is defined as
:where ? is the wavelength.

Conservation of basic radiance

Radiance of a surface is related to étendue by
:where
*n is the refractive index in which that surface is immersed;
*G is the étendue of the light beam.

As the light travels through an ideal optical system, both the étendue and the radiant flux are conserved. Therefore, basic radiance defined by
:is also conserved. In real systems, the étendue

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