Basic radiance is radiance divided by the square of the refractive index. For radiance at a fixed wavelength,Lossless propagation conserves reduced radiance along each light ray; itself is conserved only when the endpoint refractive indices agree. A larger irradiance at a smaller image can result from admitting more directions without increasing reduced radiance. Uniform wave absorption can lower radiant flux while leaving the occupied geometric etendue unchanged; an opaque aperture removes part of that occupied set. [ref]
Radiative Transfer lecture notes by John E. Greivenkamp. Section 22, slides 22-18 and 22-20: lossless radiance conservation and invariant basic radiance L/n²
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