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optical extent
etendue
product of geometric extent and the squared refractive index of a medium through which the beam propagates
Note 1 to entry: The optical extent is invariant and remains constant when the beam is propagating through successive non-diffusing media.
Note 2 to entry: See also "geometric extent".
Note 3 to entry: The optical extent is expressed in square metre times steradian (m2·sr).
Note 4 to entry: This entry was numbered 17-845 in CIE S 017:2011.