etendue

Etymology

of how “spread out” light that crosses, or is emitted by, a surface is, in terms of angle and area.]] Borrowed from French étendue (“area, expanse; size; extent, range, scale”), short for étendue géométrique (“geometric extent”).

noun

1. (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.
   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, “[https://books.google.com/books?id=EpSfAAAAMAAJ&pg=PP5 Advanced Infrared Technology [course title]]”, in The University of Michigan Engineering Summer Conferences: Fundamentals of Infrared Technology, June 6–10, 1966; Advanced Infrared Technology, June 27 – July 1, 1966; Precision Radiometry – Calibration and Instrumentation, June 27 – July 1, 1966: Three Intensive One-week Courses for: Engineers, Scientists (University of Michigan Official Publication), volume 67, number 115, Ann Arbor, Mich.: University of Michigan, →OCLC
   The light power transmitted to the detector is proportional to the product of the solid angle mathit Ω and area A in the central plane, which is imaged by the lens onto the aperture (often called the étendue U). 2003, Wolfgang Demtröder, “Spectroscopic Instrumentation”, in Laser Spectroscopy: Basic Concepts and Instrumentation (Advanced Texts in Physics), 3rd edition, Berlin: Springer-Verlag, →DOI, →ISSN, page 149
   Etendue is an optical invariant and is called by many different names. One of the names is "light-gathering power." As implied by this terminology, when light enters an optical system with smaller etendue from another optical system with larger etendue, the former optical system fails to transmit some part of the light. […] Therefore, a given EUV [extreme ultraviolet] power with a larger etendue than that of the requirement is substantially equivalent to a smaller EUV power with the etendue of the requirement. 2006, Kazuya Ota, Yukata Watanabe, Vadim Banine, Hans Franken, “EUV Source Requirements for EUV Lithography”, in Vivek Bakshi, editor, EUV Sources for Lithography, Bellingham, Wash.: SPIE Press, section 2.2.6 (Etendue of Source Output and Positioning Stability), pages 36–37