What is Multispectral ZnS?

Multispectral Zinc Sulfide (ZnS‑MS) is a water‑clear, polycrystalline IR optical material engineered to pass a very broad spectral band—from ~0.37 to 13.5 µm—so a single optic can support visible, SWIR, MWIR, and much of the LWIR. That visible‑through‑IR reach is the defining advantage versus FLIR‑grade ZnS, which is optimized for IR and not intended for true visible transparency.

ZnS‑MS is widely chosen for windows and lenses in systems that must image in the thermal band (8–14 µm) yet still benefit from visible‑band transmission—for example, to enable visible alignment or to share a “common aperture” among visible and IR sensors.

Both FLIR‑grade ZnS and multispectral ZnS start as CVD‑grown zinc sulfide: zinc vapor reacts with H₂S and deposits as microcrystalline ZnS sheets on heated graphite susceptors, with grain size controlled for strength. Multispectral grade then undergoes hot isostatic pressing (HIP), a high‑temperature, high‑pressure post‑process that removes pores and scattering centers and yields a visibly clear form with extended short‑wavelength transmission.

In practice, that single extra HIP step is what converts a good IR material into a broadband, “water‑clear” optic suitable for simultaneous visible and IR work.

ZnS‑MS vs. FLIR‑grade ZnS

Processing:

• FLIR grade: CVD ZnS used essentially as‑deposited (no HIP). Appearance is pale yellow/translucent in the visible. Selected as a tough front optic for thermal imagers, especially in harsh environments.

• Multispectral: CVD + HIP → water‑clear and broadband (adds visible/SWIR capability).

Spectral band:

• FLIR grade: published transmission band ~8–14 µm (IR‑centric).

• Multispectral: ~0.37–13.5 µm, adding the visible and deeper SWIR.

Use‑case emphasis:

• FLIR grade: Prioritizes environmental durability as a front window in the 8–14 µm band (e.g., thermal cameras in rain/sand).

• Multispectral: Prioritizes broadband imaging (VIS+MWIR/LWIR) and visible alignment, often behind a protective element or where the environment is less erosive.

Selected property contrasts:

• Thermal conductivity: ~16.7 W·m⁻¹·K⁻¹ (FLIR) vs ~27.2 W·m⁻¹·K⁻¹ (MS).

• Elastic/strength metrics: FLIR data lists a higher apparent elastic limit (on the order of 100 MPa) than MS (tens of MPa), consistent with its use as a “tough front optic.” (Exact values vary by vendor datasheet and test method.)

Practical takeaway: Choose ZnS‑MS when you must combine visible/SWIR with MWIR/LWIR in one optic or need visible‑band alignment through the same aperture. Choose FLIR‑grade ZnS when the optic’s main job is surviving weather/erosion as a front window in the 8–14 µm band.

Applications of Multispectral ZnS

• Common‑aperture imaging where a single window must pass both visible and thermal bands (e.g., systems carrying visible + MWIR/LWIR sensors behind one protective element).

• Alignment‑friendly thermal optics where visible transmission is useful during system setup/boresighting, yet the optic will operate primarily in the 8–14 µm band in service.

• General VIS–IR sensor windows and lenses requiring low scatter/low absorption across a broad band; available with 3–12 µm BBAR coatings for improved throughput.

Design & integration notes

• AR coatings are essential: With an index ~2.2 at 10 µm, uncoated two‑surface reflection is ~25% at 10 µm; broadband IR AR coatings (e.g., 3–12 µm) are routinely offered to recover throughput.

• Thermal/environmental limits: In air, keep service temperatures ≤ ~250 °C to avoid oxidation and property degradation. For elevated‑temperature or erosive front‑window duty, evaluate FLIR grade or protective sacrificial windows.

• Handling/cleaning: Follow specialty‑optic handling practices; vendors note that acetone may be used with care to remove particulates/debris.