${\displaystyle {\beta }_{\text{v}}=\frac{L_{\text{v}\text{,n}}}{L_{\text{v}\text{,d}}}}$

Note 1 to entry: The definition holds for a surface element of a non-luminous medium, in a given direction and under specified conditions of irradiation.

Note 2 to entry: This quantity is also defined spectrally and is called "spectral luminance factor".

Note 3 to entry: For photoluminescent media, the luminance factor consists of two components: the reflected luminance factor, β_v,R, and the luminescent luminance factor, β_v,L. The sum of the reflected and luminescent luminance factors is called "total luminance factor" and can be expressed by β_v,T: β_v,T = β_v,R + β_v,L.

The subscript R is used here for the reflected luminance factor because it is more intuitive than the traditional S and avoids confusion with the use of S to denote a state of polarization.

Note 4 to entry: The luminance factor has unit one.

Note 5 to entry: This entry was numbered 845-04-69 in IEC 60050-845:1987.