Calcium fluoride is the classic broadband crystalline window when an instrument must see from the UV into the mid-IR without jumping to opaque-in-VIS materials like Ge. Spectrometers, beam delivery for multi-line lasers and research instruments still specify CaF₂ when the spectral story is wide and the mechanical environment is controlled.
Spectral personality
Deep UV transmission (grade-dependent) plus usable IR into the mid-wave makes CaF₂ a bridge material. It will not own the full 8–12 µm thermal band the way Ge does; know where the long-wave edge falls for your thickness.
Mechanical and environmental
Soft relative to sapphire; sensitive to thermal shock if mishandled; coatings and mounts must respect CTE. For harsh outdoor windows, sapphire or ZnS architectures often win despite narrower or different spectral tradeoffs.
Coatings
Broadband AR across UV–IR is a coating fantasy if taken too literally — define the band that matters (for example 200–400 nm vs 3–5 µm) and accept optimisation. Uncoated CaF₂ may be acceptable in some low-power instrument paths; measure first.
For multi-instrument OEMs, CaF₂ often sits beside fused silica (UV–VIS workhorse) and Ge/ZnSe (thermal). Map the band once in the Spectral Explorer, then write one coherent window strategy into the RFQ.