Color vision is not only good for seeing surface colors, but for seeing other visual dimensions, or “modalities,” such as form, depth, material, and motion. This use of color vision relies in part on the exploitation of physical constraints that exist between the patterns of color and luminance in the natural visual world. An example of this use is the way color vision helps to distinguish variations in local illumination such as shadows and shading from variations in material properties. However for some functions, e.g., stereopsis and motion perception, color vision on its own, that is, in the absence of luminance information, requires more contrast relative to detection threshold to achieve a level of performance commensurate with that provided by luminance information.