Color images visible under UV light
The present contribution aims at creating color images printed with fluorescent inks that are only visible under UV light. The considered fluorescent inks absorb light in the UV wavelength range and reemit part of it in the visible wavelength range. In contrast to normal color printing which relies on the spectral absorption of light by the inks, at low concentration fluorescent inks behave additively, i.e. their light emission spectra sum up. We first analyze to which extent different fluorescent inks can be superposed. Due to the quenching effect, at high concentrations of the fluorescent molecules, the fluorescent effect diminishes. With an ink-jet printer capable of printing pixels at reduced dot sizes, we reduce the concentration of the individual fluorescent inks and are able to create from the blue, red and greenish-yellow inks the new colorants white and magenta. In order to avoid quenching effects, we propose a color halftoning method relying on diagonally oriented pre-computed screen dots, which are printed side by side. For gamut mapping and color separation, we create a 3D representation of the fluorescent ink gamut in CIELAB space by predicting halftone fluorescent emission spectra according to the spectral Neugebauer model. Thanks to gamut mapping and juxtaposed halftoning, we create color images, which are invisible under daylight and have, under UV light, a high resemblance with the original images.