Etendue
Measure of the "spread" of light in an optical system
Etendue
Etendue or étendue () is a property of light in an optical system, which characterizes how "spread out" the light is in area and angle. The term étendue comes from French, where it means "extent". Other names for etendue include acceptance, throughput, light grasp, light-gathering power, optical extent, and the AΩ product. From the source point of view, etendue is the product of the area of the source A and the solid angle Ω that the system's entrance pupil subtends as seen from the source. Equivalently, from the system point of view, the etendue equals the area of the entrance pupil times the solid angle the source subtends as seen from the pupil. These definitions must be applied for infinitesimally small "elements" of area and solid angle, which must then be summed over both the source and the diaphragm as shown below. Etendue may be considered to be a volume in phase space. Etendue never decreases in any optical system where optical power is conserved. A perfect optical system produces...
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