luminance factor[′lü·mə·nəns ‚fak·tər]
the ratio of the luminance of a body at a certain point and in a given direction to the luminance of a perfect diffuser under the same conditions of illumination. A perfect diffuser is a diffuser whose luminance is the same in all directions and whose reflection coefficient is equal to unity.
The concept of the luminance factor pertains to radiation that is evaluated in either radiometric or photometric units. The respective symbols for the luminance factor are βe and βv; the symbol β may be used in either case.
In recent years, the luminance factor has come to be applied not only to the reflection of light but also to the transmission and scattering of light, as well as to thermal radiation.