${\displaystyle R=\frac{{\Phi }_{\text{n}}}{{\Phi }_{\text{d}}}}$

Note 1 to entry: The definition holds for a surface element, for the part of the reflected radiation contained in a given cone with apex at the surface element, and for incident radiation of given spectral composition, polarization and geometric distribution.

Note 2 to entry: The reflectance factor is also defined spectrally and is called spectral reflectance factor, R(λ).

Note 3 to entry: The ideal isotropic (Lambertian) diffuser with reflectance or transmittance equal to 1 is called a perfect diffuser.

Note 4 to entry: For regularly reflecting surfaces that are irradiated or illuminated by a beam of small solid angle, the reflectance factor can be much larger than 1 if the cone includes the mirror image of the source.

Note 5 to entry: If the solid angle of the cone approaches 2π sr, the reflectance factor approaches the reflectance for the same conditions of irradiation.

Note 6 to entry: If the solid angle of the cone approaches 0 sr, the reflectance factor approaches the radiance factor or luminance factor for the same conditions of irradiation.

Note 7 to entry: The reflectance factor has unit one.

Note 8 to entry: This entry was numbered 845-04-64 in IEC 60050-845:1987.