A Close-Up Investigation of Halftone Color Prints

• Modeling the color reproduction of halftone prints is difficult, 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 dot size. The aim of this study is to go beyond the macroscopic approach and study optical dot gain on a micro-scale level, using colorimetric images of printed halftones. An experimental imaging system, combining the accuracy of color measurement instruments with a high spatial resolution, opens up possibilities to better study the color reproduction in halftone color prints. The main focus is to study how the reflectance values of the printed dots and the paper between the dots, Ri(Fi) and Rp(Fi), vary with the dot area fraction. Micro-scale images, i.e. when the resolution of the images is high in relation to the resolution of the halftone, allow for measurements of the individual halftone dots, as well as the paper between them. To capture the characteristics of large populations of halftone dots, histograms are computed. From the histogram data it is possible to derive the mean reflectance, R, the reflectance for the dots, Ri(Fi), and the paper between the dots, Rp(Fi). The true dot area coverage, including the physical dot gain, is computed using histogram data, as well as using line scans in the micro-scale images. A previously proposed extension of the Murray-Davies equation, incorporating Ri(Fi) and Rp(Fi), is evaluated. The model is further extended to handle color prints, predicting tristimulus values, by using 3D histograms in CIEXYZ color space. To reduce the complexity, projection from XYZ coordinates into one dimensional color distributions are used. The prediction errors of the model were found to be equivalent, or better, to that of the Yule-Nielsen model using an optimal n-factor. However, unlike Yule-Nielsen, the extended Murray-Davies model preserves the linear additivity of reflectance, thus providing a better physical description of optical dot gain in halftone color prints.

Nyckelord

Color print reproduction

Optical dot gain

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