Germanium is the default high-index workhorse for uncooled LWIR imagers (typically 8–12 µm or 8–14 µm). Its refractive index enables compact lens counts; its transmission window covers the thermal atmospheric band used by most commercial bolometer cameras. That does not mean “any Ge blank will do.”
Optical design implications
High index means strong power in thin elements, but also strong reflection at each air–glass interface unless AR-coated. Single- and multi-layer LWIR AR stacks are not cosmetic — they set average transmission across the band and drive narcissus and cold-stop efficiency in the camera. Specify the band (for example 8–12 vs 7.5–13.5) before freezing coating design.
Material and fabrication
IR-grade Ge with low absorption is assumed for imaging; polycrystalline vs monocrystalline choices affect cost, size and sometimes polish or diamond-turn strategy. Ge machines well (including diamond turning for aspheres), which is why so many LWIR aspheric singlets and hybrids are Ge-based. Hardness is lower than sapphire or some chalcogenides — handle coatings and cleaning protocols carefully for fielded systems.
Environmental caveats
Ge is opaque in the visible (useful as a cold shield in some layouts, inconvenient for visual alignment). Thermal runaway risk exists in high-power laser paths — Ge is primarily an imaging / passive IR material in most OEM RFQs, not a CO₂ focusing optic substitute for ZnSe.
Compare Ge against chalcogenide and ZnSe at your band in the Spectral Explorer, then send the print via Request a Quote.