The goal of this research is to find signature strength metrics that correspond closely to the distinctness of a target as perceived by humans, especially for camouflaged targets. We consider metrics that attempt to measure the strength of three perceptual cues: contrast, texture differences, and boundary strength. The contrast cue is measured with first-order metrics, differences in texture are measured with a second-order metric, and boundary strength is measured by computing contrast along the target-background boundary. We discuss a psychophysical experiment designed to generate quantitative measurements of perceived target distinctness for comparison with the target signature strength metrics. This experiment involves paired comparisons of image stimuli, each containing a single random target pattern embedded at a known location in a random background pattern. The data from the psychophysical experiment are compared with computed values of the target signature strength metrics, and a second-order image texture metric was found to exhibit the strongest correlation with the human data.