Propagation. Optical systems. Secondary sources
Radiometry. Photometry

Optical instruments are generally made up of a succession of diopters (regular surfaces separating media in which the refractive index is uniform) and/or mirrors.

It's easy to see that the product n ² d ² G (product of the square of the refractive index multiplied by the extent of a brush propagating through an instrument) is the same in all the media traversed. It follows that if there is no loss in propagation, through absorption in the media and at the meeting of diopters and mirrors, L /n ² is conserved.