For a thorough discussion both of the limitations of imaging '0o`<xW
optical systems in the collection of radiant energy and of the @cjhri|vH
nonimaging collectors which can be used to collect energy VVe>}
efficiently, see W.T. Welford and R. Winston, High Collection nh E!Pk
Nonimaging Optics, Academic Press (San Diego), 1989.