Printing color images visible under uv light on security documents and valuable articles
In the present invention, we propose a method of creating fluorescent color images visible under UV light. It relies on the new colorants that can be achieved by superposing ink dots, possibly at a reduced size, in order to avoid quenching effects. It also relies on juxtaposed halftoning, which ensures that colorants are printed side by side and do therefore not overlap, thereby preventing quenching effects. It also support the selection of a fluorescent set of inks which comprises at least one ink whose emission spectrum yields a color different from the standard red, green and blue colors, for example a yellow color emitting ink. The method comprises the following techniques: (a) creating new colorants by superposing carefully selected amounts of the fluorescent inks, (b) mapping the gamut of the image to be reproduced into the gamut of the resulting fluorescent colorants and (c) creating the target fluorescent color image by juxtaposed halftoning of the fluorescent colorants. Juxtaposed halftoning avoids quenching effects by creating diagonally oriented pre-computed colorant screen dots, which are printed side by side.' 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. Applications comprises the protection of security documents such as bank notes, passports, ID cards, entry tickets, travel documents, checks, vouchers or valuable business documents as well as valuable articles such as CDs, DVDs, software packages, medical drugs, watches, personal care articles, and fashion articles. And a last application is art, decoration, publicity, fashion, and night life, where fluorescent images viewed under UV illumination at night or in the dark have a strongly appealing effect.