Color Changing Prints on a Metallic Substrate

Printing on a flat metallic substrate has gained popularity in recent years. The reflection of light on a specularly reflecting metallic substrate differs strongly from the reflection of light on a diffusely reflecting medium such as paper. Direct prints on a pure metallic substrate provide bright and brilliant colors when seen under specular reflection. We adapt the classic color-reproduction workflow in order to print color images on highly specular metallic sheets. The goal is to accurately reproduce color prints on metal. The workflow relies on the ink spreading enhanced cellular Yule-Nielsen modified spectral Neugebauer model (IS-CYNSN), calibrated with printed patches measured under specular reflection. The specular measuring apparatus consists of a panel light source illuminating the sample at 25° angle and an optical fiber collecting the light reflected from the sample at the opposite 25° angle. This apparatus simulates the conditions when the sample is specularly illuminated with light coming from a window. The resulting color images printed on metal show, under specular reflection, a colorful appearance unmatched by prints on paper. Some printers such as the Epson 7900 can print diffuse white ink on top of the metallic sheet. The white ink halftone reduces the specular reflection and increases the diffuse reflection of the print. We rely on the trade-off between amounts of the white ink and the amounts of colored inks to independently control lightness under specular and under non-specular observation conditions. The corresponding color-reproduction workflow relies on a spectral prediction model specially conceived for predicting the colors under both specular and non-specular viewing conditions. By optimizing the ratio of the white and the color inks to control lightness, we are able to hide patterns or grayscale shapes under one viewing condition, specular or non-specular, and reveal them under the other viewing condition. We extend this approach to produce prints that alternate between two independent grayscale images. We discovered that anisotropic line halftones printed on the metallic sheet change color upon in-plane rotation. We propose a color-reproduction framework for printing images that change colors upon azimuthal rotation by 90o. This framework is based on the directional optical dot-gain that occurs when the line halftone printed on metal is viewed under specular reflection. This effect depends on the orientation of the line halftone. It greatly effects the chroma and the lightness of the print. We created a spectral prediction model for predicting the color of non-rotated and of 90o in-plane rotated cross-halftones formed of superpositions of horizontal and vertical cyan, magenta and yellow line halftones. Desired non-rotated and rotated image colors are mapped onto the sub-gamut enabling for the desired color change and then, by using a 6D correspondence table, are color separated to produce optimal cross-halftone ink surface coverages. The proposed color changing framework is especially effective for creating surprising effects such as image parts whose hues change, gray regions that become colorful or alternations between two independent color images. Applications could include art, advertisement, exhibitions and document security.