The present invention will be described in connection with displays that are created with colored ink but it should be borne in mind that the system could be used with a display that is created with paints, dyes, or some other color bearing material. When a pattern is going to be printed on paper with colored inks (in the so called process color printing), the color arrangement is accomplished by separately printing partial patterns respectively with four basic colors, i.e., cyan, magenta, yellow and black. Patterns of small dots or large dots or mixtures thereof are printed, depending on the optical densities of the colors to be effected. The optical density of a color is related to the ink film thickness and is a measure of the strength of the color that is identified or seen or appreciated by the viewer. If the printing is accomplished, for example by an offset printing technique, the surface of the printing plate is chemically formed into a pattern of small dots of variable areas, which hold ink therein and the ink is transferred to the paper during the printing of the pattern. If the printing is accomplished by a letterpress plate, or raised metallic islands, then the islands are the dots and the ink rests on the islands and is transferred to the paper during the printing process. If the ink flow to the printing plate is different from time to time, then as it is transferred, (by either method), the end results will be different and the colors will not appear to be the same from one printed copy to another.
In the prior art when a printer was engaged in printing a pattern and he wanted to determine whether or not the color was remaining constant he would proceed as follows. He would take a sample of the printed copy and using a static densitometer would take readings at several places along a color swatch line, make some calculations from the information that he obtained with his static densitometer and then determine what, if anything, he should do about the discrepancies between a reference optical density that he desired and the actual optical density of the copy which was being printed. The present device provides a means for automatically scanning the color swatch line. The present device provides a means for determining, during the scan, the optical densities of the colors of inks that have been laid down on the paper so that the ink flow can be changed quickly in response to the monitoring results, thereby effecting an early correction and reducing the amount of copy which may result in waste. It should be understood that while the foregoing problems have been described for the color processing technique, the same problems exist in the prior art for maintaining and monitoring solid prints of non-process colored ink, as in label or can wrapper printing.