an opaque plate is positioned in front of a patient undergoing color vision testing . a number of openings are formed in the plate . behind each disk is a hollow sphere having colored light sources mounted therein . by selectively energizing the colored light sources in a sphere , a mixture of light is presented at the corresponding transparent disk at a selected intensity . a tester can select one or more spheres to be internally illuminated by color light sources so that the patient is presented with a pattern of selectable color mixtures which checks his color vision .