For a thorough discussion both of the limitations of imaging U_B"B;ng+
optical systems in the collection of radiant energy and of the [r<lAS{ .
nonimaging collectors which can be used to collect energy l0l2fwz(
efficiently, see W.T. Welford and R. Winston, High Collection s^)wh v`C
Nonimaging Optics, Academic Press (San Diego), 1989.