Microscopic Color Measurement of Halftone Prints

• Modeling halftone print reproduction is difficult, mainlybecause of light scattering, causing optical dot gain. Mostavailable models are based on macroscopic color measurements,integrating the reflectance over an area that is large relative thehalftone dot size. The reflectance values for the full tone and theunprinted paper are used as input, and these values are assumedto be constant. An experimental imaging system, combining theaccuracy of color measurement instruments with a high spatialresolution, allows us to measure the individual halftone dots, aswell as the paper between them. Microscopic color measurementsreveal that the micro-reflectance of the printed dots and the paperis not constant, but varies with the dot area fraction. Byincorporating the varying reflectance of the ink and paper in anexpanded Murray-Davies model, the resulting prediction errorsare smaller than for the Yule-Nielsen model. However, unlikeYule-Nielsen, the expanded Murray-Davies model preserves thelinear additivity of reflectance, thus providing a better physicaldescription of optical dot gain. The microscopic colormeasurements further show that the color shift of the ink andpaper depends on the halftone geometry and the print resolution.In this study, we measure and characterize the varying microreflectanceof ink and paper with respect to properties of thehalftones, using AM and FM prints of various print resolutions.

Nyckelord

Color print reproduction

Optical dot gain

Yule-Nielsen effect

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