Maybe I’m not being clear. I don’t mean radioactive metals, and I don’t mean the “flourescent” inks that look like hi-liter markers.
Have you ever gone to a Six Flags theme park, and gotten your hand stamped upon leaving? It doesn’t look like anything is there, until you go back into the park; then they hold your hand under a black light, and the stamped shape is suddenly visible. That’s the kind of ink I’m talking about.