For a thorough discussion both of the limitations of imaging C%0<1mp
optical systems in the collection of radiant energy and of the y+?=E g
nonimaging collectors which can be used to collect energy {a]pF.^kf
efficiently, see W.T. Welford and R. Winston, High Collection $4DFgvy$
Nonimaging Optics, Academic Press (San Diego), 1989.