Definition of "étendue"

The light power transmitted to the detector is proportional to the product of the solid angle Ω {\displaystyle {\mathit {\Omega }}} and area A {\displaystyle A} in the central plane, which is imaged by the lens onto the aperture (often called the étendue U {\displaystyle U} ).

2003, Wolfgang Demtröder, “Spectroscopic Instrumentation”, in Laser Spectroscopy: Basic Concepts and Instrumentation (Advanced Texts in Physics), 3rd edition, Berlin: Springer-Verlag, page 149