When a web printing press has a digitally-controlled ink system, it can be equipped with a closed loop color control system. Video densitometers and/or spectrophotometers scan a color bar on the web which is analyzed and used to control the ink settings. In this way, color characteristics are matched to targets in a closed loop. The color bars can be either in the trim or tail edge of the sheet. Solid ink density, dot gain, print contrast, trapping, three color gray balance and up to 30 other variables can be read. Gray balance is measured in a gray patch made with cyan, magenta and yellow. Variations in gray balance indicate that the dot gain of one color is incorrect. 

By adjusting the press automatically, makeready time and materials can be reduced by 30% or more, color accuracy can be improved, and overall waste lowered. 

Additional benefits are possible if the data are used throughout the press run or compared between press runs for statistical process control. 

It is possible to measure within the image if its content is precisely known. Computer-to-plate image preparation can supply this information. Further reductions in waste can be attained by eliminating the color bar and reading directly within the image. Measurement options and speed may be reduced with this method. 

New presses are being shipped with closed loop color control systems. Recently constructed presses with digital ink controls are candidates for retrofits with closed loop color control systems.