CaF2 Optics

https://www.yasioptics.com/products/caf2-optics/

Calcium fluoride (CaF2) crystals have excellent transmittance between 250 nm and 7 μm, which is widely used in making beam splitters, windows, and lenses. It has good transmittance in the ultraviolet, visible, and infrared wavebands, so it is widely used in the technologies of lasers, ntrared otcs, ultravolet optics, and energy detectors, et cetera.