However, real-world physical light sources exhibit a finite area–angle product, corresponding to a finite etendue. Light-emitting diodes in particular exhibit a large etendue due to their extended emission area and Lambertian intensity pattern, which needs to be considered during illumination design.^{8}^{,}^{9} Solar applications, especially concentrators, are limited by the size of the sun. Lasers, especially poor quality lasers or lasers with pointing tolerances, have nonvanishing etendue, which needs to be considered. Currently, the finite etendue of the light sources is very often considered as a correction step during illumination design.^{10} That is, the system is laid out with a point-like light source and later the effect of finite source etendue is considered. Such methods have led to feedback methods in designing collimators.^{11}^{,}^{12} Unfortunately, this iterative process lacks a deeper understanding of the effect of the source extensions, and fundamental implications and limitations in particular are not well defined. Therefore, a deterministic design process for extended sources is not yet available.