Informational Difference (InDiff) is aa index that measures the difference between two image sets. Previous research has shown that it can be used to explain the results of important target recognition experiments involving human observers. In this project we present the results of various studies on the suitability of using the InDiff as a measure of target conspicuity. First, InDiff is defined and adapted to measuring the difference between two images, one containing a target on a background and the other - containing only the background. Second, we present results of two experiments involving human observers. In one experiment, observers were presented with grey level images of complex scenes. The second experiment displayed color images. Response times for target detection and recognition in these images were measured and InDiff values for the images were calculated. Correlation coefficients of 0.60 - 0.85 were found between the InDiff values and detection speed in both experiments, as well as recognition speed in the grey-level experiment. In the grey-level experiment, significant correlations were found between the probability of correct detection or recognition and a quantity based on the InDiff. Finally we discuss possible applications of these findings and suggest extension to the formalism.