L-A-Omega is the mathematical expression for collection of light within an optical system as it depends upon radiance, space, and angle.
The symbolic expression for L-A-Omega is Φ=LAΩ.
In which: Φ is the optical flow (photons per second, watts, or lumens); L is the radiance of an object (flow per area-and-angle); A is the area of the object; and Ω is the solid-angle of collection. The radiance L is constant throughout an optical system. The space-angle product AΩ also remains constant throughout an optical system unless an evanescent field is created. The space-angle product may be redistributed between an object and its image for maximum delivery of light into a detector of limited area.
L-A-Omega defines maximum collection of light through elegant manipulation of the space-angle product.