Reproducing color images with embedded metallic patterns

Abstract

By combining a metallic ink and standard inks, one may create printed images having a dynamic appearance: an image viewed under specular reflection may be considerably different from the same image viewed under non-specular reflection. Patterns which are either dark or hidden become highlighted under specular reflection, yielding interesting visual effects. To create such images, one needs to be able to reproduce at non-specular reflection angles the same colors, by standard inks alone or in combination with a metallic ink. Accurate color prediction models need to be established which model the underlying physical phenomena in a consistent manner. To meet this challenge, we propose two models, one for predicting the reflection spectra of standard inks on coated paper and one for predicting the reflection spectra of a combination of standard inks and a metallic ink. They are enhancements of the classical Clapper-Yule model which models optical dot gain of halftone prints by taking into account lateral scattering within the paper bulk and multiple internal reflections. The models we propose also take into account physical dot gain and ink spreading for standard inks as well as the low reflectance of metallic inks at nonspecular reflection angles and the poor adherence of standard inks printed on top of a metallic ink (trapping effect). These models open the way towards color separation of images to be reproduced by combining a metallic ink and standard inks. Several designs printed on an offset press demonstrate their applicability and their benefits for high-end design and security applications.

Enhancing the Specular Effect of Metallic Color Prints by Reducing the Use of Yellow Ink

“…1. We, then, show that the specular reflection of the silver ink is not visible in the presence of yellow.…”

“…3 This extension incorporates special parameters to take into account specificities of the silver ink such as large ink trapping and a raised metallic reflection spectrum in the presence of other inks. 1 These specificities increase the complexity of the model. We further refer to this model as the Clapper-Yule based MetallicArt model (CY-MArt).…”

“…With these effective colorant coverages, the YNSN model calculates the predicted reflection spectrum. Before comparing the accuracy of the IS-YNSN model and the prior art CY-MArt model, 1 we must calibrate these models. The calibration patches of the CY-MArt and IS-YNSN models are composed of the 16 solid colorants (Neugebauer primaries) as well as 4 patches for each ink spreading curve, i.e., the ink spreading curves are calibrated at 20%, 40%, 60%, and 80%.…”

“…The resulting strong specular reflection has been used to hide metallic patterns in normal CMY images, a technique called MetallicArt. 1 Using silver ink, this technique consists in determining the image pixels located within a given pattern and to replace their original CMY coverages by CMYS (cyan, magenta, yellow, and silver) coverages containing as much metallic ink as possible, while preserving the original CIELAB color under nonspecular viewing conditions. The preservation of the original color under nonspecular viewing conditions is achieved using a spectral reflection prediction model.…”

Color imaging and pattern hiding on a metallic substrate

“…They measured increases in lightness and chroma when evolving from non-specular to specular observation angles. Hersch et al [2003] created a new security feature by combining a silver ink and standard inks to print on paper color images with embedded metallic color patterns. In contrast to the present contribution, the early prediction model used in that work was able to hide these patterns only under non-specular observation angles.…”