For a thorough discussion both of the limitations of imaging (qdk
&
optical systems in the collection of radiant energy and of the 8~&v\GDkF
nonimaging collectors which can be used to collect energy E;0"1
P|S
efficiently, see W.T. Welford and R. Winston, High Collection 0@;E8^pa
Nonimaging Optics, Academic Press (San Diego), 1989.