A comment someone left on a post I made a year ago (when I was having a slightly different problem with a form 2 at a different studio) may have cast some light on this:
This makes me believe that, possibly, the optic window may just be dirty. Or in need of replacing. I’m going to do some cleaning and then run the test again, but I don’t know how to find someone who sells replacement windows, so I don’t know if that will be an option.