Camouflage is an attempt to obscure the signature of a target and also to match its background. The goal is to make the detection and recognition of the target more difficult, whether the observer is a man, a machine, or both. In this paper, we are concerned specifically with the task of target discrimination. In this context, the signature strength of a target in a sensed image is equivalent to the distinctness of the image pattern representing the target from the pattern of its specific background. We complete both first- order and second-order target signature metrics for a set of 15 test images which consist of a wide variety of naturally occurring target and background patterns. To compare the quantitative measures of target distinctness to human judgments of the same attribute, the sets images were used as stimuli in a paired comparison experiment with human observers. For each comparison, the observer was asked to choose which of the two images possesses a target pattern that is more distinct from its background. The raw judgments from 20 observers were used to estimate scale values for the stimuli, indicating relative amounts of perceived target distinctness in the images. Of the metrics considered, a second-order metric based on a model of image texture was most strongly correlated with the scale values. These results are most applicable to the areas of camouflage assessment and design.