${\displaystyle \tau =\frac{{\Phi }_{\text{t}}}{{\Phi }_{\text{m}}}}$

Note 1 to entry: Transmittance 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 α is absorptance and ρ is reflectance.

Note 3 to entry: Transmittance, τ, is the sum of regular transmittance, τ_r, and diffuse transmittance, τ_d: τ = τ_r + τ_d.

Note 4 to entry: The transmittance has unit one.

Note 5 to entry: This entry was numbered 845-04-59 in IEC 60050-845:1987.