What is Closed-Loop Color Process Control for Digital Presses

Periodic monitoring of the aim characteristics (required colorant and overprint values) of a digital press is an important prerequisite for ensuring consistent color reproduction. The remainder of the necessary and sufficient operations for commendable color quality is the ability to correct colorant densities to meet aim values. Each of these two operations can be accomplished either manually or automatically via hardware and software. The former is the traditional process, which leaves lots of margin for operational cost reduction and increased throughput by means of automation. The manual process involves

  • Interrupting press operations
  • Pulling tear sheets
  • Making manual measurements of color bars
  • Entering colorant density correction values via keyboard

This sequence of monitoring and correction is both part of make-ready time and the active print run. In a manual operation, make-ready and the production print run are separate and distinct operations. With an automated process, color make-ready and run time press calibration fold into one continuous operation from press startup to print run completion.

A closed-loop automated color control process requires a high-speed color measurement instrument (in-line spectrophotometer) mounted on the press to measure, compute, and command the corrective density adjustments electronically. This is accomplished by intermediate color correction computations and system feedback via look up table modification. These operations must happen fast enough to keep up with press speeds. The key operational benefits of automated color control are

  • Reduction in make-ready time
  • Reduction of media and ink or toner waste during make-ready
  • Increase in productivity due to avoidance of press interruptions
  • Increase in overall throughput due to continuation of press operation even after media and colorant changes have been made

The last item above refers to on-the-fly color monitoring and control even when colorants and paper stock with measureable and visually noticeable color shifts have been introduced into the same print run. As a catch all, it is important to note that eliminating subjective color judgment frees press operators to perform other important tasks during print production.

The main components of an automated closed-loop color system integrated with a digital press include the following:

  • Multi-band in-line spectrophotometer
  • On-board high speed computer and operating system
  • Integrated feedback electronics which can control colorant laydown
  • Software application which monitors and displays real time color correction performance

Controlling colorant laydown implies that it is possible to send corrected colorant values to press control points which will either increase or decrease the density on paper to match a desired color or gamut of colors. The integrated feedback electronics for controlling colorant laydown are already a part of most web and sheet-fed digital presses. The automated system avoids manual keyboard entry and provides a correction data path directly from the on-board computer in a hardwired fashion to the print engine. The most likely entry portal would be the printer’s RIP.

The simplest form of color control is in one-dimensional look up table transformations for each of the cyan, magenta, yellow, black, or spot color (SC) densities separately to a color corrected value. C to C’, M to M’, Y to Y’, K to K’ and SC to SC’ A more complex form of color control is required if there are spectral shifts in paper and/or colorants or if overprint values cannot be corrected by one-dimensional mappings. In this case the spectral measurements from the spectrophotometer would be used to generate a CMYK to CMYK’ profile to compute new colorant values.

In addition to these on press components, a color bar is printed on the media along the direction of the press run. It consists of shadow, mid tone, and highlight gradations of each colorant as well as overprints with known spectral characteristics. There may also be one or more spot colors included in the color bar to maintain desired densities of special colors such as those used for company logos. The color bar is only an approximation of the actual colors in the page, and some work doesn’t leave any room for trim. So, to ensure that important colors within a page are accounted for and not be too dependent on color bars, another admirable degree of freedom for the system is the inclusion of spectrophotometer motion control along the width of the press. This would allow the measurement and correction of specific elements of the printed page such as solid spot colors in a logo or critical solids printed by means of process color.

To round out the system, a useful quality assurance tool is a software application for continuous display of how well colors are controlled for consistency and accuracy throughout a run. Its purpose is to show and record on going press status as well as trending data showing the direction colors may drift prior to correction.

Do You Need Automated Closed-Loop Color Control as Part of Your Press Operation?

The following are the main value drivers for implementing closed-loop color control on a digital press.

  • excellent color reproduction quality throughout an entire print run by means of on-the-fly automated measurement and feedback to correct drift in color densities and variations in colorants and paper stock
  • consistent color reproduction for multiple independent runs of the same printed product from week to week or year to year
  • consistent color reproduction quality from job to job, operator to operator, and press to press
  • automated on-the-fly color correction without press interruption
  • on press generation of ICC color profiles
  • minimization of time and material waste to calibrate press during make ready period
  • measurement and reporting of uniformity of colorant densities across the width of the printer
  • spot color and process color monitoring in the work
  • generation of a print run color quality verification report for the print buyer from the print service provider

If one or more of these value proposition components are critical to your operation, then automated closed-loop color monitoring and control should be an important consideration for future system upgrades.

By Hapet Berberian