For a thorough discussion both of the limitations of imaging Pksr9"Ah
optical systems in the collection of radiant energy and of the MgJ5FRQ
nonimaging collectors which can be used to collect energy q'a]DJ`
efficiently, see W.T. Welford and R. Winston, High Collection Nsq=1)
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Nonimaging Optics, Academic Press (San Diego), 1989.