IR polycrystalline, a structure with multiple crystals applied beyond 400 NM.

The typical characteristic is the presence of distinct grain boundaries, the number of grain boundaries is greater than one, mostly between two and four, and the number is not controllable.

The permeability of polycrystals is the same as that of single crystals, the chemical properties are the same and the physical mechanical properties are not significantly different from those of single crystals, but the optical delay properties are not as good as those of single crystals.

Because of the homogeneous properties of calcium fluoride and the high yield and low price of polycrystals, they can be used for protective windows, single lenses or single prisms, but for high power laser applications or for lenses in a set, single crystals are often recommended.