An investigation into the interactions of certain visual attributes of a four-color reproduction produced on a sheet-fed offset lithographic printing press

The development of a closed loop press control system has been the goal of many press manufacturers in recent years. These closed loop systems are designed to collect data from sample press sheets by measuring certain process variables which are considered to be indicators of press control. The theory is that by controlling the variation of these indicator variables, it is possible to control the color variation of the halftone overprints of the reproduction. The problem in the design of these automatic control systems, however, concerns which process variables should be considered indicators of press control. This study was undertaken to determine which process variables in sheet fed offset lithography can be considered indicators of press control in the halftone overprints of the press sheet. This thesis involves a series of correlation analyses which study the interellationships between several process variables of the reproduction. These process variables are: solid ink density of the process colors, density of screened process colors, hue and gray value of halftone overprint colors, hue and gray values of a near neutral gray halftone overprint, and hue and gray values of a critical halftone overprint color. The sample press sheets used in this thesis were taken from industrial production so that the results of the correlation studies of the process variables could be considered typical of those in industry. The experimental results indicated that the variation of the solid ink density of the process colors at the color bars was not a true indicator of the variation which occurred in the screened process colors. It was possible for the density of the solids to remain very close to the standard while the screened process colors varied in a large amount or in the opposite direction from the solids. This indicated that the density of the screened process colors was controlled by variables other than solid ink density. The experimental results also indicated that the density of the screened process colors may vary in large amounts from the standard and continue to produce halftone overprints with hue and gray values comparable to the standard. Also, the variations of the hue and gray values of the halftones were not consistent within the same press sheet. It may be concluded that further study is necessary which would clarify the proper indicator variables to be used in the control of the offset lithographic press. From the results of this study, it appears that proper control of the press is attainable through visually matching the halftones on the press sheet to those on the standard. Even though the process variables of this study showed a marked amount of variation, the hues and gray values of the halftone overprints remained visually stable. If an automatic closed loop press control system is to succeed, it must be able to evaluate the hues and gray values of the halftones. Proper adjustments can then be made to the offset press to match the halftones of the press sheet to the standard