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

integral taken over the whole beam of the elementary quantity, d*G*, defined by the equivalent formulae:

where d*A* and d*A*' 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 d*A* and d*A*'; _{} is the solid angle subtended by d*A*' from a point on d*A*

Unit: m^{2}·sr

NOTE For a beam propagating through successive non-diffusing media, the quantity *Gn*^{2} , where *n* is the refractive index, is invariant. That quantity is called “optical extent”.

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