Abstract
Authors have been continuing to investigate the colorimetric characteristics of process color prints and proofs produced under various printing conditions. This paper reports measured data and analyzed results of these data. These results obtained can be summarized as follows:
1) Regarding the 3 color prints, three-dimensional colorimetric values for color patches where their ink values of the one color (e.g. Cyan) are held fixed and others are arbitrary have a planner relationship and lie on a flat plane in the L*a*b* color space.
2) The planner relationship for an ink F (F: Cyan, Magenta or Yellow) can be expressed by the following equation. L*=αF(F)a*+βF(F)b*+γF (F) where, F is an ink value of Cyan, Magenta or Yellow. Coefficients of αF (F), βF (F) and γF (F) can be approximated by quadratic functions of ink value F, and their curves of coefficients have almost same form regardless of rendering systems. The mean value of color differences (ΔEs) between measured values and predicted values for specified ink values is less than 4.0 for offset processes, newsprints and proof print.
3) Regarding the 4-color prints, the same relationship as 3-color prints can be observed when F and K values are held fixed and others are arbitrary. When replacement of coefficients αF (F)→αF (F+K-F×K-δF), βF (F)→βF (F+K-F×K-δF) and γF (F)→γF (F+K-F×K-δF) is performed, coordinates of Colorimetric values for color patches of 4-color prints have a planner relationship referred to in Result 1) where, the δF (additional value) is a small value for correction.
These results made possible to describe the output device profile by simple equations accurately and to provide the gray balance conditions. Moreover, we could obtain the mutual conversion algorithm between CMY and CMYK ink values and the replacement rule between color inks and black ink for UCR and achromatic process by making clear the planner relationship for 4-color prints.
