Some Colorimetric Properties included in the Color Characterization Data of Process Prints.

In the ISO/TC130 committee, various efforts have been undertaken to make common the color characterization data for the offset printing using the ISO 12642 Output Targets. The authors participated in printing test project in Japan, produced test proofs and obtained various color characterization data including some combinations of papers, inks and screens. In this paper, the qualitative properties of measured colorimetric data of the process prints and the relationship between values of CMYK and color coordinates expressed by CIELAB values are explained on the basis of the results of our investigation. First, we studied the colorimetric characteristics of proofs certified as meeting the requirements of JAPAN COLOR Conditions. The measurements had been performed by using printed color patches on the proofs where dot values were specified in ISO 12642 Graphic technology-Prepress digital data exchange-Input data for characterization of 4-colour process printing. The results of our studies of the characterization data can be summarized as follows: 1. Regarding the color patches of 3-color process prints for which the dot values of C, M, and Y are specified by ISO 12642, all CIELAB values of patches where the dot values of the one color (e.g. C) are the same and the others (e.g. M and Y) are variable, lie on a flat plane.2. These phenomena can be observed for other combinations.3. The flat planes can be expressed by the simple equationsL * = ( (C) + ( (C) a* + ( (C) b*... for each C value L * = ( (M) + ( (M) a* + ( (M) b*... for each M value L * = ( (Y) + ( (Y) a* + ( (Y) b*... for each Y valueand the coefficients values of (, (, ( can be approximated by quadratic functions of C, M, or Y dot values. [5] By using these equations, the trials of the conversion between CIELAB values and C, M, and Y dot values are performed and their results are evaluated by calculating (E between predicted CIELAB values from specified CMY dot values and measured CIELAB values. The comparison shows the (E is less than 5.0 for the offset prints on Type 1 paper. For color patches including black inks, the relationship between K values and coefficients values is investigated and it is made clear that the CIELAB values of the patches where C and K values are the same and M and Y values are variable lie on a flat plane as well as in the 3-color prints. Moreover, the plane is the same as the plane of C = constant where C (value of 3-color print) = C* (value of 4-color print) + K-Q, and Q is related to the halftoning technologies (e.g. dot gain). The Q value is analyzed in detail in this paper and has been confirmed by test prints. Also a subjective comparison of color patches one of which has the specified C M Y dot values and the other has the CMY dot values calculated from the same CIELAB values for the specified CMY dot values has been performed with prints produced by using the Pictrography. Consequently, we can make clear the colorimetric characteristics of process prints and the results will contribute to the color reproduction technique.