Calcium fluoride (CaF2) is often used in spectroscopic windows and lenses due to its high transmission from 250nm to 7?m. Its low absorption and high damage threshold makes it a popular choice for excimer laser optics. Calcium fluoride's low index or refraction allows it to be used without an anti-reflective coating. The Knoop hardness of calcium fluoride is 158.3. CaF2 Window is manufactured from vacuum UV grade calcium fluoride are commonly found in cryogenically cooled thermal imaging systems.