Conservation of radiance
In
telecommunication, a
conservation of radiance is a basic principle of optics, that no passive optical
system can increase the quantity
L /
n 2, where
L is the radiance of a
beam and
n is the local
refractive index.
Note: "Conservation of radiance" was formerly called "conservation of brightness" or the "brightness theorem."
Source: from Federal Standard 1037C and from MIL-STD-188