For a thorough discussion both of the limitations of imaging d@#!,P5` optical systems in the collection of radiant energy and of the [wWip1OR nonimaging collectors which can be used to collect energy K??%Qh5l+C efficiently, see W.T. Welford and R. Winston, High Collection f]L`^WU
Nonimaging Optics, Academic Press (San Diego), 1989.