integral taken over the whole beam of the elementary quantity dG defined by the equivalent formulae
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′; 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.
|