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The Narcissus Effect In Infrared Optical Scanning Systems

In a typical infrared optical scanning system, the surface temperature of the detector is usually much less than the ambient. The narcissus effect occurs when the detector, through unintended reflections off internal lens surfaces, sees sources at temperatures other than the background ambient. These sources usually are reflections of the detector itself, hence the term narcissus. The resulting display shows abrupt changes in the background radiation as a function of field position. A technique that evaluates the narcissus effect in terms of narcissus equivalent temperature difference (NARCAT) has been developed and programmed for the computer. Computer results for several infrared optical scanning systems indicate that predicted and measured values of NARCAT agree well. The theory is based on the principle that radiance along any geometrical ray bundle does not vary. 1 The narcissus-induced variation of this radiance is found to be the arithmetic product of three factors, viz., (1) image-forming, (2) surface spectral reflec-tance, and (3) temperature difference. Therefore, to suppress the narcissus effect, one simply minimizes these factors. Examples illustrating both the computational approach and the suppression techniques are given.