For a thorough discussion both of the limitations of imaging UWd=!h^dt
optical systems in the collection of radiant energy and of the Y2~nBb
nonimaging collectors which can be used to collect energy =Jyu4j *}
efficiently, see W.T. Welford and R. Winston, High Collection &7K?w~
Nonimaging Optics, Academic Press (San Diego), 1989.