Biological evolution has adapted human vision to terrestrial light contrasts. Earthly scenes have a typical bimodal contrast histogram, with a reflection mode and a shadow mode. Consequently, human sensitivity to gray-scale differences also is bimodal. Luminance differences are most discriminable at the modal intensities of the terrestrial contrast distribution so that vision conveys maximum information about the world. By inverting this biologic, an image can be computationally adapted to the human visual system. Using Paul Whittle's model of gray-scale sensitivity as a basis, distinct pixel intensities in image data are mapped to optimally discriminable displayed luminances. Since this approach is scene dependent and display dependent, the optimum gray scale must be recomputed for each displayed image, background luminance and display environment.