Brightidea

Hi, I'm currently in the process of designing a product. It needs to work like invisible ink, but instead of reacting to UV light, I need it to show up when exposed to a normal bright light source. For instance if this clear ink was used on a piece of paper, then photocopied, it would show up on the print. Although the ink on the original piece of paper would remain invisible. I'm not sure if any chemicals used during the process of film developing would be useful? The chemical must be safe to use and be non permanent if used on glass. Any comments or suggestions would be greatly appreciated. Thanks