Zinc sulfide is what you buy when the program needs more visible than germanium can offer and still has to reach MWIR/LWIR. Raytheon industrialized CVD ZnS in parallel with CVD ZnSe in the mid-1970s (Harris, SPIE 1999, ch. 6). Yellow CVD ZnS is the IR grade. Multispectral “clear” ZnS is typically HIP-treated CVD (the Cleartran / MS-ZnS family) so visible scatter drops enough for boresight and dual-band sensing. Name the grade. “ZnS window” on an RFQ is not a grade.
Harder than ZnSe, worse at 10.6 µm
ZnS is the rain-and-sand crystal relative to ZnSe. It is also usually higher absorption and scatter at the CO2 line than laser-grade CVD ZnSe. Do not swap it onto a kW cutting head to “save cost.” Li’s CINDAS compilation (J. Phys. Chem. Ref. Data 13, 103, 1984) is still the index table people fit; use the grade datasheet for the lot you will cut.
Coatings
Dual-band AR (visible + IR) is a coating design, not a polish. Hard-carbon / DLC overcoats show up on outdoor prints; qualify abrasion and humidity separately from T. Use the Spectral Explorer at both wavelengths, then the quote form with both curves stated.
References. Harris, D.C., Materials for Infrared Windows and Domes, SPIE (1999), ch. 6 CVD ZnS/ZnSe and HIP multispectral ZnS. Li, H.H., J. Phys. Chem. Ref. Data 13, 103–150 (1984). Crystran ZnS / MS-ZnS data.