I was referring to fused quartz that shows a UV cutoff <200nm. I don't think the scanner platten is made of that, as opposed to polished glass that will pass UVA to an extent, but not UVB or UVC. Quartz would be prohibitively expensive for a scanner and of little value, as the bulb is coated to block short wavelength UV. Fisher Scientific has quartz 6" X 6" for about $230. The scanner is 9X11.
Modern rolled window glass does not cause enough optical distortion to matter on the PCBs I make.
John
Edit: Some plastics such as polypropylene and polyethylene, also pass UVA, even UVB, but one must be careful as UV blocking agents are often added to increase UV stability. Data on UV transmission of specific plastics is hard to find. I've tried to get that information for the films I use to make my transparencies, but the manufacturers just will not give it to you.