New G7 Master Certification Levels Explained

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This reserved article originally appeared in CHROMiX ColorNews Issue 60 on September 20, 2016.

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The beauty of gray

A gray scale ramp

One of my early jobs at a large photo lab was to go out and take test pictures that we would use to compare printers. A fun assignment to be sure - but ironically these were not very exciting photos for most of what I shot were pictures of concrete and driveways, gray barns, that sort of thing. That gray helped us to hone into subtle differences between machines like no bright subjects could. Some find that a knowledge of film scanners is helpful in seeing the value of gray balancing. It's sometimes easier for a drum scanner operator to understand and embrace G7 than for an old-time press operator who is still steeped in density and TVI curves.

But embrace they did, judging by the massive interest and adoption of G7 in recent years. This methodology has been taken on whole-heartedly by printing industry giants as well as small print shops who know enough to take advantage of modern innovations. As the co-producers of IDEAlliance Curve, the very first implementation of G7 software in 2008, we have ridden the wave of adoption throughout North America and the rest of the world. It has been gratifying to see that we have been instrumental in helping so many printers make their lives better while coming into the modern age.

The original software was written primarily to provide a standardized means of calibrating printing presses, but as it has found a wider circle of adoption, we have added more features that make the software popular for digital press, inkjet printers, special ink printers, dye sub printers, and the list keeps growing.

The industry has embraced G7 so thoroughly that it's no longer enough to just say "Yeah, my printer has been G7ed." Now there are three levels of compliance with the G7 specification depending on the kind of printing that is done.

G7 Grayscale

This is the level that we are used to seeing, found in the IDEALink Curve, Curve2 and Curve3 applications. A 300-patch P2P target print is printed on a stable printing system, and the measurements of that target are used to correct the ink curves to bring the printing system into alignment with the G7 ideal neutral density curve.

This is the fundamental magic of G7: regardless of the printing technology, if we can ensure that a printer or press prints the neutral tone ramp well, then all the other colors will tend to fall into line more easily.

Aligning the printing of the various different printing methods in a plant to this same neutral aim point is a tremendous step forward in getting these disparate printers to match. While achieving this G7 level will naturally help all the colors to be consistent, the only thing that's required at this level is for the K and CMY neutrals to be aligned to the G7 tone curve and the CMY neutrals gray balanced.

While very helpful in many ways there are obvious limits to what this grayscale method can do alone. If a customer were to wander into any old print shop that is proudly "G7 Compliant", he may be surprised that his saturated art work is not matching his expectations, even while the work is G7 compliant.

G7 Targeted

Solids and overprints

Somebody had the good idea that you can do a lot more than just match an ideal gray curve. You can also get your primary and secondary solid inks to match some industry-standard spec at the same time. This is where G7 Targeted comes in.

G7 Targeted is achieved when G7 Grayscale is achieved, plus the solid ink measurements for primaries and secondaries (CMY and RGB) and the substrate color are also within spec to one of the G7-based targeted color spaces.

This can be done according to the absolute white point, or using a substrate-relative condition.

One item that's not well known is that G7 Targeted compliance is not limited to only the reference print conditions in ISO 12647-2 or in ISO 15339. Any G7-calibrated data set can be used as a G7 reference print condition.

Those who want to can use their own color reference in place of the standard CPRC's but their files must be verified as being compliant with G7 grayscale. In the future, the full release of Curve4 will provide a means of utilizing user-imported references and ensuring they're G7 compliant before doing so.

Now, a printer can let customers know that they not only can hit G7 grayscale, but they can also achieve a higher level of compliance and their strongest colors can hit, for example, the specific Lab values for "GRACoL 2013".

There is still a more stringent level of compliance that can be reached however.

G7 ColorSpace

All the patches of an IT8 target are used for G7 ColorSpace

G7 ColorSpace includes all the requirements of the G7 Targeted level (and therefore the G7 Grayscale level) and adds the matching to an entire reference print condition. This level shows that you are maintaining tight tolerances throughout the whole color space. An entire IT8.7/4 target is compared against the targeted color space and the average of all 1617 patches needs to be within a narrow range of tolerances. This assures that the printing system can indeed reproduce the entire color space, and not just the primaries & secondaries.

Again, like G7 Targeted, the tolerances for G7 ColorSpace can be relative to either the absolute white point or the substrate-relative aim values.

The G7 ColorSpace level is further divided into two separate purposes, for Sheetfed Offset printing, or for Proofing. There are slightly more stringent requirements for proofing. One requirement at the ColorSpace level for proofing is that all patches of the IT8.7/4 be within an average dE2000 of 2.

Obviously, everyone wants to aim for the highest compliance level that they can, but the level you'll be aiming for will largely be dependent on the kind of printing that's being done. For example, someone printing onto ceramic tile with an electrophotographic process, using non-standard toner colors will likely be aiming for G7 grayscale. Almost every kind of CMYK printing can be calibrated using G7, but this kind of process will not be capable of matching the standard CMYK process colorants, which are more likely to be used with G7 Targeted or G7 ColorSpace.

One might aim for G7 Targeted if printing with a offset press, and G7 ColorSpace might even be achievable by using good equipment and careful process control.

For more details concerning the actual tolerances for the three levels, see the IDEAlliance document:

To see at what level your shop will pass, get Curve4 Verify!

Thanks for reading,

Patrick Herold

See also:

Curve3 - The Evolution of G7 Calibration

G7 Myths

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