For a thorough discussion both of the limitations of imaging pis`$_kmwV
optical systems in the collection of radiant energy and of the x$.^"l-vX
nonimaging collectors which can be used to collect energy :tB1D@Cb6
efficiently, see W.T. Welford and R. Winston, High Collection /}fHt^2H
Nonimaging Optics, Academic Press (San Diego), 1989.