Calcium fluoride

Calcium fluoride (CaF2) is an insoluble ionic compound of calcium and fluorine. It occurs naturally as the mineral fluorite (also called fluorspar), and it is the source of most of the world's fluorine. It reacts with concentrated sulfuric acid to produce hydrogen fluoride:
CaF2(s) + H2SO4(l) ? CaSO4(s) + 2 HF(g) This is the reaction used to produce hydrogen fluoride in industry.

Applications
Calcium fluoride is commonly used as a window material for both infrared and ultraviolet wavelengths, since it is transparent in these regions (about 0.15 µm to 9 µm) and exhibits extremely weak birefringence. Nevertheless, at wavelengths as low as 157 nm, which are interesting to semiconductor manufacturers, the birefringence of calcium fluoride exceeds tolerable limits. This may be overcome by minimizing birefringence by optimimizing the growth process. It is particularly important as an ultraviolet optical material for integrated circuit lithography. Canon also uses artificially-crystallized calcium fluoride elements in some of its L-series lenses to reduce light dispersion. As an infrared optical material, calcium fluoride is sometimes known by the Eastman Kodak trademarked name Irtran-3, although this designation is long since obsolete.
Uranium-doped calcium fluoride was the second type of solid state laser invented, in the 1960s. Peter Sorokin and Mirek Stevenson at IBM's laboratories in Yorktown Heights, New York, achieved lasing at 2.5 µm shortly after Maiman's ruby laser.