The roles of scale, contrast and spectral difference in visual detection and recognition was investigated. Geometrical target arrays were produced and projected using a rear projection apparatus. Scale was varied using combined photographic reduction and slide projection. Contrast levels were adjusted in projection of lithographic target arrays using neutral density filtration arrangements in the projection apparatus. Experimentally obtained cumulative probability of detection and combined detection and recognition was examined relative to established combinations of target scale, contrast and spectral difference. A method for the characterization of target pair differences was offered and analyzed. The relation of scale, contrast and perceptual target difference was analyzed and a hierarchy of scale importance in simple geometrical targets was clarified
