Robert Chung, Fred Hsu

Abstract
Process color printing has three chromatic inks (C, M, Y). Colorimetric properties of the two-color overprint solids (R, G, B) are measured to determine the color gamut of the process ink set. Non-process colors, as defined by the Pantone Color Library, have more than a thousand ink formulations. There are too many possible two-color overprint solids to be measured. Colors of non-process color overprints are unknown until they are printed. While ‘Overprint Fill’ is available in Adobe Creative Suites that allows one spot color overprinting to the other, there is no color management solution to ensure that the overprint solids, as displayed, correctly matched that as printed. This paper describes a spectral-based solution based on paper, 1st ink, 2nd ink, and a generalized ink trapping factor, t, to predict color of overprint solid. Color differences between predicted and printed two-color overprint solids, resulted from wet-on-wet (offset) ink transfer are discussed.

Keywords: Color, Overprint, Spectrophotometry, Ink trapping
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Michael Dattner, Heinz Mantler, Jorge Rodriguez-Giles, Peter Urban

Abstract
We propose an application oriented colour prediction model for offset, which considers the effects of light scattering and optical brightener on the perception of printed colour. This model considers all underlying physical phenomena nearly without empirical parameters. It is based on the new developed wavelengths-dependent-area-coverage approach, which considers every current process specific parameter by considering the complete spectral information in an extended area coverage approach. This effective wavelengths-dependent-area-coverage (wda-coverage) is directly related to the constant nominal area coverage and allows the calculation of excellent spectral predictions for colour halftone prints. Optical brightener and the super positioning of inks are considered in the calculation of the reproducible colour space.

Keywords: Offset, Spectral colour prediction model, Light scattering, Wavelength dependent area coverage, Brightener
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Li Yang

Abstract
Perfect light diffuseness inside the medium, such as a paper sheet, is the fundamental assumption when applying the Kubelka-Munk (K-M) model. In reality, this pre-condition is seldom fulfilled, which is unfortunately either not well noticed or ignored by users. Nevertheless, this is little troublesome as long as it is applied to media of similar light diffuseness. When applied to media of significantly different light diffuseness great discrepancies between theoretical predictions and measurements may occur. Besides the intrinsic optical properties, there are other factors that affect light diffuseness inside the medium, such as illumination geometries, medium thickness, etc. In this work, we proposed a solution that enables the K-M model to be applicable across media of different light diffuseness. In the proposed model, the light distribution is explicitly expressed with a parameter, b, whose value equals to unity (b=1) for a perfect diffuse light and zero (b=0) for a collimated light distribution, respectively. The proposed model was verified by applying to papers of different light diffuseness, caused by different illumination geometries and different thickness. The spectra of light reflection as well as light transmission predicted with this model agree very well with measurements.

Keywords: Extended Kubelka Munk model, Light scattering & absorption, Partial light distribution.
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