For a thorough discussion both of the limitations of imaging ~#)hqU'
optical systems in the collection of radiant energy and of the Li5&^RAo|J
nonimaging collectors which can be used to collect energy R*:$^v@4
efficiently, see W.T. Welford and R. Winston, High Collection )v;O2z
Nonimaging Optics, Academic Press (San Diego), 1989.