| integral taken over the whole beam of the elementary quantity dG defined by the equivalent formulae
X
where dA and dA' are the areas of two 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'; X 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 G · n2, where n is the refractive index, is invariant. That quantity is called the optical extent.
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