Used for IR windows and lenses in the thermal band (8 to 14 microns) where maximum transmission and lowest absorption is required. Also selected for use where visible alignment is an advantage. Zinc Sulphide is produced by synthesis from zinc vapour and H₂S gas, forming as sheets on graphite susceptors. It is microcrystalline in structure, the grain size being controlled to produce maximum strength. Multispectral grade is then Hot isostatically pressed (HIP) to improve the mid IR transmission and produce the visibly clear form. Polycrystal ZnS is available.