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Summary:
Measuring the color of ink on paper with a spectroradiometer has the advantage that the calibration can be done independent of illuminant and so as to minimize metamerism. The disadvantage of the approach lies in the amount of data needed for calibrations. We have explored means of reducing the bulk of data, specifically, the number of calibration patches required by a spectral scanning device to characterize a proofing system or ink-press-paper combination. For example, we asked whether the reflectance spectrum of solid cyan over magenta could be obtained as the product of a separate reflectance spectra of solid cyan and solid magenta. Not surprisingly, the answer is No, yet we pursued the inquiry in an effort to identify factors contributing to a failure of additivity of log reflectances, for which we might compensate in synthesizing the reflectance spectra of complex overprints. We have compared a press and a proofing system for which trap may be presumed to be ideal (3M's Matchprint I). Apart from trap, discrepancies between calculated and measured spectra can be attributed to a minimum reflectance which appears to depend on both wavelength and ink and is probably the effect of differences in opacity (scatter), interlayer reflections, and gloss. Experiment suggests that internal reflection and light scattering are the most important effects.