Sasan Gooran, Mahziar Namedanian, Henrik Hedman

Abstract
Printed dots appear bigger than their reference size in the original bitmap. This is because of the physical and optical dot gain. In order to overcome the problem original images are compensated for dot gain. The compensation is usually done by using a dot gain curve for each colour separation. In this paper we firstly show that using only one dot gain curve works well for black, but not for any of the other three colours, i.e. cyan, magenta or yellow. We also present a new approach to calculate colour values where three different curves are used for each colour separation. In order to evaluate the proposed approach we compare the results of our method with the results when only one dot gain curve is used for each colour, both for Murray-Davies and Yule-Nielsen models. In the case of only one dot gain curve for each separation we use the curve that gives a minimized ∆ELab using least squares method. The experiments and calculations show that our approach gives a better approximation of the resulting colour coordinates.

Keywords: Dot Gain, Colour Calculation, Halftone Print
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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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