The Color between the Dots

Reflectance models of halftone images, such as the Murray-Davis equation for monochrome and the Neugebauer equations for color, generally are not satisfactory predictors of average reflectance or tristimulus values (TSVs), even when used with the Yule-Nielsen nfactor. CIE colorimetric measurements of halftone dots and the paper between the dots for cyan, magenta, and yellow wax thermal transfer halftone images showed that the CIE tristimulus values of the dots and the paper are a function of the printed dot fractional area. The dependence of dot and paper TSVs on area was empirically modeled by a power function. Model exponents between 0.264 and 0.392 were found for the paper, and the exponents ranged from about 10-13 to 0.401 for the dot. A key finding was that the paper TSVs are linear mixtures of paper and a limiting TSV, whereas the dot TSVs are linear mixtures of the same limiting value and the solidarea TSVs. The limiting TSV was determined from the spectral product of the paper and the colorant layer transmittance. The empirical models for paper and dot TSVs were used to predict the X, Y, and Z TSVs for cyan, magenta, and yellow halftone patches. CIE L*a*b* coordinates were predicted to within an average color difference of 2.95 for the cyan and magenta, and 4.49 for the yellow, both within the variability of the particular printer.