Abstract: Light biasing of a vidicon target is optimized to effect a consistent rate of response to instantaneous changes in the intensity of light from an object to be imaged. The optimum bias lighting level is determined by adjusting the bias lighting to a predetermined level and producing a first and second series of video frame signals representing a gray scale image focused onto the target. The difference in magnitude between a selected frame signal in the first series and one in the second series is determined and recorded. This difference determining procedure is repeated for a succession of bias lighting levels, and the bias lighting for the target is then set at the level which effected production of the smallest difference determined.