For a thorough discussion both of the limitations of imaging AZmABl optical systems in the collection of radiant energy and of the ,g R9~k, nonimaging collectors which can be used to collect energy Kn]c4h}@b5 efficiently, see W.T. Welford and R. Winston, High Collection T)IH4UO Nonimaging Optics, Academic Press (San Diego), 1989.