For a thorough discussion both of the limitations of imaging uZ( I|N$
optical systems in the collection of radiant energy and of the f>ohu^bd
nonimaging collectors which can be used to collect energy mh]'/C_*<w
efficiently, see W.T. Welford and R. Winston, High Collection 6lO]V=+
Nonimaging Optics, Academic Press (San Diego), 1989.