For a thorough discussion both of the limitations of imaging %?].(
Lc
optical systems in the collection of radiant energy and of the v LN KX;9
nonimaging collectors which can be used to collect energy vA_,TS#Bo
efficiently, see W.T. Welford and R. Winston, High Collection _|12BVq
Nonimaging Optics, Academic Press (San Diego), 1989.