G7 method or GRACoL/SWOP colorimetry?
G7 method or GRACoL/SWOP colorimetry?
We are in the process of getting our shop to print to G7 methodology. In this process we realized our ink was not meeting the required LAB values so we tested some different inks and made a switch. We also realized that our stock is out of specification (L*94, A* 1.5, B*-7) we do not have the option of switching papers so my question is can you adjust for the paper color, if so what is the best process? We have already made curves for the stock using the Idealliance curve software. We are using standard SWOP ICC profiles.
Maybe just to clear one thing up, the "G7 Method" isn't necessarily about targeting specific solid ink or paper colorimetry. You can calibrate your press/proofer/whatever using the G7 methodology and not care one wit about either of those things. G7 as a methodology is strictly about "NPDC" or the tone curve and gray balance embodied in the Neutral Print Density Curve.
What you're referring is more about using the G7 method to target your printing to either GRACoL or SWOP. In G7 terms, that's GRACoL/SWOP Targeted where you're attempting to get the ink colorimetry to either the "G7" GRACoL or SWOP values.
Having said all that, I'm curious about your paper. Based on the Lab values you're giving, it doesn't fall into either GRACoL or SWOP. The Lab values you're giving sound more like an uncoated offset paper. In any case, I would guess you have a paper with a fair amount of optical/UV brighteners given the negative b* and positive a* value. If that's the case, there isn't (yet) a specification you can accurately target, especially SWOP where the paper spec is either neutral (SWOP3) or decidedly "warm" (SWOP5). Even GRACoL, the one that uses the "blueist" stock (a*0 b*-2), isn't anywhere close to your paper. Like I say, your numbers sound a lot like an uncoated offset stock. Getting back to your inks, it might've been a hasty conclusion that your previous inks were "non-compliant" as I doubt any normally compliant inks could print in compliance on that paper....your magenta ink would have to be decidedly "warm" and your yellow ink would have to quite strong to overcome the negative b* of your press stock and appear to be in compliance. I see it all the time in press rooms where an ink set will print perfectly fine on a normal gloss coated stock (b* around -2 to -3.5) but then move to a "brightened" uncoated offset and you can't hit anything in compliance...it's the paper not the ink.
You appear to be in that no man's land of wanting to use the G7 method for calibration (which you certainly can) but without an exact specification or colorimetry that you can follow that matches your paper and print conditions. If it's about proofing for this situation, this is the one rare time I would recommend that you forget about trying to use a standard data set (GRACoL1/SWOP3/SWOP5) and simply profile the press in it's calibrated state and use that in your proofing system. I would probably still recommend using one of the standard profiles for doing your RGB-CMYK conversions/separations but beyond that you'll have to be content that you're using the G7 method on press, just without any particular specification to follow. If your paper is indeed an uncoated offset stock, hopefully this will be addressed in the future if/when a standard data set for uncoated paper is published by the IDEAlliance.
Regards,
Terry