G7 Calibration for Effective Process Control - Part 3

Common Misconceptions
A common misconception regarding G7 calibration is that it will restrict the color gamut of the printer, or otherwise limit the printer from producing its maximum gamut for the ink and media selected. However, this is not true. The principal reason behind this belief is the confusion over visual appearance and color gamut, the notion that two printers calibrated to produce prints with a common visual appearance must likewise produce a common color gamut. But, G7 defines only a common visual grayscale relative to the printer’s native white point (media) and black points (K and CMY process gray). It does not define or otherwise restrict the native process solid densities of the printer, nor does it restrict its gamut. A printer with a very wide dynamic range (one with a very high white point and very dense black point) will still produce a larger gamut than one with a lower white point or less dense black point after both are calibrated to G7; however, once G7 calibrated, the two printers will produce images with very similar visual appearance – the difference is that the printer with the larger gamut will produce images more vibrant and saturated, but still visually similar to the one with the smaller gamut. In the parlance of G7, such a printer could be called G7 Extreme.
 
To understand this, imagine a three-dimensional color gamut, a volume of color; a printer with a larger gamut will contain a larger volume of color. As the volume expands, the colors become more saturated; the intensity, or chroma of the colors increases at the periphery of the gamut. Running vertically down the middle of the gamut is a line representing the neutral (gray) axis, from media white to blackest printed black. All in-gamut colors are defined relative to this neutral axis; two printers when calibrated to G7 aims will produce images with the neutral axis of their respective gamuts very closely aligned, which will more closely correlate all of the colors common to both gamuts. Neither printer will produce a noticeable color cast, and thus will appear visually neutral – the brain will perceive images from both printers as visually similar even if discrete colors within each image are colorimetrically different.
 
Another common misconception is that G7 calibration prior to profiling is unnecessary since the ICC profile should correct for neutral density. While technically true, this approach can be problematic for several reasons. First, achieving G7 compliance is not the same as maintaining it. For inkjet printing in particular, G7 calibration is an excellent basis for process control. With few exceptions, there is no way to ‘dial in’ color on an inkjet printer – all control over color reproduction resides in software, and in most cases that relies on an ICC profile. Generating an optimum ICC profile is nontrivial; the final result can very often be influenced by parameters selected prior to creating the profile, and the effect of those parameters can sometimes be ambiguous. ICC profiling software from different vendors, or even different versions from the same vendor, can offer different parameters and produce different results. Sometimes the selection of parameter values can be subjective, and very often different operators using the same software will generate profiles that produce somewhat different output. Moreover, profile color targets are big, and may contain from close to 1000 to well over 2000 patches.
 
Correcting for day-to-day color drift with frequent reprofiling can be not only very time consuming, but can also introduce errors and inconsistencies of its own. On the other hand, G7 calibration is quick, with virtually no parameters to configure, and yields a simple pass or fail outcome. Re-calibrating back to G7 aims when color drift begins to occur is equally quick – a simple reiteration of the existing calibration curves. When an ICC profile is built for a printer already calibrated to G7 aims, the calibration acts as a foundation for the profile. Except for some extreme cases, color drift can be corrected for by simply reiterating the calibration curves, which requires minimal operator skill and avoids the need for reprofiling. For the print shop owner, that is most often the ROI justification for adopting G7.
 
Another issue associated with achieving G7 conformity though the ICC profile, rather than by calibrating before profiling, is that the color target used to generate the profile lookup tables may print with an obvious color cast (see Fig. 1). Should that happen, which is common, the target will lack definite neutral patches and the profiling software will need to estimate the ink ratios that constitute a neutral grayscale. While this should still produce a reasonable profile, the potential accuracy of the profile may be diminished. The greater the post-linearization cast, the greater the reliance on the profiling software to fix the target values, and the greater the risk that the accuracy of the profile will be adversely impacted. If, of the other hand, the printer is G7 conformant prior to profiling, there is less reliance on the profiling software, and G7 compliance can be tested before and after profiling as a safeguard against profile-induced errors. If the printer matches G7 aims prior to profiling, and still matches those same aims after the profile is applied, the likelihood of having a poor profile is greatly reduced.
 
One caveat to G7 is that it is based on a standard CIE D50 illuminant; subjective (visual appearance) acceptance is only valid when viewed in a properly calibrated light booth. Like any colorimetrically-derived method, G7 is subject to issues of metamerism and color constancy. Viewing two G7 conformant prints produced using ink and media with significantly different spectral properties, under anything other than proper D50 illumination can yield distinct visual differences. For this reason, proofs and prints should be viewed in a calibrated light booth, and proofs should include aim points for objective measurement and comparison by instrument. 

This article first appeared in the July/August 2014 issue of SGIA Journal.