Leveraging G7-based Calibration for Effective Process Control

This articled originally appeared in the July/August 2014 issue of SGIA Journal.

Effective Color Management
When we think of color management, we tend to think of measuring color targets and building ICC profiles. When we talk of color management, we tend to talk of instruments, neutral balance and ΔE values. And when color management fails, we tend to blame the hardware, software or the operator. But while software, instruments and well-trained operators are all critical to the color workflow, color management must be built on a solid foundation of process control to be effective.

Consider the myriad Fortune 500 brands we encounter every day; their products all share a common trait called consistency. For example, Starbucks® has over 19,000 stores worldwide, nearly 40,000 employees and sells over $10 billion worth of coffee each year. And yet, regional taste preferences notwithstanding, every barista at any given Starbucks can produce a near identical Frappuccino® due largely to the implementation of effective process control (the secret is the water).

Making coffee on the scale of Starbucks is a manufacturing process; so too is printing. By integrating straightforward process checks into your workflow, you can ensure consistent and predictable results, curb waste and reduce overhead costs. The key is to integrate effective controls into your production flow: first, establish pass/fail criteria based on common aim points; next, define a universal known good condition that each printer can be calibrated to; implement control checks during production; and finally, archive your data to isolate variations over time.

Control Begins with Calibration
An ICC profile is critical to color management. When you create an ICC profile, your profiling software generates a series of lookup tables based on the way your printer reproduces color at that moment; the ICC profile is a model of how your printer is performing at a given point in time. Should the performance of the printer change due to any combination of ink or media change, environmental variation, or maintenance or mechanical wear, the validity of the profile, and thus the accuracy of your color matches can fall into doubt. The goal of process control is to establish a known good condition for the printer prior to generating the ICC profile. The known good condition is one that can be reliably calibrated back to should a change cause color drift, maintaining the accuracy of your profiles throughout production.

Use of aim points establishes the known good condition. G7® is a method of calibrating any four-color process device, including offset and digital presses, large format inkjet printers, digital proofers, color laser printers, flexographic and gravure presses, and dye-sublimation printers to a common set of colormetrically-derived aim points intended to produce prints with a common, visually-consistent appearance. G7 is not color management, per se, but it does remove the subjective and often ambiguous definition of what defines a visually faithful reproduction, replacing it with an objective set of aim points based on native media white point (Dmin) and CMY process black point (Dmax).

It does this by defining the colorimetric relationship of the neutral points lying between Dmin and Dmax necessary to produce a visually neutral print density independent of the ink and substrate being used. This is done for both K and CMY process grayscale, and is referred to as a Neutral Print Density Curve (NPDC).

A key aspect of G7 calibration is that it takes the color of the substrate into consideration when building the NPDC. For example, if the substrate has a yellow cast, that same amount of yellow is removed from the Yellow NPDC, thereby removing the influence of the substrate yellow and the subsequent yellow cast it would produce in the printed image. This is how G7 can calibrate all devices to a common visual appearance, regardless of gamut, and establish a known good condition for each.

A simulated GRACoL proof (above center) is show next to two simulated print samples from the same inkjet device. The linearized print (left) shows an obvious green cast, a not uncommon problem with many commercial inkjet inks. The G7- calibrated print (right) shows the green cast removed; the visual appearance of the print more closely matches the GRACoL proof (center). The G7 calibration was performed in lieu of linearization; no extra steps were added to the workflow. Also, no color management was performed, save for an initial set of primary ink limits that were defined prior to calibration to avoid excess ink saturation at print time (the same limits were applied to the linearization sample).

Coming up, Part 2...