G7 tolerances

[jstotz]

Can someone tell me where to find the tolerances that are used to judge proofs and press sheets for the G7 qualification process?

 

[jaed]

G7 Overview | IDEAlliance

There are a bunch of links on this page that should be helpful.

 

[jstotz]

I looked through those and there isn't really anything that says what tolerances are actually used for for the proofs and press sheets submitted for the qualification process.

 

[jaed]

Do your proofs have the control stip on them? The whole Idea behind the G7 method is to print to a neutral grey with your process colors CMY. Once your proofer is calibrated every proof should have a copy of the control strip printed on the side of it. If you want to check and see if it is printing neutral grey you need a spectrophotometer and you check the grey patches looking at all 4 channels and you should see something along the lines of K30 C30 M30 Y30, it probably won't be that perfect but thats the general Idea.

 

[gordo]

Can someone tell me where to find the tolerances that are used to judge proofs and press sheets for the G7 qualification process?

It is stated in several places in the GRACoL literature and it is based on ISO 12647-2
For the solids the tolerance is Delta E of 5 or less for all colors.

best, gordo

 

[Colorblind]

You can go this link : SWOP | Certification: Certified Systems and download the "Hard Copy Proof Application Data Sheet" PDF of any listed certified vendor, under the "type 1" column. Adding to what Gordo mentioned, CMY midtone grey (50c40m40y) and white paper patch have to be under 3 DeltaE

 

[jstotz]

Thanks for the info. Maybe I should have provided more details. I'm not really concerned about the proofs. I was more concerned about doing a press test for GRACoL on a UV press. The green we are getting is 9 delta-E out but the other colors are close. It looks like we'll need to run it on the conventional press.

We've passed our G7 Master Printer in past years, (on a conventional press) but we never got any feedback about how close we were. Did we have a great sheet or did we just squeak by? I was trying to find out how they evaluate the sheet. For example, are they checking only the solids, traps and grays? I would assume they read the P2P target. If so, is there an acceptable number for the NPDC curve values? For the entire set of TVI swatches? Do they read the IT8 chart? If so, do they look at the average for the whole chart? For the best 90%? Do they look at the sigmas? Do they allow a higher delta-E for the dark 3-color swatches than they do for the bright colors?

Colorblind, I looked at the link you provided, but I had always thought that these documents were for manufacturers to get their devices certified, not necessarily the tolerances for a sheet submitted for the G7 Master program.

 

[Deleted member 16349]

It is stated in several places in the GRACoL literature and it is based on ISO 12647-2
For the solids the tolerance is Delta E of 5 or less for all colors.

best, gordo

Gordon,

Do you have an idea of what the approximate density tolerances would be for the Delta E of 5 tolerances for the solids?

Just curious to see how wide they might be.

Thanks.

 

[Colorblind]

Colorblind, I looked at the link you provided, but I had always thought that these documents were for manufacturers to get their devices certified, not necessarily the tolerances for a sheet submitted for the G7 Master program.

There is apparently a little more info on these PDFs such as a description of the steps required to produce the sample proofs they submitted for certification. While they show that they meet the specification tolerances, they omit to disclose the actual numbers they were able to get... But you're right, these documents are for proofing system vendors to get Gracol C1 - SWOP C3-C5 certified.

 

[gordo]

But you're right, these documents are for proofing system vendors to get Gracol C1 - SWOP C3-C5 certified.

I believe that application data sheets show how to setup a specific vendor's proofing device to output the certified proof. It is applicable to both vendor and printer.

best, gordo

 

[gordo]

Gordon,

Do you have an idea of what the approximate density tolerances would be for the Delta E of 5 tolerances for the solids?

Just curious to see how wide they might be.

Thanks.

No, I'd love to know as well. There are a few things that seem to remain secret about G7 and GRACoL 7 that no one who has gone through the process will talk about.
For example;
1) What are the center point SIDs that they arrived at once they hit the required CIELab values?
2) What SID variation is represented by a CIELab Delta E value of 5?
3) What were the base dot gain curves before G7 and what were they after?
4) Why does the GRACoL press operator guide specify that printing the targets should result in "measurements are within 1.5 delta C* of target" - but does not explain what "C*" is nor why they don't use L*a*b*

best, gordo

 

[ScannerMan]

Thanks for the info. Maybe I should have provided more details. I'm not really concerned about the proofs. I was more concerned about doing a press test for GRACoL on a UV press. The green we are getting is 9 delta-E out but the other colors are close. It looks like we'll need to run it on the conventional press.

We've passed our G7 Master Printer in past years, (on a conventional press) but we never got any feedback about how close we were. Did we have a great sheet or did we just squeak by? I was trying to find out how they evaluate the sheet. For example, are they checking only the solids, traps and grays? I would assume they read the P2P target. If so, is there an acceptable number for the NPDC curve values? For the entire set of TVI swatches? Do they read the IT8 chart? If so, do they look at the average for the whole chart? For the best 90%? Do they look at the sigmas? Do they allow a higher delta-E for the dark 3-color swatches than they do for the bright colors?

There are different levels of G7 Compliance, G7 Grayscale, G7 Targeted, G7 Colorspace and G7 Extreme.

For what your asking , G7 Grayscale they would look at the P2Ps and see how good your gray balance is....delta e
On the G7 Targeted they would look at P2Ps and also IT8s checking that you not only hit gray balance but your solids and traps.
Its all in the G7 how to 2009.
And when you did your last certification the G7 Expert should have gone over everything with you on how well you did or didnt do.

 

[rich apollo]

The tolerances are still under development by the Print Properties Group. As ScannerMan stated there are different levels of compliance. I've attached a chart from a draft version of the tolerance document from March of 2010. I'm probably gonna' get whacked for leaking this information.

 

[gordo]

Rich,

Can you explain what delta C*, delta F*, and delta L* are?
Also, why are they being used rather than Delta E? What are their respective benefits? Perhaps how they are calculated?

thx, gordo

 

[ScannerMan]

No, I'd love to know as well. There are a few things that seem to remain secret about G7 and GRACoL 7 that no one who has gone through the process will talk about.
For example;
1) What are the center point SIDs that they arrived at once they hit the required CIELab values?
2) What SID variation is represented by a CIELab Delta E value of 5?
3) What were the base dot gain curves before G7 and what were they after?
4) Why does the GRACoL press operator guide specify that printing the targets should result in "measurements are within 1.5 delta C* of target" - but does not explain what "C*" is nor why they don't use L*a*b*

best, gordo

My take on Question #1 is that every inkset will have a slightly different density once you hit the desired CIELab values, so they dont want to nail that one down....and thats why CIELab works so well. Its defining the color not density.

Question 2 is same as #1, it all comes down to the individual process and they will all be slightly different.

Question 3 is a Linear curve, after depends on press / paper / ink. Ive done all types of press / paper combinations and rarely do they ever come up the same for compensation curves

Question 4 I was only able to see this on the second to the last page of the press operators guide,tolerance for gray balance
and I have no idea Was fun trying to find the guide though !

You will not find Density target Aims, Density variation and TVI numbers for G7, its only for process control after the calibration and thats it.

 

[jstotz]

I'm pretty sure delta L is the L* difference without worrying about a* and b*. I would guess C is chroma. Maybe delta F is delta a and b without L? We care about hue but ignore lightness?

 

[gordo]

View attachment 2500

My take on Question #1 is that every inkset will have a slightly different density once you hit the desired CIELab values, so they dont want to nail that one down....and thats why CIELab works so well. Its defining the color not density.

I'm interested in printers' experience not specified targets to hit. So, what are the center point SIDs that different printers arrive at once they hit the required CIELab values?

Question 2 is same as #1, it all comes down to the individual process and they will all be slightly different.

OK, but again I'm looking for printers' experience. What SID variation is represented by a CIELab Delta E value of 5? I'm hoping for an answer something like: "To hit the correct CIEL*a*b* solid for Cyan we end up at a density of 1.25. We are within a delta e of 5 if we maintain densities +/- .02 of our center point density."

Question 3 is a Linear curve, after depends on press / paper / ink. Ive done all types of press / paper combinations and rarely do they ever come up the same for compensation curves

I think you misunderstand my question - it can't be a linear curve. I'll rephrase What were the base dot tone reproduction curves for a number 1 coated paper before G7 and what were they after G7?

Question 4 I was only able to see this on the second to the last page of the press operators guide,tolerance for gray balance
and I have no idea Was fun trying to find the guide though !

One of the problems with G7 documentation is clarity.

You will not find Density target Aims, Density variation and TVI numbers for G7, its only for process control after the calibration and thats it.

Again, that's not the info I'm after. What I'm trying to find out should be very simple - based on the user's experience what SIDs did they arrive at, what were the on press tone reproduction curves pre and post G7.

Have you implemented G7? If so can you provide your data?

best, gordo

 

[gordo]

I'm pretty sure delta L is the L* difference without worrying about a* and b*. I would guess C is chroma. Maybe delta F is delta a and b without L? We care about hue but ignore lightness?

"pretty sure" "guess" ???? those should not be the reaction of a manufacturer to reading their own industry specifications. (I share your reaction).

I would hope that someone involved in writing these documents starts writing them for their intended audience with more clarity and exactness.

best, gordo

 

[jstotz]

Here is a link that describes Delta-F, it is a measure of gray balance, a and b without L.

ColorWiki - Why and How of G7 Calibration

Delta-C is Chroma, and there is also a Delta-H for Hue.

ColorWiki - Delta E: The Color Difference

Thanks for all the responses.

 

[rich apollo]

Rich,

Can you explain what delta C*, delta F*, and delta L* are?
Also, why are they being used rather than Delta E? What are their respective benefits? Perhaps how they are calculated?

jstotz answered the "what" questions. The "why"s, as I understand them, are:

∆F deals with 3-color overprints - grays. As jstotz said, ∆F is just like ∆E without concern for L*. I think it would be considered different than chroma because chroma is the distance from absolute neutral while ∆F is from a unique point that is not necessarily absolute neutral.

∆L* deals with NPDC, or tone response. In this case you're looking at the reflectivity (¿density?) of the 3-color grays. One of the primary ideas for G7 is that we can adjust the plating to achieve a specific tone response curve. In the GRACoL_2006 colorspace a 50c, 40m, 40y, 0k hits 60, 0, 0. So on your qualification sheet your 50, 40, 40, 0 must hit an L* of 62-58 (61.5-58.5 for a proof).

∆C? Are you referring to ∆H? One of the requirements for ISO 12647 compliance is that you hit the solids to within 5 ∆E(ab), BUT the ∆H cannot be any greater than 2.5. In simplest terms, we're more concerned with the hue than with chroma or lightness.

It's also important to note that these are for qualification ONLY. The document states that these are too tight to consistently maintain on press.

So, what are the center point SIDs that different printers arrive at once they hit the required CIELab values?

You'll find that, generally, the SIDs for a number 1 sheet (well, a number 2 - I'm not sure I've ever seen a number 1 sheet) will fall around 1.35c, 1.35-1.4m, 1-1.10y, and 1.8k, BUT THESE ARE NOT HARD AND FAST NUMBERS. YOU WANT TO HIT THE L*a*b* TARGETS. Solid ink density doesn't describe color.

As to TVI - I think that will vary quite a bit depending on the ink package, paper traits, environmental variables, blanket surface, ink-form roller size and condition, plate grain, fountain solution, lunar phase, wind-speed and direction, and whether or not you saw a black cat on the way to the press calibration.

G7 grew out of the inadequacy of ISO 12647 and SWOP to achieve a consistent appearance. TVI and SID do not describe color. They also don't describe your overprints, which I will argue are more important in color reproduction than the solids. In my experience, achieving TVI and SID targets did not yield neutral grays.

It was a funny conversation I had once when I was describing G7 calibration to an old scanner operator. To her, the idea of balancing the grays was not novel at all. That's when it occurred to me that Don Hutcheson is an old scanner operator/engineer/tech and he's an avid photo buff. We're just doing, on press, what scanner operators used to do.