Paper Adjusment

[mamarcus]

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.

 

[tmiller_iluvprinting]

The G7 How To book explains how to adjust the specified solid CIELab values for the effects of non-standard paper color. pgs. 57-58
Best regards,
Todd

 

[TerryWyse]

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

 

[meddington]

The problem with optical brighteners and G7 is the spectrophotometer illuminant may excite brighteners differently than your viewing lights, and you may end up with gray balance curves that do not result in a visually neutral result. When creating your curves, its possible to get a more visually accurate result by changing the white point in idealink curve...increasing the negative b* value so the curves aren't adjusted quiite so strongly. This is a purely subjective process though as you have to take the visual result into consideration. Not something easy to get right the first time, particularly on an offset press.

 

[TerryWyse]

The problem with optical brighteners and G7 is the spectrophotometer illuminant may excite brighteners differently than your viewing lights, and you may end up with gray balance curves that do not result in a visually neutral result. When creating your curves, its possible to get a more visually accurate result by changing the white point in idealink curve...increasing the negative b* value so the curves aren't adjusted quiite so strongly. This is a purely subjective process though as you have to take the visual result into consideration. Not something easy to get right the first time, particularly on an offset press.

Mike, would you be an advocate of using a UVcut spectro for G7/gray balance calibration? I've been thinking about this lately and have considered using a UVcut spectro with SpotOn! and using UVcut when measuring P2Ps.

My current method with the P2Ps is to use an iSis with Measure Tool and measure BOTH "no filter" and UVcut and average them together....with most "brightened" gloss coated house stocks, the result is a nearly perfect a*0/b*-2-2.5 with these papers (they typically measure about a*+.5/b*-3.5 with no filter).

I'm thinking UVcut may actually be the preferred method for figuring gray balance but can't commit to it just yet, hence the mixed no filter/UVcut measurements. Did a job recently on 5 different sheetfed presses using this method and it worked out nicely.

Terry

 

[meddington]

Mike, would you be an advocate of using a UVcut spectro for G7/gray balance calibration? I've been thinking about this lately and have considered using a UVcut spectro with SpotOn! and using UVcut when measuring P2Ps.

I thnk uvcut (uvx) can be used, but I don't have the data to say that it would be better or worse than uv included (uvi) for all situations.. I have tried uvcut on paper with moderate amounts of OBs and the results compared to uv included mesurement, despite showing clearly different Lab values, were curves that were nearly identical. Likely because all measurements are skerwed in the same direction. Heavier amounts of OBs can cause larger differences between uvx and uvi. And likely more problematic when mixing uvx for curve calculation with uvi for process control. Need to make sure everybodys speaking the same language.

My current method with the P2Ps is to use an iSis with Measure Tool and measure BOTH "no filter" and UVcut and average them together....with most "brightened" gloss coated house stocks, the result is a nearly perfect a*0/b*-2-2.5 with these papers (they typically measure about a*+.5/b*-3.5 with no filter).

That's really interesting and hadn't occurred to me...due to that fact that I don't have an isis. this seems like it could be effective in dealing with the problem of the original post.

Of course the same issue will exist if there is a disconnect between the spectro illuminant and the viewing illuminant and its difficult to arbitrarily say that any filtration will cure the issues. I think ideally more robust solutions will be necessary.


Mike

 

[mamarcus]

I would like to thank everyone for there response it was very helpful as always.

Terry it is an coated offset stock (the Carolina Cover series by International Paper). I did find a stock that we had in house that does meet Gracol \ SWOP specs of L*95, A*0, B*-2 and the results were good using SpotOn to measure the press sheets. I was able to hit all my Primaries and the NPDC so if I am understanding you correctly that there is not a way to compensate for the optical brighteners to achieve the numbers published by Gracol and SWOP?