étendue

Noun

etendue (uncountable)

1. 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.
   • The topics include […] carefully outlining the concepts of etendue, finesse, and their theoretical implications; methods of spectral, spatial, temporal, and polarization discrimination; and application of these topics to search and track systems.1966, “Advanced Infrared Technology [course title]”, in The University of Michigan Engineering Summer Conferences: Fundamentals of Infrared Technology, June 6–10, 1966; Advanced Infrared Technology...

   Synonyms:

   AΩ product, acceptance, geometric extent, light-collecting power, light-gathering power, light grasp, optical extent, throughput