Color Repeatability of Spot Color Printing

A methodology that quantifies variation as well as deviation of spot color printing is developed. Two types of color variations, i.e., spatial (side-to-side) and temporal (begin-to-end), are described. The use of the cumulative relative frequency (CRF) of DE, from colorimetric data collected across the width of the signature, was used to describe spatial variation or uniformity of spot color printing. When colorimetric data are measured from the same printing location over time, we can use the same CRF method to depict the temporal variation or consistency of a press run. The deviation of spot color is determined by the color difference between the average colorimetric values of the temporal samples and its aim point. To demonstrate the methodology, spatial uniformity between a gravure press and a sheet-fed offset press of the same job was compared. If the spatial variation is large and its magnitude is systematic, visual discontinuity of overlapped edges becomes noticeable. The phenomenon is termed, spatial creeping. DE(ab) and DE(00) formulas are compared for their abilities to predict visual color differences. This methodology provides us a means for quantifying spot color repeatability of any printing process.