In addition to implementing the G7 method and the Gracol standard, I am looking into characterizing a press that may in some ways “print better than Gracol”. Another scenario would be “as good as we can print on an uncoated sheet” but there doesn’t seem to be any official uncoated standards.
I know the first step is to run some P2P and IT/8 targets with linear plates. However, without a standard choose in the Curve1 software, how do I build a curve? How do I determine what a “correct” sheet will look like? Do I decide on some amount of dot gain? Or some particular Lab values for some of the tints? Is there some formula for calculating any of these based on the solid densities? How did they come up with the official numbers for the Gracol standard?
The short answer is, no “standard” is required for G7 calibration.
The norm for a G7 calibration would be targeting it for a specific colorimetry such as GRACoL. SWOP, etc…but that’s not a requirement since the G7 NPDC tone curve/gray balance is calculated independent of any chosen colorimetry standard.
Bottom line (for uncoated), just have your pressman run their ink densities as high as they feel comfortable running on that stock and calibrate based on that. I would advise that, during the press run, you keep an eye on the 100% CMY patch or the 3/4 tone (“Sc” patch) neutral gray and see that it’s in balance. If the pressman can run their densities where they feel comfortable while at the same time maintaining gray balance, it will make your life easier when coming up with the correct G7 curves.
BTW, there are some standards for uncoated ink colorimetry in the ISO 12647-2 document (Paper Type 4) but I generally find their values a bit too conservative for typical printing on uncoated offset stocks. YMMV.
There’s something I should’ve added to my response that might be helpful.
If you’re familiar with Lch, the “friendlier” version of Lab, then you’ll understand what I’m talking about…
…if it were me and I was doing a G7 calibration with “unknown” ink colorimetry, I would likely target the same hue angle as something like GRACoL as long as the result wasn’t totally out-of-bounds in terms of gray balance. By targeting the hue angle of the primary (CMY) and secondary (RGB) inks, the result will be something visually quite close to GRACoL, albeit with lower density and chroma because of the uncoated stock. Usually a “flatter” looking blue (C+M) is more tolerable to a customer than a blue that is either quite magenta (purple) or cyan compared to a standard GRACoL proof.
