Etendue

The topics include […] carefully outlining the concepts of etendue, finesse, and their theoretical implications; methods of spectral, spatial, temporal, and polarization discrimination; and application of these topics to search and track systems.

The light power transmitted to the detector is proportional to the product of the solid angle #92;mathit#123;#92;Omega#125; and area A in the central plane, which is imaged by the lens onto the aperture (often called the étendue U).

Etendue is an optical invariant and is called by many different names. One of the names is "light-gathering power." As implied by this terminology, when light enters an optical system with smaller etendue from another optical system with larger etendue, the former optical system fails to transmit some part of the light. […] Therefore, a given EUV [extreme ultraviolet] power with a larger etendue than that of the requirement is substantially equivalent to a smaller EUV power with the etendue of the requirement.