For a thorough discussion both of the limitations of imaging JTA65T{3
optical systems in the collection of radiant energy and of the "N>~]
nonimaging collectors which can be used to collect energy i=jwk_y
efficiently, see W.T. Welford and R. Winston, High Collection Q'>pOtJG*J
Nonimaging Optics, Academic Press (San Diego), 1989.