Is CaF2 transparent?
Calcium Fluoride (Caf2) is a highly transparent optical material that is commonly used in applications such as spectroscopy, thermal imaging, and excimer laser optics. It has a broad transmission range from the ultraviolet to the mid-infrared region and is highly resistant to thermal and mechanical shock.

What is the difference between single crystal and polycrystalline thermal conductivity?
In general, polycrystals with randomly oriented crystals contain many interfaces. Hence polycrystals exhibit lower thermal conductivity than single crystals

CaF2 Elements' Applications:
Thermal imaging systems,
Spectroscopy, 
Excimer lasers, 
Fourier analysis,
Testing instruments, 
Gas liquid laser system, 
Astronomical telescope.  

Highly recommended CaF2 Optics from Yutai Optics:
0.5"1"2"CaF2 Windows,
CaF2 Lens,
CaF2 Domes,
CaF2 Prisms.

CaF2 transmission curve: