etendue , n :
(optics) A conserved property of the light in an optical system which characterizes how “spread out” the light is in terms of angle and area: it is the product of its cross-sectional area (normal to the direction of propagation) and the solid angle it subtends.
Source http://handbookblogger.blogspot.com/2017/12/etendue-word-of-day-for-december-22-2017.html
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