Patent ID: 8127680

Claim:
A method of controlling quality of a printed image of a color printing press that prints a color print image on a subject to be printed by: adjusting ink feed amounts from a plurality of ink fountains using a plurality of ink fountain keys, the ink fountains respectively containing printing inks of a plurality of basic colors that are different from each other; supplying the inks of basic colors that have been adjusted for the corresponding ink fountain keys to a plurality of printing plates provided corresponding to the plurality of ink fountains; and sequentially printing a plurality of images of the basic colors on the subject to be printed, the images being formed respectively with the supplied inks of the plurality of basic colors, the method comprising the steps of: measuring gray Lab values of a printed matter that has been printed; calculating ΔL, Δa, and Δb that are respectively differences between the measured Lab values and predetermined target gray Lab values; correcting Δa based on Δb; thereafter correcting ΔL based on a corrected value Δa 1 of Δa; calculating C, M, and Y halftone density differences, respectively, based on Δb, the corrected value Δa 1 and a corrected value ΔL 1 obtained by correcting ΔL; converting the calculated C, M, and Y halftone density differences into ink density differences, respectively; and adjusting the ink feed amounts for the corresponding plurality of ink fountain keys based on the converted ink density differences; wherein: the correction of Δa is carried out by converting Δb into a Y halftone density difference for gray, and calculating a variation amount Δa′ of Δa that has varied due to the conversion, the correction of ΔL is carried out by obtaining the corrected value Δa 1 by adding the variation amount Δa′ to Δa, and converting the corrected value Δa 1 into an M halftone density difference for gray, the calculation of C, M, and Y halftone density differences is carried out by calculating a variation amount ΔL′ of the Y halftone density difference and ΔL that have varied due to the conversion, obtaining the corrected value ΔL 1 by adding the variation amount ΔL′ to ΔL, converting the corrected value ΔL 1 into C, M, and Y equivalent amount halftone densities for gray, setting the converted C, M, and Y equivalent amount halftone densities as C, M, and Y halftone density differences, calculating target halftone density values by adding actual measurement values of the C, M, and Y halftone densities to the C, M, and Y halftone density differences, and calculating the C, M, and Y halftone density differences by subtracting the target halftone density values from the actual measurement values of the C, M, and Y halftone densities, and the calculated C, M, and Y halftone density differences are converted into the ink density differences.