For a thorough discussion both of the limitations of imaging 6' 9ITA
optical systems in the collection of radiant energy and of the B,@<60u
nonimaging collectors which can be used to collect energy 'nK(cKDIG
efficiently, see W.T. Welford and R. Winston, High Collection ICJp-
Nonimaging Optics, Academic Press (San Diego), 1989.