Modeling color reproduction in halftone printing is difficult, mainly because of light scattering, causing optical dot gain. Most available models are limited to macroscopic color measurements, averaging the reflectance over an area that is large relative the halftone dot size. The reflectance values for the full tone ink and the unprinted paper are used as input, and these values are assumed to be constant. An experimental imaging system, combining the accuracy of color measurement instruments with a high spatial resolution, allows us to measure the individual halftone dots, as well as the paper between them. Microscopic reflectance measurements reveal that the micro-reflectance of the printed dots and the paper between them is not constant, but varies with the ink area coverage. By incorporating the varying micro-reflectance values of the ink and paper in an expanded Murray-Davies model, we have previously shown that the resulting prediction errors are smaller than for the commonly used Yule-Nielsen model. Moreover, unlike Yule-Nielsen, the expanded Murray-Davies model takes into account the varying micro-reflectance for the printed dots and the paper, thus providing a better physical description of optical dot gain in halftone prints.
In this study, we further extend the proposed methodology to handle color prints, predicting tristimulus values for prints with multiple and overlapping colorants. After converting the microscopic images of halftone prints into CIEXYZ color space, tristimulus values for the paper and the different combinations of ink are computed from CIEEXYZ histograms. From the microscopic images we can also compute the physical ink area coverage for each of the Neugebauer primaries, which typically differ from the nominal one, due to physical dot gain. The result is an expanded Neugebauer model, taking into account how the tristimulus values of the paper, the primary inks and the overlapping secondary colors, vary with the total ink area coverage. Experimental results confirm the accuracy of the proposed methodology, when compared to measurements using a spectrophotometer. The results have shown that the variation of the micro-reflectance of the Neugebauer primaries is large, and depends on the total ink area coverage. The results further show that the way that the micro-reflectance vary is also strongly dependent on the surrounding inks, because of light scattering in the substrate.
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