Microscopic Color Measurements of Halftone Prints

Abstract

Modeling halftone print reproduction is difficult, mainly because of light scattering, causing optical dot gain. Most available models are based on macroscopic color measurements, integrating the reflectance over an area that is large relative the halftone dot size. The reflectance values for the full tone 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 color measurements reveal that the micro-reflectance of the printed dots and the paper is not constant, but varies with the dot area fraction. By incorporating the varying reflectance of the ink and paper in an expanded Murray-Davies model, the resulting prediction errors are smaller than for the Yule-Nielsen model. However, unlike Yule-Nielsen, the expanded Murray-Davies model preserves the linear additivity of reflectance, thus providing a better physical description of optical dot gain. The microscopic color measurements further show that the color shift of the ink and paper depends on the halftone geometry and the print resolution. In this study, we measure and characterize the varying micro-reflectance of ink and paper with respect to properties of the halftones, using AM and FM prints of various print resolutions.