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glossary

Solid angle: two-dimensional angle in the three-dimensional space measuring how large an obect appears from a viewing point
Aperture : diaphragm limiting the number of light rays that are able to propagate
Etendue: describes the envelope in which the light from a source passing through an aperture is wrapped; for a small source and a small aperture in air or vacuum, product of the apparent area of the source seen from the aperture by the solid angle of the aperture seen from the source ; in an equivalent way, product of the apparent area of the aperture seen from the source by the solid angle of the source seen from the aperture