Specular color imaging on a metallic substrate

We propose a full reproduction workflow for printing color images on metallic substrates. It relies on an ink spreading enhanced cellular Yule-Nielsen modified spectral Neugebauer model, calibrated with 35 color samples printed on the metal film and measured under specular reflection. The calibration accounts for the different phenomena contributing to the reflectance of halftone prints on metal: specular reflection by the metal film substrate of light traveling across the inks, illumination and viewing geometry, shadowing effect induced by the ink and difference in surface structure between the inked and non-inked metal halftone elements. The model enables predicting printable colors with an average CIELAB ∆E94 error of 1.7. Thanks to the model, the metal print gamut is established and a 3D table provides the correspondence between printable metallic colors viewed under specular reflection and the corresponding ink surface coverages. The input sRGB gamut is mapped into the print gamut. At halftone image generation time, surface coverages of the inks yielding the desired gamut mapped input colors are obtained from the 3D table. These ink surface coverages yield the ink separations that are halftoned and printed. The resulting color images printed on a silver substrate viewed under specular reflection reproduce the hues at a high degree of fidelity. The luminance of metallic prints under specular observation is generally higher than the luminance of paper under the same illuminating conditions. Therefore, the printed metallic colors appear more colorful. Such metal prints are attractive for design, art and publicity. Their high brightness immediately strikes the observer and transmits the message incorporated into the reproduced picture or artwork.