BaF2 (Barium Fluoride) optical lens

BaF2’s transmittance at 200nm is relatively low (0.60), but at 500nm it goes up to 0.96-0.97 and stays at that level until 9μm, then it starts falling off (0.85 for 10μm and 0.42 for 12μm). The refractive index is about 1.46 from 700nm to 5μm. Barium Fluoride is used as a pre opacifying agent and in enamel and glazing frits production. It is also used in metallurgy, as molten bath for refining aluminum.

Highly recommended BaF2 Optics from Yutai Optics:
BaF2 lens,
BaF2 windows.