For a thorough discussion both of the limitations of imaging !Dd'*ee-; optical systems in the collection of radiant energy and of the N&8$tJ(hhx nonimaging collectors which can be used to collect energy u]ms~rO efficiently, see W.T. Welford and R. Winston, High Collection f)sy-o! Nonimaging Optics, Academic Press (San Diego), 1989.