A GREEN MATERIAL MAY TURN BLUE; a pale blue material may turn purple and a white material may turn red as different colors of light shine upon them. This photochromic effect is a property of certain types of inorganic materials. Among those that have been studied are single crystals of calcium fluoride doped with rare-earth elements, and strontium titanate doped with transition metals, and the alkali halide, potassium chloride. If these materials are to be used as light-sensitive media, their sensitivity to light, their rate of switching color and their resolution are of particular importance. Once the photochromic mechanism is completely understood, its technical applications may range from computer memories to three-dimensional television.