Etendue or étendue (`ay-tahn-doo`) is a property of light in an optical system, which characterizes how `spread out` the light is in area and angle. From the source point of view, it is the product of the area of the source and the solid angle that the system`s entrance pupil subtends as seen from the source. Equivalently, from the system po...
Found on
http://en.wikipedia.org/wiki/Etendue
Also called the throughput of an optical system, it is the product of its entrance aperture and the solid angle in which light can be accepted at that aperture.
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http://www.intl-lighttech.com/support/glossary
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