Inks that are invisible under daylight convert to a strong color when illuminated with UV-light of a defined spectral peak (254 nm, 365 nm). Fluorescent inks, which re-emit the absorbed radiation immediately and phosphorescent inks which re-emit light with a delay and therefore afterglow, are in the program. A large range of hand held light sources allow for instant detection. Mono- or bi-luminescent inks are available.