For a thorough discussion both of the limitations of imaging lK;|ciq"c7 optical systems in the collection of radiant energy and of the fQ33J> nonimaging collectors which can be used to collect energy T}d%X MXq efficiently, see W.T. Welford and R. Winston, High Collection A:Pp;9wl Nonimaging Optics, Academic Press (San Diego), 1989.