Next time you have or come across a dead computer screen, (some) flat screen TVs, photo frame or whatever, don't just dispose of it - take it apart - particularly the screen itself. You will find inside sheets of "useful" material including usually a film or thin sheet that is a perfect "ground glass" with a shiny surface and a non-shiny surface. Usually also various other sheets of clear and transluscent plastic material to hoard away "just in case".
I have used this material to cut up and put in the film gate of cameras to check the focussing.
I have used this material to cut up and put in the film gate of cameras to check the focussing.