The Effects of fluctuations in solid ink density on color variation for conventional and grey component replacement separations when printing process color on newsprint

A color separation technique known as Gray Component Replacement, or GCR has been an option available to high-end drum scanner operators for the past eight years. Recently, GCR has been made available by lower-end color system vendors as well. GCR consists of removing the least predominant process color ink in a unit area, reducing the remaining colored inks by the same amount and replacing the gray component of the color with black ink. One of the reported benefits of the GCR technique is that more consistency can be realized during a pressrun. If this is in fact true, the first publication printers who would surely utilize the technique would be newspapers. The problem of consistency is particularly acute for newspapers. There are a number of reasons for this, but the primary ones relate to the types of materials used. Newspapers use the cheapest paper and ink and in many cases, they print with old or poorly maintained presses. They would surely welcome any process that offered better reproductive quality and more consistency, provided the investment was justifiable. Since their present color separation systems allow for the use of GCR in the majority of cases, its potential in the area of newspaper printing was investigated in this study. One factor determining consistency during a pressrun is the control of ink film thickness. As ink film thickness fluctuates, color variation takes place. This study compared the amount of color variation realized for a conventional separation and GCR separations as ink film thickness was varied on press. A separation was made conventionally and with five levels of GCR (however only 40% and 80% could be used for analysis). A Macbeth color checker, comprised of twenty-four color patches, was used as the test target. Ink film thickness was varied to three fluctuation magnitudes. Twenty-five press sheets were chosen from the fluctuation cycles. A spectrophotometer was used to obtain color difference magnitude data (AE) between the ink variation sheets and a sheet with nominal solid ink densities for both the conventional and GCR separations. A mean AE value was tabulated for each group. The GCR mean values were compared to the conventional mean value. Using both the z-Statistic (for multi-sheet comparisons) and the Student\u27s t-Statistic (for single sheet comparisons) tests were performed to determine if the difference in color variation between the conventional separation and the GCR separations was significant statistically. The t-Tests indicated that less color variation was realized for the GCR separation if the level of GCR was substantial (80% as opposed to 40%). Where the data was deemed valid, no cases existed where less color variation was realized for the conventional separation. The initial z-Test (when the number of samples was 192) revealed less color variation for the GCR separations and higher color variation for the conventional separation. The final z-Test (including all sample press sheets) revealed much less color variation for the GCR separations compared to the conventional separation. The amount of color variation was higher for the GCR separations compared to the conventional separation only in cases where the black ink film thickness was varied. However, in these cases the difference between the conventional and GCR color variation means (mean AEs) revealed that the difference was not statistically significant