Conservation of radiance
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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