For a thorough discussion both of the limitations of imaging ~RBa&Y=Mb
optical systems in the collection of radiant energy and of the &5B/>ag1!
nonimaging collectors which can be used to collect energy @LS%uqs
efficiently, see W.T. Welford and R. Winston, High Collection n!Y}D:6c6
Nonimaging Optics, Academic Press (San Diego), 1989.