Abstract. The proposed new spectral reflection model enhances the classical Clapper-Yule model by taking into account the fact that proportionally more incident light through a given colorant surface is

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.

We propose a new spectral prediction model as well as new approaches for modeling ink spreading which occurs when printing ink layer superpositions. The spectral prediction model enhances the classical Clapper-Yule model by taking into account the fact that proportionally more incident light through a given colorant surface is reflected back onto the same colorant surface than onto other colorant surfaces. This is expressed by a weighted mean between a component specifying the part of the incident light which exits through the same colorant as the colorant from which it enters (Saunderson corrected Neugebauer component) and a component specifying the part of the incident light whose emerging light components exit from all colorants, with a probability to exit from a given colorant equal to that colorant surface coverage (Clapper-Yule component). We also propose two models for taking into account ink spreading, a phenomenon which occurs when printing an ink halftone in superposition with one or several solid inks. Besides the physical dot gain present within a single ink halftone print, we consider in the first model the ink spreading which occurs when an ink halftone is printed on top of one or two solid inks. In the second more advanced model, we generalize this concept to ink halftones printed on top or below solid inks. We formulate for both ink spreading models systems of equations which allow to compute effective ink coverages as a combination of the individual ink coverages which occur in the different superposition cases. The new spectral prediction model combined with advanced ink spreading yields excellent spectral predictions for clustered-dot color halftone prints, both in the case of offset (75 to 150 lpi) and in the case of thermal transfer printers (50 to 75 lpi).

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.