= Basic radiance
{title2=$L/n^2$}
= Reduced radiance
{synonym}
<Basic radiance> is <radiance> divided by the square of the <refractive index>. For <radiance> $L$ at a fixed <wavelength>,
$$
d\Phi=\frac{L}{n^2}\,dG.
$$
Lossless propagation conserves <reduced radiance> along each <light ray>; $L$ 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. <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]{ref}
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