Method and apparatus of controlling quality of printed image for color printing press

An apparatus of controlling quality of a printed image of a color printing press that includes a plurality of ink fountains respectively containing printing inks of a plurality of basic colors that are different from each other and a plurality of ink fountain keys for adjusting ink amounts fed from the corresponding ink fountains, and that prints a color print image on a subject to be printed by supplying the inks of basic colors, whose feed amounts 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 respectively with the supplied inks of the plurality of basic colors, the apparatus comprising:
• a controlling unit that is provided with:
• measurement means that measures gray Lab values of a printed matter that has been printed;
• calculation means that calculates ΔL, Δa, and Δb that are respectively differences between the Lab values measured by the measurement means and predetermined target gray Lab values;
• first correction value calculation means that corrects Δa based on Δb;
• second correction value calculation means that corrects ΔL based on a corrected value Δa1 that has been calculated by the first correction value calculation means;
• halftone density difference calculation means that calculates C, M, and Y halftone density differences, respectively, based on Δb, the corrected value Δa1 and a corrected value ΔL 1 obtained by correcting ΔL;
• ink density difference conversion means that converts the C, M, and Y halftone density differences calculated by the calculation means into ink density differences, respectively;
• and
• ink feed amount adjustment means that adjusts the ink feed amounts for the corresponding plurality of ink fountain keys based on the converted ink density differences;
• wherein;
• the first correction value calculation means converts Δb into a Y halftone density difference for gray, calculates a variation amount Δa′ of Δa that has varied due to the conversion, and calculates the corrected value Δa1 by adding the variation amount Δa′ to Δa,
• the second correction value calculation means converts the corrected value Δa into an M halftone density difference for gray, calculates a variation amount ΔL′ of the Y halftone density difference and ΔL that have varied due to the conversion, and calculates the corrected value ΔL1 by adding the variation amount ΔL′ to ΔL, and
• the halftone density difference calculation means converts the corrected value ΔL1 into C, M, and Y equivalent amount halftone densities for gray, sets the converted C, M, and Y equivalent amount halftone densities as C, M, and Y halftone density differences, calculates 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 calculates the C, M, and Y halftone density differences by subtracting the target halftone density values from the actual measurement values or the C, M, and Y halftone densities.