Optimal Design of Ink Spectra for Multiple-Ink Color Reproduction

Conventional four-color printing systems are limited by a relative small color gamut, poor color constancy, and a high degree metamerism. In this dissertation, these problems were addressed by increasing the ink number, optimizing the spectral ink curves, and the lookup tables for multiple-ink printing. First, a computer simulation was performed to investigate the optimum combinations of different ink number to maximize the color gamut for halftoning printing. By analyzing the color gamuts achieved by real ink sets and virtual ink sets, the conclusions were drawn that the color gamut can be improved by increasing ink number and optimizing spectral ink curves. Second, an investigation of color inconstancy on several ink-jet printers showed that the color inconstancy could be a serious problem for some printers. A computer simulation was performed to investigate optimal ink sets to minimize color inconstancy while maintaining a large color gamut. The relationship between ink shape and the color constancy of prints was analyzed. The results show that the center wavelength and width of an ink spectral curve are very important for the color constancy of prints. When the number of inks exceeds three, there is the usual color-management one-to-many mapping problem. In order to further solve the color inconstancy problem, an algorithm was developed for multi-ink printing in which the one-to-many mapping problem was overcome by selecting ink combinations with the best color constancy. The algorithm was evaluated on both real and virtual ink sets. The results proved that the new color separation algorithm can improve color constancy, significantly. Therefore, color constancy can be improved by both ink design and lookup table creation. Third, in order to make printed reproductions that match original artwork under different illuminants, an optimization strategy was developed to optimize ink sets to minimize the spectral error between reproduction and artwork paints. The results showed that as the ink number increased the accuracy of spectral reproduction improved. A different optimization was performed for minimizing the degree metamerism from illuminant D50 to Fl 1 and from D65 to A. The results showed that the optimal ink sets can decrease the metamerism between sets of illuminants and maintain the accuracy of colori metric reproduction simultaneously