The traditional method of color calibration of a digital press is a manual operation, which falls into two logical categories. The first is a one-time task during the make-ready operation and the second is on going during the press run. With an automated process, color make-ready and run time press calibration fold into one continuous operation from press start-up to print run completion. A high-speed color measurement instrument (in-line spectrophotometer) mounted on the press will continuously monitor actual color printed values and compute and command corrective density adjustments electronically. This is accomplished by intermediate color correction computations and system feedback fast enough to keep up with press speeds. 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 via look-up-tables in the printer's raster image processor.
Color control for digital presses is an excellent candidate for automation because it can provide improved quality along with operational cost reduction and increased throughput. An additional benefit is a short-term return on investment for the print service provider. The following is a summary of the 5 most important reasons to consider automated closed loop color monitoring and control.
Reason 1: Excellent color reproduction quality throughout an entire print run while increasing throughput and minimizing material waste - At the beginning of a print production run, the closed loop system will measure color bar values and command modified values to meet prescribed densities. This will provide for a shortened make-ready period for color calibration with minimum paper and ink waste. All of this will take place on press without interruption of the press to pull a single sheet. The result is replication of offset color quality with a digital press.
Reason 2: Consistent color reproduction for multiple independent runs of the same printed product - Automated color monitoring control will ensure consistent color results from week to week, month to month, and year to year. It will also ensure consistency from job to job, operator to operator, and press to press.
Reason 3: On press generation of ICC color profiles - If there are significant changes to color reproduction due to colorant or media changes, a spectrophotometer which has horizontal motion control across the width of the press and an on board processor will be able to measure a rectangular array of color patches (profile target); generate a full ICC compatible device color profile on press; and electronically load the profile into a look-up-table for automated correction.
Reason 4: Spot color and process color monitoring in the work - An additional benefit of horizontal motion control for the in-line spectrophotometer is that the operator can periodically measure, monitor, and, if necessary, correct spot colors and special process colors (logos, etc.) in the work. It would be compatible with on-press inspection system cameras to allow a press operator to place a cursor on critical color locations to be monitored in the work within the camera field of view. This would identify the exact locations where the spectrophotometer would make measurements and report and enable required corrections.
Reason 5: Generation of a print run color quality verification report from the print service provider - A critical component of an automated system is the quality control software application that monitors, displays, and records production run color data for the operator, quality control personnel, the pressroom manager, and the print buyer. A report can be provided to show customers the quality history of their jobs including:
- periodic reporting of print job performance includes delta E, density and dot gain derivation from the tolerance values
- detailed statistical performance reports for a single press run, or multiple press runs of the same job
- analytic tools which allow users to run in-depth trend analysis of specific colors in print jobs that are printed on multiple presses, across different facilities, and that are printed on a recurring basis