For a thorough discussion both of the limitations of imaging ;,8 )%[
optical systems in the collection of radiant energy and of the Xh}S_/9}5
nonimaging collectors which can be used to collect energy X=+|(A,BdY
efficiently, see W.T. Welford and R. Winston, High Collection @(bg#
Nonimaging Optics, Academic Press (San Diego), 1989.