A spectral model was derived to predict the spectral reflectance factor of colors formed using a color proofing system simulating offset printing. A first-order model was based on the spectral Neugebauer equation modified by the Yule-Nielsen correction in which n was assumed to vary as a function of wavelength. The n[z] and effective dot areas were optimized using primary (cyan, magenta, yellow, and black) halftone tints. Systematic errors were observed. The systematic error behaved in a similar fashion to the phenomenon of ink trapping. Because ink trapping,.ink spread, ink mixture and variance of mechanical dot gain were negligible for this proofing system, this is an optical effect to be referred to as optical trapping. An interaction model was derived that compensated for optical trapping. Adding the optical trapping effect to the first-order model significantly improved model prediction to an average ΔE[*][ab], of 2.2 with a maximum of 5.5. A simple black printer model was derived for an inversion of the forward model that aimed to provide a similar black amount with a conventional color-separation method and colorimetric match applying a concept of under-color removal (UCR) in original density space with tone reproduction curves of a gray scale. Using the Simplex method, the modified spectral Neugebauer model with the black printer model was inverted to build a backward model. Influences of the dot area transform function obtained from the backward model were compared with those from a conventional method for an evaluation of similarity. A desktop drum scanner was colorimetrically characterized using a spectral reconstruction model for a reflective photographic material to build a concatenated device profile in which digital counts of a scanned photographic reflection print were the input and those of the proofing system were the output. Performances of the concatenated device profile were evaluated for practical use. The average ΔE*[ab] error from the profile was 2.1 including colors outside of the proofing system's color gamut.
Department, Program, or Center
Chester F. Carlson Center for Imaging Science (COS)
The Journal of Imaging Science and Technology 42N2 (1998) 99-114
RIT – Main Campus