Patent ID: 8085438

Claim:
A method for producing a fluorescent halftone color image which is invisible under daylight and visible under UV light, using fluorescent inks and superpositions of fluorescent inks as fluorescent emissive colorants, the method comprising the steps of (a) selecting a set of at least two fluorescent inks and superpositions of said fluorescent inks as base fluorescent emissive colorants invisible under daylight; (b) by relying on emission spectra of said fluorescent emissive colorants, determining, using a color prediction model, under ultraviolet illumination the gamut produced by juxtaposed halftones of said fluorescent emissive colorants in a colorimetric space, and with said color prediction model creating correspondences between emitted colorimetric values and surface coverages of said fluorescent emissive colorants; (c) obtaining gamut mapped input image colors located inside the gamut formed by said fluorescent emissive colorants by gamut mapping of said input image colors into said gamut; (d) converting said gamut mapped input image colors into colorant surface coverages using said correspondences between said emitted device-independent colorimetric values and said fluorescent emissive colorant surface coverages; and (e) creating fluorescent color screens invisible under daylight according to said colorant surface coverages by juxtaposed halftoning, (i) creating side by side surfaces of said fluorescent emissive colorants according to ratios of their surface coverages; (ii) rasterizing said side by side surfaces of the fluorescent emissive colorants into juxtaposed emissive colorant halftones and inserting them according to their surface coverages into corresponding juxtaposed screen element library entries; and (iii) during the creation of said fluorescent halftone color image, accessing according to said emissive halftone colorant surface coverages said corresponding juxtaposed screen element library entries and retrieving the emissive halftone colorant to be printed at a current position; and where side by side surfaces of said fluorescent emissive colorants maximize the range of emitted intensities and saturations of said emissive colorant halftone colorants by avoiding undesired colorant superpositions yielding quenching effects which may reduce and distort emitted colorant colors.