Color vision testing can be accomplished by several means, each of which has its virtues and limitations. The most common test of color perception is the pseudoisochromatic plate. This is a card composed of a field of dots of varying hue and saturation within which persons with normal color vision can see some figure, but persons with a color vision defect cannot. Alternatively, some pseudoisochromatic plates are constructed so that color normals see one figure and color defectives another. These tests rely upon either the reduced hue discrimination or the reduced saturation discrimination of color defectives as a means of identification of these color defectives. Unfortunately, the illuminant under which the test is conducted is critical, and the diagnostic value of this type of test is limited.