Let us consider a source of light - or focal plane - whose area A uniformly emits radiations of intensity Iv within a frequency band 5v. Assuming that the emitting surface is centered and perpendicular to the axis of an optical system, the total amount of energy per second W reaching the entrance pupil of area A, located at distance d from the source, is represented by the inverse square law
This relationship is well known in spectroscopy and photometry (Pecker and Schatzman , Sterken and Manfroid ). The solid angle Q of the field of view is Q = A!/d2 (Fig. 1.18), so the energy is represented by
In the expression of the energy W, the product A Q of the geometrical terms is called the optical Étendue after Jacquinot  who formalized this quantity as an invariant through a perfectly transmissive optical system. The optical Étendue is generally denoted
Aperture solid angle
Focal plane area
Field solid angle
Was this article helpful?