For a thorough discussion both of the limitations of imaging {h *Pkn1
optical systems in the collection of radiant energy and of the 5<KY}
nonimaging collectors which can be used to collect energy _T[m YY
efficiently, see W.T. Welford and R. Winston, High Collection >s+*D =k
Nonimaging Optics, Academic Press (San Diego), 1989.
