For a thorough discussion both of the limitations of imaging AR)&W/S)7,
optical systems in the collection of radiant energy and of the m-;u]X=a
nonimaging collectors which can be used to collect energy `<
VoZ/v
efficiently, see W.T. Welford and R. Winston, High Collection mWO=(}Fb\
Nonimaging Optics, Academic Press (San Diego), 1989.