The stuff youre talking about where forensic scientists spray stuff, they're spraying Luminol to detect trace amounts of blood because it reacts with the iron in the blood traces and makes it luminescent enough for it to be photographed in a darkened room even without UV.
Luminol is a chemical that reacts with hemoglobin to glow where very small amounts of blood might have been. That's usually sprayed, for detecting much smaller concentrations than what would show up under a UV light. That might be what you're thinking of.