Narcissus: reflections on retroreflections in thermal imaging systems

Narcissus, a composite of self-images of a cryogenically cooled detector array, is an effect peculiar to scanning thermal imaging systems. It occurs when the detectors sense variations in the amount of background radiation reaching them via reflections from lens surfaces. Paraxial surface-contribution formulas derived from Lagrange invariants, coupled with the system spectral response, determine narcissus in terms of its equivalent scene temperature difference (NARCΔT). The equations give the intensity and size of the narcissus ghost in terms of paraxial ray data at the contributing surfaces and the system f/No. This Gaussian formulation has sufficient radiometric accuracy to make extensive ray tracing unnecessary.