geometric extent (of a beam of rays) [G]

integral taken over the whole beam of the elementary quantity, dG, defined by the equivalent formulae:


where dA and dA' are the areas of 2 sections of an element of the beam separated by the distance l; Θ and Θ' are the angles between the direction of that elementary beam and the normals to dA and dA';  is the solid angle subtended by dA' from a point on dA

Unit: m2·sr

NOTE For a beam propagating through successive non-diffusing media, the quantity Gn2 , where n is the refractive index, is invariant. That quantity is called “optical extent”.

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