Color Convergence Under Closed-Loop Control


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Steve Tiltman, John Seymour, and Jeff Krueger


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Photo, Member: 
Photo, Non-Member: 


The subject of convergence of color to a target provided for a closed-loop color control system in the web offset printing environment has long been mired in a combination of subjectivity and variability. Subjectivity resulting from such things as differing human perception of good color, and variability coming from a wide array of circumstances and configurations, including the range of available cut-off or impression lengths utilized. For those involved in the development and use of such closed-loop color control systems, this subjective and variable base has been one on which foundations for measurement and comparison have been difficult to establish. Consider the statement 'it takes 3,000 copies to get good color' and its intrinsic lack of scientific credibility. How much paper is wasted during those 3,000 copies and what is an objective measure of good color?

For the purpose of this study, influence on measurement resulting from human subjectivity was eliminated through the introduction of the concept of Theoretical Good Color. This is when 80% of all ink zones on all ink fountains are within +/-0.10 density points of the target. Variability as a consequence of differing cut-off or impression lengths was removed by converting all lengths to meters. The elimination of these factors prepares the way for meaningful analysis as it places distributed closed-loop color control systems on equal footing.

A large collection of closed-loop color control system data was gathered from web offset presses produced by two major manufacturers, the presses were distributed around Europe and the USA, and encompassed a wide range of variability in press crews, press impression lengths, printing environments and printed work. The collected data was analyzed and studied with the intention of identifying the remaining variables which most influence the performance of a closed-loop color control system as it brings color from its starting condition to theoretical good color.

The analysis revealed two main influences on the length of substrate wasted as color converges on a target:

  • The quality of the output produced by the presetting system used to set the ink fountain roller speed and ink key positions--referred to in the study as Preset/Ratchet Quality.
  • The length of paper passing through the closed-loop measuring device between subsequent samplings of the printed product--referred to in the study as Measurement Period.

By manipulating these two influences on the speed of color convergence, it may be possible to exert significant control over the amount of wasted substrate.

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