I'm looking for 'watermark' and or invisible ink. That only shows up when held to a light source. For example say I printed trees on a white sheet of paper, and when I held said image up to a light source, a sun would appear, or an image that was otherwise invisible. I've looked around and also herd things about photo emulsion (have no idea how this relates to watermarks), screenprinting, and stamps with regards to watermarks. Any and all help/suggestions appreciated.