Zinc sulfide earns its place when a program needs more visible or near-visible utility than Ge can offer, while still reaching into MWIR/LWIR. Multispectral windows, some targeting pods and dual-band sensing heads often land on ZnS (or ZnS variants) before ZnSe — especially when visible boresight or camera-through-window alignment matters.
Material families
Not all ZnS is equal. “Clear” grades emphasise shorter-wave transmission and cosmetic quality; other process routes prioritise IR performance and durability. Engineering RFQs should name the grade family and the full spectral requirement — not only “ZnS window.”
Coatings
Dual-band AR (visible + IR) is a common request: enough visible for assembly and alignment, enough IR for the sensor. Expect tradeoffs — optimising 550 nm and 10 µm simultaneously is a coating design problem, not a blank polish problem. Hard carbon or durability overcoats may appear in outdoor specifications; qualify abrasion and humidity separately from spectral transmission.
Versus ZnSe
ZnSe often wins pure high-power CO₂ focusing; ZnS often wins multispectral and durability-balanced windows. Do not swap them by habit. Scatter, hardness and absorption spectra differ enough to move MTF or laser damage thresholds.
Use the Spectral Explorer at dual-band wavelengths, then quote with both spectral curves stated on the RFQ.