${\displaystyle \rho =\frac{{\Phi }_{\text{r}}}{{\Phi }_{\text{m}}}}$

Note 1 to entry: Reflectance 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 τ is transmittance.

Note 3 to entry: Reflectance, ρ, is the sum of regular reflectance, ρ_r, and diffuse reflectance, ρ_d: ρ = ρ_r + ρ_d.

Note 4 to entry: The reflectance has unit one.

Note 5 to entry: This entry was numbered 845-04-58 in IEC 60050-845:1987.