Calcium fluoride (CaF2) is able to withstand maximum temperatures of up to 800°C in dry atmosphere. In fact, this material is used quite commonly in cryogenically cooled thermal imaging systems. Due to its high transmission from 130nm to 10μm, it is also used in spectroscopic windows and lenses. Calcium fluoride’s low refractive index allows it to be used without an anti-reflective coating.
The Knoop Hardness of CaF2 is 158.3. Out of all fluoride crystal materials, CaF2 is known to be one of the hardest materials. This is the reason why it is particularly well-suited for use in the manufacturing of a wide range of different lenses, prisms and mirror substrates.In addition, calcium fluoride is both physically stable and chemically inert, which means that windows made of it are highly resistant against any impact.
At high temperature, calcium fluoride crystals are quite weak and react very sensitively to stress