OST Photonics CaF2 Crystals

https://www.ostphotonics.com/products/caf2-calcium-fluoride-crystals.html

Calcium fluoride is often used in spectroscopic windows and lenses due to its high transmission from 250nm to 7μm. 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. Calcium Fluoride (crystran CaF2) windows manufactured from vacuum UV-grade calcium fluoride are commonly found in cryogenically cooled thermal imaging systems.