A real-time optical correlator employs a spatial light modulator (SLM) to record ongoing, changing scenes. Current SLMs are quite useful in this role although some suffer from limited dynamic range and, therefore cannot respond fully to such variations as light changes on the target. In this paper, a method is described for preprocessing an image before it is impressed upon an SLM. The processed image, in effect, alters the transfer characteristics and serves to make the image relatively invariant with changes in scene and environmental target conditions. The restructured image will appear invariant to the SLM - or at least quite constant - and, therefore, invariant to the optical matched filter residing in the memory. The correlator operates as though the target and scene conditions were fixed, confined to more acceptable, narrowed conditions.