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etendue noun

  /ˌeɪ.tɑnˈdu/ , /ˌeɪ.tɒnˈd(j)uː/
  • (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.
 геометричен фактор
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