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.

The magic lens

“…Classical "invisible ink" writing using for instance lemon juice is only revealed when exposed to a catalyst such as heat. More recent computational approaches use metallic inks to embed images that only appear under specular reflection [Hersch et al 2003], or print with specialized inks that are only visible under UV light [Hersch et al 2007]. Band Moiré images [Hersch and Chosson 2004] encode a hidden image in interfering Moiré patterns caused by superimposing transparent sheets.…”

Printing spatially-varying reflectance

“…Hersch et al [2007] created gamutmapping and halftoning methods for printing images with custom fluorescent inks: the printed images are invisible under daylight and indoor lighting, but become visible when illuminated with UV light. Most relevant to our work is the method of Hersch et al [2003], who combined a metallic ink with standard inks to print color images with embedded metallic patterns. Such printed images have locations that exhibit a particular amount of specular reflection.…”

Printing spatially-varying reflectance

“…Hersch et al [2007] created gamutmapping and halftoning methods for printing images with custom fluorescent inks: the printed images are invisible under daylight and indoor lighting, but become visible when illuminated with UV light. Most relevant to our work is the method of Hersch et al [2003], who combined a metallic ink with standard inks to print color images with embedded metallic patterns. Such printed images have locations that exhibit a particular amount of specular reflection.…”