Grey balance test

[gordo]

If it is allowed, please post the results of the Auraia DM screening test, or your observations. I have researched Auraia-II DM screening and it would be great to have a first hand account if it truly delivers on the claims its creators make. Thanks Gordo!
Regards,
Todd

Unfortunately the paper I'm working with didn't heed my advice. I suggested that they implement Auraia-II DM over a few weeks. However, after one test run on a live section of the newspaper they decided to drop AM screening completely and switch over to Auraia-II DM. So far (3 weeks) they have not had any press issues with the screening. The press operators love it. Their ad department loves it, a major advertiser who had been ready to pull a weekly insert due to quality issues has written them a letter of appreciation for what they are now printing.
They didn't monitor their ink consumption before so there is no way to see if ink consumption is less. My experience with Staccato suggests that Auraia-II DM will reduce ink consumption more than Staccato did. Perhaps more than a 25% reduction.
I have no way to check the gamut to see if it is larger - I suspect it is based on my experience with Staccato.
Auraia-II DM does reduce the visibility of misregistration - a big issue for them.

If you PM me and pay for shipping and handling I would be happy to post you a copy of the paper.

best, gordo

Note: I am in no way compensated/employed by Hamillroad Software the creators of Auraia-II DM.

 

[tmiller_iluvprinting]

The 3/C gray patch can be used as a process control metric. A single measurement tells you if your print is gray balanced (important for a gray balance calibrated device), and if you are successfully reproducing your targeted NPDC. The 3/C gray patch is something that IMHO is not used enough in the press room by press operators.
Regards,
Todd

 

[Deleted member 16349]

The 3/C gray patch can be used as a process control metric. A single measurement tells you if your print is gray balanced (important for a gray balance calibrated device), and if you are successfully reproducing your targeted NPDC. The 3/C gray patch is something that IMHO is not used enough in the press room by press operators.
Regards,
Todd

What should the operator do if the gray patch is not gray? That is not such an easy task to determine and therefore not so good as a process control tool. Yes, can tell if something is not perfect but not what kind of move needs to be done to get it back to gray. That is how I understand it.

 

[Alois Senefelder]

Gray Bars

Gray Bars

Gentlemen,

More PDFs on the use of Gray Bars in Newspaper Printing





Regards, Alois

 

[gordo]

This is a two part response to this thread. First some background then my answers to a few posts.

I think that there may be a lack of understanding of the mechanics of what happens on press and as a result, perhaps an appropriate emphasis is placed on print condition attributes.
First of all offset presses do not drift. Once the appropriate ink/water balance is achieved the press stabilizes in its laydown of ink. The ink film (SIDs) are stable but do vary. You can see this clearly in the below chart of the SIDs variation through a 10,000 sheet press run.



The SIDs go up and down within the SID tolerance range by about +/- 0.03 points. They don't drift i.e. start increasing or decreasing in one direction. The press operator will make minor adjustments to maintain SIDs in the center point of the tolerance. A shift of +/- 0.03 points will show up in the color of 3/C grey patch - however it's irrelevant information. A shift of +/- 0.05 to 0.10 points will show more strongly in the 3/C grey patch however, that is still within typical print standards tolerance range. Even then, there will be little to no change in the appearance of the live image area as I have demonstrated and which you can prove to yourself.

Although they are getting much better - the density of ink laid down by a press is affected by the amount of ink used in each ink zone. Put another way, SIDs are affected by inline ink usage. That is why great pains are taken in test forms to even out the ink usage across the sheet. E.g.:



and



Press operators do their makeready by coming up to the appropriate SIDs and ink water levels (basically the only control they have beside, (somewhat) speed and pressure). They then check the integrity of the halftone dots which tells them things like over emulsification, packing, slur, doubling, etc. If the SIDs are correct and the dots have integrity that tells them that the press is in a nominal state.

So:

Gordo, I think that your point that the 3-color gray is too sensitive to density changes has some merit. Although I also see the validity in an early warning that is more sensitive than the page content.

As per my explanation above I do not see it having any validity at all.

I'm curious how you feel about it's use to indicate that the TVI is out of whack even if the SIDs are correct? Indicating that the press is not printing the same way it was when the curves were made. Is that sort of problem more worth the operator's attention?

Press operators don't have time to measure TVI, besides, if the target patches are in the color bar the measurements will have little to no value (inline inking issues). The integrity of the dots is what the press operator is concerned about since that and the SID levels determine if the press is operating properly. This assumes that prepress has delivered plates that allow the press to run in a stable manner with the appropriate tone reproduction.

You could just as easily conclude that the measured density deviations from the SID targets are not worth the press operator compensating for.

If the SID variations are within the appropriate tolerance range then the press operator keeps his hands off.

The 3/c gray patch offers a visual (and measurable, but lets stick with visual) "alarm" that the press SIDS are deviating from the norm.

He's already measuring the SIDs, there is no need to use a second reference. I also prefer that the operator use a target that requires measurement than one that is subjective.

The SIDs themselves are not so much visual, but the measurable result of them should be just as "alarming" to the press operator, should he/she be monitoring it.

Absolutely.

I think I've already answered the other points you make.

best, gordo

 

[Chasfinch]

an issue we ran into where we had multiple issues going on and it showed within the trapping within the color bar. The magenta ink was too tacky and wrong color. So the 3 color patches on the color bar looked wrong.

As I discussed this with the pressman he said something that is key. We use the color bars as a tool. We adjust to make the print look correct.

SO I think while gordo has a point that the 3 color grey has little effect on what is desired final outcome - it is a tool pressman can and should use as a warning sign.

So everyone is right LOL

 

[meddington]

Sorry Gordo, but I'm not seeing any empirical data that proves that a 3/c gray patch is any more or less disassociated with the live image area than SID. It may be a more sensitive metric, redundant/secondary, or subjective when visually analyzed, but it it can be useful for a quick check. Inline issues would skew both the 3/c patch and the associated SIDS, where the 3/c patch would be skewed until SIDs were balanced. One could argue that balancing this gray patch is irrelevant if the image area isn't effected, though one could use the same argument that SID balancing would be unnecessary.
Moreover, Often SID and 3/c gray patches are placed in live areas of packaging jobs, where I feel they have more value as a QA tool. Often there is not room for all SID and TVI patches, but a gray patch can yield much data here.

 

[gordo]

Gentlemen,

More PDFs on the use of Gray Bars in Newspaper Printing

Regards, Alois

That's the spirit!

It's important to note that the PDFs appear to come from the days of UCR separations (when there was a better correlation between the 3/C grey patch and the live image area) rather than the present world of GCR and especially ink optimization maximum GCR separations (where there is no correlation between the 3/C grey patch and the live image area).

Also note that the PDFs do not offer any evidence for the validity of the statements made. It's more anecdotal opinion.

My snipped version with comments:

“Gray bars are assembled to be an easy aid for an operator to, without the use of any instrument, determine the amount of the inks deposited on the substrate in respect to each other.”

[GP] The amount of inks refers to density and appears to support the arguments made by others against what I've demonstrated.

“The idea behind the gray balance field is that the right amount of the three chromatic inks. cyan, magenta, and yellow, will produce a neutral colour.”

"The use of gray bars for monitoring the amount of ink has become very popular in the newsprint industry lately even though using the naked eye only an imbalance between the amount of inks is easily detected."

[GP] Yup.

"The approach of using the gray-bars for monitoring the amount of ink is to:
1. Detect an erroneous amount of one or more of the chromatic inks, seen as a colour cast in the gray balance field.
2. Detect an erroneous amount of all the chromatic inks if compared to the black ink seen as a difference in darkness between the gray balance field and the black field."

[GP] Number 1 can be determined by measuring the SIDs. Number 2 refers to tone reproduction - TVI. This was important in the old days because dot gain was used as a process control metric. Supplied film was linear and measured with a transmission densitometer. Then dot gain was measured on the press. If you are running at the correct densities but your TVI is out of tolerance then you know there is a problem somewhere in the process.

"The use of gray balance fields does only reveal an erroneous relative amount of ink in respect to each other. The overall level of the ink have to be determined in another way."

[GP] I.e. measuring SIDs.

"A result of measuring the amount of ink on gray-bars is sensitive to erroneous dot sizes. It is therefore vital that dot-sizes on the printing plates are correct. Otherwise the operator can be misled to compensate for an erroneous halftone dot-size by changing the density. This will cause incorrect reproduced colours in areas with other halftone dot-sizes."

[GP] As I wrote, monitoring dot gain was a key metric for process control.

"Experience shows that despite the easiness for anyone to detect an unbalance between the inks it is not always easy, even for a skilled operator to decide what action to take to reduce the deviation from the correct amount of ink. This leads to a conclusion that operator needs support in form of an instrument suggesting what to do."

[GP] This seems to say that if the press operator sees an imbalance in the grey bar he hasn't got a clue as to what to do about it. I.e. the information is useless to him. Hmmmm.

One of the great things is that many newspapers now include grey balance bars on their pages (an noted exception is USA Today which only includes solid patches). The grey balance bars are used because they are easy to hide in the editorial content. In my experience and in talking to newspaper publishers and their suppliers, they are not used to measure grey balance. They are used to measure SIDs. The densitometer reports the densities of the grey patches and the press operator has targets for those densities based on what they were when the press was set up.
If you still buy a daily paper, it's an interesting exercise to monitor the grey balance bars. You'll find that it bears little relationship with the live image area and is heavily influenced by inline ink issues.

best gordo

 

[gordo]

Sorry Gordo, but I'm not seeing any empirical data that proves that a 3/c gray patch is any more or less disassociated with the live image area than SID.

Don't you think that's a problem, especially given the claims made for the value of the 3/C grey patch? What I've tried to do is provide information which anyone can use to test it for themselves (since the industry organizations don't seem to have done it). I've even offered to include test elements supplied by forum members that I'll run on my next press test. So they can see for themselves.

One could argue that balancing this gray patch is irrelevant if the image area isn't effected,

That's what I'm arguing.

though one could use the same argument that SID balancing would be unnecessary.

No. SIDs and water balancing are critical to the offset lithographic process. It's a narrow window that if exceeded (too much or too little ink) causes the process to fail. This is a mechanical issue, not a color issue.

Moreover, Often SID and 3/c gray patches are placed in live areas of packaging jobs, where I feel they have more value as a QA tool. Often there is not room for all SID and TVI patches, but a gray patch can yield much data here.

Well, there's a heck of a lot lot of packaging that does not print with CMYK inks. So what do you use? Oh, oh, solid patches?

best, gordo

 

[Deleted member 16349]

SIDs and water balancing are critical to the offset lithographic process. It's a narrow window that if exceeded (too much or too little ink) causes the process to fail. This is a mechanical issue, not a color issue.

I would not say that it is a narrow window at all. It may seem that way but what is the metric to say that it is a narrow window? The impression that it is a narrow window can be due to many factors. One could be the strength of the ink where a small variation in ink feed would result in a noticeable density change or it could be in the amount of variation of the ink feed itself.

It is easy to see if it is the ink strength that is causing the issue. Plot an ink mileage curve that plots density vs ink volume. Draw horizontal lines at the upper and lower limits of the density tolerance targets and the density aim target to the curve. Then draw vertical lines down from the curve to the ink volumes related to those points on the curve. This will then show the relative amounts of ink above and below the target aim points.

For some inks I was involved with the change in ink volume for +/- 0.05 density points was about +/- 8% in change of ink volume or weight. I know from some curves of newspaper inks, that tolerance range would result in something like +/- 12% change in ink weight. So a total range of 16% or 24% for newspaper inks.
To me that is not a narrow window. Therefore the problem must be with the ink delivery consistency of the presses that is responsible. By the way, it has nothing to do with the water. Water does not show up in density values and anyway it evaporates away eventually. Water just affects the delivery rate of ink into the system.

 

[Deleted member 16349]

The grey balance bars are used because they are easy to hide in the editorial content.

They are used to measure SIDs. The densitometer reports the densities of the grey patches and the press operator has targets for those densities based on what they were when the press was set up.

I may be understanding these statements incorrectly but if it means that the density values of the gray bar are used to adjust the CMY inks, then there is a problem in principle with this idea.

If one reads the CMY densities of a gray patch, these density values are NOT directly related to the individual CMY inks used to print that gray patch. It is not a trivial problem in attempting to determine the ink levels and direction needed for correction by measuring a printed screen.

 

[meddington]

Don't you think that's a problem, especially given the claims made for the value of the 3/C grey patch?

Yes, I think it's a problem and you have a point, but I don't agree it's limited to grey patches. Your initial post implied that a 3/c gray patch is more disassociated from the live image area than solid ink density. My point is that this is disingenuous.

What I've tried to do is provide information which anyone can use to test it for themselves (since the industry organizations don't seem to have done it).

The test you proposed in this post is better suited to test the stability of imagery separated with different levels of GCR than of the merits of the control elements.

No. SIDs and water balancing are critical to the offset lithographic process. It's a narrow window that if exceeded (too much or too little ink) causes the process to fail. This is a mechanical issue, not a color issue.

I think one can push density quite a bit without failure, however my point was that a press operator wouldn't be as inclined to hit target density if the process were functioning normally and the imagery was acceptable...any more or less than if monitoring a 3/c patch. Further, if SIDs were balanced to targets, one would expect the 3/c gray to be nominal.

Well, there's a heck of a lot lot of packaging that does not print with CMYK inks. So what do you use? Oh, oh, solid patches?

Solids and screens when possible. Solids alone are not very helpful for QA post printing. Much of our imagery is still process color however, even when present in expanded gamut work.

 

[gordo]

Yes, I think it's a problem and you have a point, but I don't agree it's limited to grey patches. Your initial post implied that a 3/c gray patch is more disassociated from the live image area than solid ink density. My point is that this is disingenuous.

I'm not trying to be disingenuous (i.e. not candid or sincere, typically by pretending that one knows less about something than one really does).

The test you proposed in this post is better suited to test the stability of imagery separated with different levels of GCR than of the merits of the control elements.

If the control elements do not reflect the characteristics of what they are supposed to control then, by definition, they do not control or reflect that which they say they control.

I think one can push density quite a bit without failure, however my point was that a press operator wouldn't be as inclined to hit target density if the process were functioning normally and the imagery was acceptable...any more or less than if monitoring a 3/c patch. Further, if SIDs were balanced to targets, one would expect the 3/c gray to be nominal.

The density range is an ink film thickness between about 1 and 1.25 microns (as published by Heidelberg and others). That is a density range of about +/-.30. Pretty narrow IMHO.

Solids and screens when possible.

Yes.

Solids alone are not very helpful for QA post printing. Much of our imagery is still process color however, even when present in expanded gamut work.

I agree. For post printing forensics (i.e. when presswork fails) the more data points you have to refer to the better. But that's a different issue than production presswork.

gordo

 

[Green Printer]

Gordo

You have certainly struck a cord with this. I agree with your conclusions and test. Ink under your nails speaks much louder. Too many don't understand all the variables.
Good printing is controlled choas manufacturing with defined limited boundries. With all of the variables, ink, wet split transfer, ink film thickness, rollers, stripes, plates, plate grain structure,fountain solution, temperature, blankets, plate to blanket pressure, blanket to substrate pressure,washups, SID,press speed are just a few control points. When all of these and others are working together correctly you get great looking printing if not you get all these discussions.

 

[Deleted member 16349]

The density range is an ink film thickness between about 1 and 1.25 microns (as published by Heidelberg and others). That is a density range of about +/-.30. Pretty narrow IMHO.

Gordon, can you clarify this range a bit. It does not look right to me.

Does the statement mean that with a range of 0.25 microns (from 1 to 1.25 microns) of ink film, one would have a range of 0.60 (+/- 0.30 ) density points? This would imply a fairly narrow range but I don't think the stated ranges are correct.

It would be good if we all understand what the real ranges are. My view being that the range for ink is wide with respect to the density tolerance range. You feel that it is narrow and it might be due to wrong data.

It is a concern for me since over 20 years ago I developed an algorithm to predict density changes from one density value to another based on the change in the percent of ink feed. This was also confirmed on press. The algorithm accounted for inks with different strengths.

 

[tmiller_iluvprinting]

What should the operator do if the gray patch is not gray? That is not such an easy task to determine and therefore not so good as a process control tool. Yes, can tell if something is not perfect but not what kind of move needs to be done to get it back to gray. That is how I understand it.

When the densities of the 3/C gray patch are measured simultaneously the density values can be used to estimate how much correction may be needed. For example, if the neutral density aim is .54(- paper) and when measured with a densitometer, set to measure all colors simultaneously (the scanning spectro on the press console may be used also) the values are .54 Cyan, .54 Magenta, and .59 yellow, the yellow ink settings may need to be reduced, or, maybe there is a TVI issue? The alarm bells have sounded.......
Regards,
Todd

 

[Deleted member 16349]

When the densities of the 3/C gray patch are measured simultaneously the density values can be used to estimate how much correction may be needed. For example, if the neutral density aim is .54(- paper) and when measured with a densitometer, set to measure all colors simultaneously (the scanning spectro on the press console may be used also) the values are .54 Cyan, .54 Magenta, and .59 yellow, the yellow ink settings may need to be reduced, or, maybe there is a TVI issue? The alarm bells have sounded.......
Regards,
Todd

My point was that the readings, such as the yellow at 0.59 is not directly related to the yellow ink. That is the problem.

 

[rich apollo]

Water does not show up in density values and anyway it evaporates away eventually. Water just affects the delivery rate of ink into the system.

I'm sorry, Erik, but I disagree with that statement completely. I've seen far too many faulty sheets that were the result of water problems; pH is wrong, tanks are contaminated, conductivity is wrong, plate grain isn't carrying the water correctly, incompatible ink and fountain solution, et cetera.

------------------------

The 3c gray patch is all about balance. So, while it does not give a direct indication of ink film thickness, you can monitor the visual density of the gray patch to get an idea. That visual density is less important than the balance of the inks. You can get away with running light or heavy as long as the CMY are consistent with one another.

I would also argue that the 3c gray patch is less dissociated from the sheet content than solid patches. It's not all that often that you will run a single-channel solid as part of the live work in process printing. Solid patches give no indication of the interaction of the inks. That's not to say that solid patches are not useful, and overprint patches even more so.

Gordo, you know I respect you and your opinions, but I don't think your conclusion is valid in the examples you posted. You demonstrated the image stability that can be gained with the use of GCR. The one image that was not optimized reflected the same issues that the gray patches displayed. To perform your test properly you will need to forego the optimization process altogether.

Running without instruments like your client is, is a great opportunity to make use use of 3c gray control patches. They will be able to pick up,on potential issues more rapidly than they would otherwise.

Again, I would encourage you and Erik to contact Andrew Yan at Dragon Tone. Very interesting work.

 

[meddington]

I'm not trying to be disingenuous

Fair enough, but to intentionally skew the magenta density up by +0.20 (a density measurement that screams “out if tolerance”), then point at at the 3/c gray patch as being not valid doesn’t seem legitimate. The entire colorbar would be reflecting this imbalance, though perhaps measurably rather than visually.

If the control elements do not reflect the characteristics of what they are supposed to control then, by definition, they do not control or reflect that which they say they control.

A gray patch no more a control element for gray balance than a speedometer is a control element of speed (I used the term “control element” first, so I’ll take the blame). However, even with heavy GCR/image optimization, there can be color builds that are more sensitive to fluctuation. Moreover, one could compare the results of the 3/c gray patch to gauge how close reproduction is to its “calibrated/fingerprinted” condition rather than associate it to the live image area.

The density range is an ink film thickness between about 1 and 1.25 microns (as published by Heidelberg and others). That is a density range of about +/-.30. Pretty narrow IMHO.

That seems conservative to me. I would say the true thickness range would be closer to twice that, but I’ll conceed the point. Even with a range of +/- 0.30, that’s still a rather wide window.

 

[Deleted member 16349]

Again, I would encourage you and Erik to contact Andrew Yan at Dragon Tone. Very interesting work.

Rich, I had a look at Andrew's web site and found some of his patent info. At this time, I don't see that it is anything special but I could be wrong. The site does not really say much about what the concept does in any detail. It for sure is not in a direction I would like things to go.