Fluorides - Calcium, Barium, Magnesium

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 optmimizing 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.

Barium Fluoride is transparent from the ultraviolet to the infrared, from 150-200 nm to 11-11.5 µm, and can be used as a material to make optical components such as lenses. It is used eg. in windows for infrared spectroscopy, in particular in the field of fuel oil analysis. Its transmittance at 200 nm is relatively low (0.60), but at 500 nm it goes up to 0.96-0.97 and stays at that level until 9 µm, then it starts falling off (0.85 for 10 µm and 0.42 for 12 µm).

Magnesium fluoride is transparent over an extremely wide range of wavelengths. Windows, lenses, and prisms made of this material can be used over the entire range of wavelenths from 0.140 μm (ultraviolet) to 8.0 μm (infrared). The cost of producing optical elements from this material—as of 2004 one vendor charged nearly $500 for 25-mm diameter magnesium fluoride lenses and windows—limits its use to specialized applications. As an infrared optical material, it is sometimes known by the Eastman Kodak trademark Irtran-1.