This auction is for a round zinc selenide window,  27.4mm diameter by 1.3mm thick.   Unlike most of the zinc selenide windows that you see on ebay, which are coated for CO2 at 10.6 micron wavelength, this uncoated lens can be used at any wavelength from 600nm to 20 microns, with just some reflectivity loss.  You can also use it at brewster's angle with a polarized infrared beam with no loss at all.

These windows are perfect for building your own broadband far infrared polarizer.  Just mount from 2 to 6 of them in opposing pairs at brewster's angle, and you have a polarizer that goes from 600nm to 20 microns wavelength.  Using four of these windows will give a 200:1 extinction ratio, and using six windows will give a 3000:1 extinction ratio.

This optic is brand new in the original packet.  A zinc selenide window like this will cost you $200 or more new, so here is your chance to save.

Be sure to visit my Ebay store, Cheap-Optics, for lots more great deals on mirrors, optics, Newport stuff, and other optical mounts.