α + ρ + τ = 1 except when polarized radiation is observed, where ρ is reflectance and τ is transmittance.

Note 3 to entry: The absorptance has unit one.

Note 4 to entry: This entry was numbered 845-04-75 in IEC 60050-845:1987.