We show that ideal flux collectors in the form of systems of axially symmetric systems which concentrate flux from a given angular extent with no loss in étendue are impossible; on the other hand, systems approaching this limit very closely can be designed.
© 1978 Optical Society of America
W. T. Welford and R. Winston, "On the problem of ideal flux concentrators," J. Opt. Soc. Am. 68, 531-534 (1978)