INK-PAPER INTERACTION A study in ink-jet color reproduction

An ink jet printing system consists of three fundamental parts: inks, printing engine, and substrates. Inks are materials creating color by selectively absorbing and scattering the visible illumination light. The printer acts as an ink distributor that governs the ink application. Finally, the substrate acts as a receiver of the inks and forms the images. Ink setting on the substrate is a complex process that depends on physical and chemical properties of the inks and the substrates, and their bilateral interactions. For a system consisting of dye based liquid inks and plain paper, the ink moves together with the liquid carrier before the pores absorb the liquid. This process contributes to serious ink spreading on the surface along the paper fibers. At the same time the ink spreads down into the pore structure. This causes severe dot deformation, physical dot gain and ink penetration. Understanding the consequences of these phenomena and above all being able to characterize their impact on color reproduction is of great importance. Moreover this knowledge is fundamental for finding solutions to ink-penetration related problems. This thesis presents studies of some important issues concerning image reproduction quality for dye based ink-jet printing on ordinary plain paper (office copy paper), such as ink penetration, optical dot gain, and even physical dot gain. The thesis begins with theoretical developments to the Kubelka-Munk theory, which allows one to study even non-uniform ink penetration into the substrate. With the knowledge of scattering and absorption coefficients and ink thickness, reflectance can be computed by solving differential equations. Three forms of ink penetration, uniform, linear, and exponential have been studied. A method is then presented for obtaining fundamental properties of the inks from spectral reflectance measurements, like the scatteringand absorption-power of inks, ink layer thickness, and ink mixing scheme for the generation of secondary colors. The method is further developed for modelling the ink penetration in printing systems consisting of dye based liquid inks and plain paper. By combining the spectral reflectance measurements with theoretical simulations, quantities like the depth of ink penetration is determined. These quantities, in turn, are used to predict the spectral reflectance of prints. Simulated spectral reflectance values have been in fairly good agreement with experimental results. Models dealing with light scattering inside the substrate resulting in optical dot gain for halftone printing, in the case of existing ink penetration, have been developed for both monoand multi-color printing. It is shown that the optical dot gain leads to higher color saturation than predications from Murray-Davis approximation. Additionally, tentative studies for physical dot gain were made. Finally, an evaluation of the chromatic effects of the ink penetration for printing on office copy paper has been carried out based on both experimental data and simulations. It is found that ink penetration has a dramatic impact on chroma and hue of the color, and the color saturation is significantly reduced