quotient of the luminance of a surface element in a specified direction,
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,
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.
Note 6 to entry: This entry was numbered 17-717 in CIE S 017:2011.