For a thorough discussion both of the limitations of imaging 5dhy80|g]
optical systems in the collection of radiant energy and of the Y9gw
('\w
nonimaging collectors which can be used to collect energy r{B28'f[
efficiently, see W.T. Welford and R. Winston, High Collection 1szObhN-l
Nonimaging Optics, Academic Press (San Diego), 1989.