Buying 2nd hand CaF2 - a cautionary tale

[JMC]

A few weeks ago I was on a 'well known internet auction site' and I came across someone in Russia selling 3 calcium fluoride windows. Each of them in metal housings, and some back story about them coming from a Russian military research site. Anyway, they looked like reasonable prices for CaF2 windows, and I made him an offer. Price for them was ok, I thought, although the shipping made me wince a bit. Given I have the transmission measurement capability now with my Ocean Optics setup, I thought I would run them through and see how much light they were letting through. Firstly, the 3 windows - Large, Medium and Small in the picture below (scale is in cm).

 

This is what the transmission spectra look like;

 

Strange I thought, surely CaF2 is transparent to UV and there is a lot going on here, especially with the small window. So a quick look and I find that there are different grades of CaF2 for different applications, and the IR grades often have impurities which lead to absorption bands in the UV. Here's a picture from a website by Crystans;

 

It's a bit of a low res image unfortunately, but the scale along the bottom goes from 200 to 400nm in 50nm jumps. The UV grade CaF2 is pretty smooth all the way along, but the IR grades have varying amounts of absorption. Here's the website, in case it's of interest (http://connect.physicsworld.com/optics-and-lasers/crystran-newsletter-march-2017/2004426.article).

 

So, a cautionary tale, not all CaF2 is the same.... To be honest I got these because they were quirky. Am I annoyed they're not UV grade, not hugely. Would have been cool if they were, but c'est la vie.