the card come with 2 peices of polarized film taped to card stock, and folded over in a greeting card fashion. The front half of the card stock has a square cut out to expose the polarized film (like a window), and the back side of the card does not… the polarized film is just taped to the card stock with no window.
So, when you sandwich your fused test piece between them, and hold it up to a light… of course, no light at all comes through because the back of the card stock is solid (no window). I can’t see a thing on any of my tests.
I watched the Kaiser YouTube video on how to do it, but the video never exposes the back side of the card to see if the window is cut out behind the film. Shouldn’t both the front and the back have a window cut out, with the film taped over it?