radiance factor

β_e; β

quotient of the radiance of a surface element in a specified direction, L_e,n and the radiance of the perfect reflecting diffuser or perfect transmitting diffuser identically irradiated and viewed, L_e,d

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

Note 2 to entry: Radiance factor is equivalent to reflectance factor or luminance factor when the cone angle is infinitely small, and is equivalent to reflectance when the cone angle is 2π sr. These quantities are also defined spectrally and called spectral radiance factor, β(λ), and 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 photoluminescent media, the radiance factor contains two components, the reflected radiance factor, β_R, and the luminescent radiance factor, β_L. The sum of reflected radiance factor and luminescent radiance factor is the total radiance factor, β_T: β_T = β_R + β_L.
The subscript R is used here for the reflected radiance 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 5 to entry: The radiance factor has unit one.

Note 6 to entry: This entry was numbered 845-04-68 in IEC 60050-845:1987.

Note 7 to entry: This entry was numbered 17-1015 in CIE S 017:2011.