J. Andres, R. D. Hersch
Color Research & Application, Vol 39, No 4, 331–340 (2014)
We propose a method for creating authenticable color images under UV excitation backlight by printing an invisible luminescent white emissive ink layer on the verso side of a transmissive substrate and a classical cmy image on the recto side of the substrate. In order to obtain a backlit image whose colors are as close as possible to the original image colors, we map the input image sRGB color gamut into the gamut formed by the emissive white source attenuated by the classical ink halftones. The relationship between surface coverages of the classical cmy ink halftones and the resulting backlit colors is obtained by accounting for the transmission of the emissive white through the paper substrate and through the classical cmy ink halftones. The transmittance of the classical ink halftones is modeled by a new halftone absorbance prediction model. The lightness range of the luminescent backlit color gamut is expanded by printing a black and white UV-absorbing instance of the original color image in superposition with the luminescent white emissive layer and in registration with the cmy image printed on the recto side. Luminescent backlit color images provide a high anticounterfeiting security, as they combine a verso printed invisible luminescent ink layer, a verso printed black and white instance of the original image and a matching recto printed cmy instance of the original image. The resulting luminescent backlit image colors are close to the original image colors only when observed in transmission mode under UV light.