For a thorough discussion both of the limitations of imaging ]^^mJt.Iv
optical systems in the collection of radiant energy and of the :u53zX[v
nonimaging collectors which can be used to collect energy %,*{hhfu
efficiently, see W.T. Welford and R. Winston, High Collection co%ttH\ n
Nonimaging Optics, Academic Press (San Diego), 1989.