For a thorough discussion both of the limitations of imaging U]8 @
optical systems in the collection of radiant energy and of the K3CTxU(
nonimaging collectors which can be used to collect energy G!%Cc0d"7
efficiently, see W.T. Welford and R. Winston, High Collection t;^NgkP{$
Nonimaging Optics, Academic Press (San Diego), 1989.