Zinc Sulfide (ZnS) is an IR window material and a transparent semiconductor with a large direct band gap. It also possesses piezoelectric, photoconductive and electroluminescent properties. Zinc Sulfide is produced by a Chemical Vapor Deposition process (CVD). Two grades of ZnS are available:
Regular (FLIR) grade Zinc Sulfide is a polycrystalline optical material and has reasonable hardness. FLIR grade ZnS has yellow colour and is translucent in the visible range. It is used as deposited without further treatment. The material is often is used for IR windows and lenses in the thermal band (8 to 14 microns) as a tough front optic in thermal imaging systems, particularly those subjected to harsh environments.
Multispectral grade Zinc Sulfide is treated after growth with a special process to eliminate microscopic voids and defects which occur in the regular grade material. This extends the transmission with low absorption and scatter to include visible region to infrared range. Clear grade ZnS is used for Multispectral applications as external windows or domes that require a single aperture or beam path for several wavebands like systems they have a visible camera, a mid- and long-wave detector and a laser rangefinder.