CaF2 vs. Sapphire windows

The Edmund optics catalog says "Calcium Fluoride is also often found in cryogenically cooled thermal imaging systems". Why is it prefered
for this purpose? I might want to put an IR window in a cell that would be filled with liquid nitrogen; this would mean considerable thermal stress. Sapphire is strong, cheap, hard, and undamaged by moisture (you could wipe condensation off of it with no worries) but is sold thinner than other materials, thus increasing the thermal gradient.
For that matter, what sort of O-ring would be usable in a cell that will be filled with LN2? I don't mind some leakage, I just don't want the window or the O-ring to crack.
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Paul Ciszek wrote:

Calcium fluoride is used because it transmits the wavelengths used in this type of imaging system (somewhere in the 3 to 5 micron range, IIRC). However I don't think the window itself is not subjected to the LN2 temperatures.
Don't think any elastic-type o-rings would stand up to LN2. Remember the argument about space shuttle o-rings? And that was just at 0 C! If NASA can't afford better o-rings than that, neither can you ... or they don't even exist.
Best to design the vessel so that any seals are far removed from the cold portion. Thin-walled stainless provides enough thermal resistance for this.
Mark
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