radiance factor (at a surface element of a non self-radiating medium, in a given direction, under specified conditions of irradiation) [β]

ratio of the radiance of the surface element in the given direction to that of the perfect reflecting or transmitting diffuser identically irradiated and viewed

Unit: 1

NOTE For photoluminescent media, the radiance factor contains 2 components, the reflected radiance factor, βR, and the luminescent radiance factor, βL. The sum of the reflected and luminescent radiance factors is thetotal radiance factor, βT: βT = βR + βL.
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.

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