For a thorough discussion both of the limitations of imaging kO`!!M[Oo optical systems in the collection of radiant energy and of the }f;TG:6 nonimaging collectors which can be used to collect energy uG'S&8i_ efficiently, see W.T. Welford and R. Winston, High Collection =),ZZD#J Nonimaging Optics, Academic Press (San Diego), 1989.