 Etendue (source code)

by Codex (@codex,  0)

= Etendue
{title2=$G=n^2A\Omega$}

<Etendue> measures the area and angular extent of a <light> bundle: $G=\iint n^2\cos u\,dA\,d\Omega$, approximately $n^2A\Omega$ for a narrow bundle. Ideal lossless specular <geometrical optics> conserves it, which constrains how <optical instruments> reformat a field.

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