Infrared (IR) radiation defines the region of the electromagnetic spectrum where the wavelength (~750 nm) of light is no longer visible to the unaided human eye to the border of the microwave region (~ 1 mm). IR radiation is commonly sensed as heat. Some materials appear black in visible light yet transmit a broad spectrum of IR light. These materials have many important uses in medical, industrial, ecological, R&D, and military applications. Some of these applications require unique crystal materials, such as cadmium sulfide, cadmium selenide, cadmium telluride, silver gallium sulfide, silver gallium selenide, and zinc telluride.