Colorbars, print contrast, tints, relating to G7

Prepper

Well-known member
I have a question relating to colorbars and reading them with curves applied. Maybe I'm doing this wrong but I've always applied curves to everything on the plate including the colorbars. Recently we had an issue on press where we wanted to check print contrast of yellow and while the scanner reference is set to what we normally used for Gracol and those tolerances, now I'm wondering what would we set it to or even how would we know what it should be on jobs with G7 NPDC curves applied to the colorbars?

Should we not be applying the curves to the colorbars? But if not, then we'd be measuring gray balance there without the curves applied so that doesn't work either, right?

So, I guess if print contrast is too low, indicating filled in, then the gray balance at 75% would be off in the affected color.
Same way with monitoring TVI also, if 50% gray is off, the TVI is off from what it should be for G7. No way to really monitor TVI separately is there? My yellow curve is set to 52% at 50 and my cyan is 44% at 50, so what would the reference point for 50% tint be set to in the press scanner? Doesn't really matter about that does it, just gray balance, which tells you whether your tints are right.

Get back to where I was before, telling the pressman he could run with just a gray balance bar for CMY gray and K and some solids just for keeping it even during the run. He doesn't agree with that one though. :)

Thanks
 

rich apollo

Well-known member
You're correct, the curves must be applied to the colorbar. Instead of monitoring dot gain, you'll measure the densities of the quarter-tone, mid-tone, and three-quarter-tone gray and black patches.
 

alibryan

Well-known member
Think of it like this; you're changing the curve for the press to compensate for dot gain. Some presses need more adjustment in prepress than others but the idea is that you're trying to match the color in the digital file (non-variable) to the color reproduced on the press (variable). Whatever curve you apply to compensate for color coming off the press, needs to be in the grey balance areas as well because those are also achieved by printing dots.

I believe that when you put in the numbers for the curve it also applies to the (percentage) dots in the color bar. What you don't want to do is change the slur bars or dot gain measuring patches because if you do that, what does the press operator have left to use for a guide on press?
 
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gordo

Well-known member
What you don't want to do is change the slur bars or dot gain measuring patches because if you do that, what does the press operator have left to use for a guide on press?

Just a side bar...
Press operators don't need slur bars - every element in the presswork serves as a slur indicator.
Press operators don't usually measure dot gain as a guide. Typically, if they are used at all, dot gain patches are use forensically if a job gets pulled.
 

alibryan

Well-known member
As an operator I prefer to have slur and dot gain patches. I know and appreciate the value of having them on every job possible. If I change a blanket or packing or anything at all, the first thing to tell me if I have a printing problem with that unit (or if my printing is exactly what it should be) is the slur and dot gain patches. I'm talking about the ones that also have a 98, 99 and a 1 and 2 percent dot area. These are best served at the tail of the sheet (not the side) so you can tell if there's any kind of sheet distortion going on.

I've worked at shops that have them incorporated into the color bar on every plate and I've worked at shops that don't use them at all. Quality can be achieved both ways but running without them leaves you nothing to look at to instantly know the quality of the image being printed.

Simply put;

A full color bar including solid, trap, slur, gain and grey scale patches is the single most important tool a press operator has at their disposal for consistent color reproduction on press.
 
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aqazi81

Well-known member
Can a press operator control dot gain?
Gordo... I want to know your opinion on this.
 

gordo

Well-known member
Can a press operator control dot gain?
Gordo... I want to know your opinion on this.

How you ask a question can affect the answer.

"Dot gain" is a derived value from the ratio of the measured density of a solid patch of ink, a tint of that ink (halftone screened or not), and a non inked area (the raw substrate).

The press operator's actions are directed towards laying down and even film of ink by controlling ink density (through volume, fountain solution emulsification, pressure, etc) - since, in this context, ink density is pretty much the only control that a press operator has.

Since ink density affects both the density of the solid patch of ink tint and a tint of that ink - the press operator's actions can affect the derived dot gain values.

It's important to understand and remember that the densitometer (or spectrodensitometer) that is used to do the measurements does not measure the actual physical size of halftone dots. It measures tone values (solid ink, tint of ink, substrate) and their ratios and reports their ratio as either "dot" gain (more correctly "tone" value increase) or dot area (more correctly "tone" area). That is why the term "dot gain" is not used much in print specifications literature.

It's also important to understand that dot gain is not a target to hit. Instead, the measured dot area (tone area) is the target to hit. This is because different dot gains can result in the same final measured dot area. This is perhaps why G7 dropped dot gain targets and went to density values of tints.

So, during a production run, the press operator's actions are directed towards laying down and even film of ink by controlling ink density. The press operator's actions are not specifically directed towards controlling dot gain. That being said, dot gain values may have some value as a diagnostic tool if the presswork is pulled because it fails to meet expectations.
 
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alibryan

Well-known member
Mechanical dot gain on a printing press:

"On an offset printing press ink is transferred from the printing plate to the blanket and from the blanket to the paper. Each time the dots get squashed a little bit, increasing the physical diameter of the printed dot. The ink that is used, the fountain solution, the blanket, the pressure (over/underpacking) and the speed at which the press runs all influence this type of dot gain. When ink is absorbed in paper, this occurs both vertically (into the paper) and horizontally which again increases the dot diameter. This effect is more pronounced on uncoated than it is on coated paper."

Without the presence of accurate dot gain or slur patches in a color bar, it is very difficult to determine how much gain is being experienced during a press run. These patches are not typically used to determine ink density as much as they are used to determine actual print quality.

Some of these patches are very useful for instantly determining the quality of print visually - without the use of a densitometer - and can give the operator an accurate idea of the image quality being printed.
 
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gordo

Well-known member
Mechanical dot gain on a printing press:

"On an offset printing press ink is transferred from the printing plate to the blanket and from the blanket to the paper. Each time the dots get squashed a little bit, increasing the physical diameter of the printed dot. The ink that is used, the fountain solution, the blanket, the pressure (over/underpacking) and the speed at which the press runs all influence this type of dot gain. When ink is absorbed in paper, this occurs both vertically (into the paper) and sideways, which again increases the dot diameter. This effect is more pronounced 0n newsprint than it is on coated paper."

Without the presence of accurate dot gain or slur patches in a color bar, it is very difficult to determine how much gain is being experienced during a press run. Those patches are not typically used to determine ink density as much as they are used to determine actual print quality.

The definition is correct although it does not mention optical dot gain (which can be the greater portion of the gain).

Your conclusion, IMHO, is not correct.

Press operators don't typically determine how much gain is being experienced during a press run because that is not primarily what they're concerned with. They are concerned with managing the press so that it does what it is designed to do (see my previous post). If you, as a press operator, honestly measures dot gain during the run then you would be very much in a small minority of print operators.
If the press has been set up correctly then the presswork will align with the proof once the press is functioning normally. That includes tone reproduction (dot gain).

As far as slur is concerned, every element on the press sheet can be used to identify slur/doubling. Having those targets in just one small area of the press sheet is of very limited value. (check out this posting for more info: The Print Guide: What the press operator is scrutinizing )
 
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alibryan

Well-known member
"The definition is correct although it does not mention optical dot gain (which can be the greater portion of the gain).

Your conclusion, IMHO, is not correct."



Optical dot gain:

"When light hits the printed surface, it becomes slightly diffused around the dots. The human eye (as well as measuring devices) perceive this as a darkening. Dots appear to be larger than they really are."



All I was saying to the OP to begin with is that dot gain and slur patches need to be without a curve applied so they can be accurately measured - visually or with a densitometer - to help determine print quality. Without them you can't be as accurate in your judgement of printed image. Here are some other factors that cause dot gain:


1. Ink and water balance.
2. Blanket construction or other properties.
3. Blanket height.
4. Roller settings.
5. Bearer pressure settings.
6. Plate wear.
7. Ink temperature.
8. Piling.

And more, most of these can be controlled by the operator but are harder to determine without some type of 'guide', as in patches in the color bar.

I understand that your experience in the industry is extensive and wouldn't want to debate you in any area except that I don't know how much press operating experience you actually have. As a press operator I don't claim to know what the majority of other operators are doing, and I don't know how that would be determined to begin with. I do know that I can instantly tell how good an image coming off the press is by looking at these patches. They're obviously not the only thing I look at but they show me things that no other patches can.

On another note; if these patches aren't printed in the first place then how could you use them on a job that is pulled for forensic evaluation?
 
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gordo

Well-known member
All I was saying to the OP to begin with is that dot gain and slur patches need to be without a curve applied so they can be accurately measured - visually or with a densitometer - to help determine print quality.

Normally, if dot gain patches are included, then whatever curve was applied to the live image area is also applied to those patches. Otherwise the patches in the color bar would not reflect what is happening to their equivalent areas in the live image area.
I have been in shops where the dot gain patches in the color bar are run linear (while the live image area is curved). That can work if the printing is consistent in terms of content and substrate (e.g. web publications, newsprint).


Without them you cannot be as accurate in your judgement of printed image. As a press operator I don't claim to know what the majority of other operators are doing, and I don't know how that would be determined in the first place.

My opinion is based on running presses, study, testing and visiting several hundred shops around the world. But I could be wrong.

I understand that your experience in the industry is extensive and wouldn't want to debate you in any area except that I don't know how much actual press experience you have.

Ran a Heidelberg - Speedmaster SM 102 but was also technical director of a large west coast sheetfed shop.

I do know that I can instantly tell how good an image coming off my press is by looking at these patches. They're obviously not the only thing I look at but they show me things that no other patches can.

See my posts on that subject.

That link you posted is an article written by you?

Yes I wrote the article. The photo of the press operator was taken at a newspaper plant in Oregon after converting their presswork to FM screening.
I have been fortunate in being able to visit hundreds of shops around the world to observe operations and ask the same questions of management, prepress, and pressroom. Before opening my big mouth I do first try to understand, from their perspective, how and why they operate the way they do. It's quite fascinating to hear their answers and (often lack of) reasoning behind the way they work.

______________________________________

"We see things not as they are - but as we are."
 
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alibryan

Well-known member
As I said I understand your extensive experience, even more so now than before your last post. I hope I didn't offend you in any way and if I did I certainly didn't mean to.

I don't understand your opinion on press operators not needing gain or slur patches. I stated before that I prefer them (and why) and I still do.

It's only my opinion but I think it's valid. If this forum only needed one poster's opinion (or indeed, the majority) it wouldn't be an open forum and that would possibly defeat the purpose.
 
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gordo

Well-known member
As I said I understand your extensive experience, even more so now than before your last post. I hope I didn't offend you in any way and if I did I certainly didn't mean to.

No offence taken at all.

I don't understand your opinion on press operators not needing gain or slur patches. I stated before that I prefer them (and why) and I still do.

It's only my opinion but I think it's valid.

My background and experience does not mean that what I say is correct. As I wrote - I could be wrong. This industry is full of misinformation, error, and opinions that are just that - opinions not based on objective data and proper testing. (don't take that as a shot against you).
Your opinion is valid because it works for you and that's fine. I tend to question opinions because I like to know if they are well reasoned out or they are held just because that's what is commonly accepted. As a result my questions can therefore seem aggressive or offensive in a forum context. In a face to face situation I'm more like that Columbo TV detective. Just asking questions to try and understand how the person has arrived at the opinion they hold.
The other dimension is that printing is a technical/mechanical activity. Should technical/mechanical activities be guided by opinion or by technical/mechanical criteria? Should they be discussed as matters of opinion? Or in terms of technical/mechanical principles?
Slur targets in the color bar indicate slur/doubling in the color bar and that's true. But the live image area, which is inspected and monitored by the press operator (especially during makeready where the problem will likely appear), will show the problem wherever it occurs on the sheet. As a result, most press operators that I have spoken to would prefer the slur target areas to be used for SID targets.

If this forum only needed one poster's opinion (or indeed, the majority) it wouldn't be an open forum and that would possibly defeat the purpose.

You are absolutely correct.

In an earlier post you asked "On another note; if these patches aren't printed in the first place then how could you use them on a job that is pulled for forensic evaluation?"

Good point.

I wrote: "dot gain values may have some value as a diagnostic tool" - note the word "may" - they may or may not have some value.

Here's my line of thinking.

Let's say the dot gain patches are included in the color bar.
The press operator does the makeready and comes up to the appropriate SIDs, ink/water balance, squeeze, etc.
In my experience, he then visually compares the press sheet to the proof fully expecting the two to align with perhaps a few minor SID moves.
Let's say that they don't align and he figures altering SIDs won't improve the alignment (or would be excessive and result in other issues arising). So the job is pulled and everything goes back to whoever is responsible for doing the (forensic) diagnosis.
Does having dot gain patches help figure out why the job failed? They might show that dot gains were not hitting the targets - but that fact is already known since the job got pulled because it did not align with the proof.
Let's say the dot gains were higher (or lower) than the target (BTW I've never seen dot gain values displayed at a press consul as a target for the press operator - just SIDs). What usually happens in the forensic process is that things like SIDs, the dot formation, plate tones/calibration, curves, proof setup, etc. are checked to determine where the point of failure occurred. The press dot gain whether higher (or lower) than the target is still not relevant since it doesn't tell you anything that you don't already know.

Does that make any sense?


______________________________________
"We see things not as they are - but as we are."
 
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Cornishpastythighs

Well-known member
Our closed loop ink density control equipment measures Density, Dot Gain, Trap and Spectro. It does this for every scan and not once have I seen any of our pressman check anything but Density and Spectro DeltaE.
I personally like to view trap as this lets me know how my process in performing but the only specs we have at the press are for Standard ink densities nothing for trap or dot gain. I have used slur/gain targets before but mainly to let me know if the back of the blanket is loose.
 

alibryan

Well-known member
To end my input in this discussion, the slur, double, and gain targets I am talking about are intended for visual inspection, not auto scanners. There are no specs (that I am aware of) to refer to, only operator experience. If, as an operator you are interested in the print quality at a glance, these are ideal.

The OP didn't specify what system they are using at the press, only that the press operator didn't agree with the OP's opinion on what was needed to run jobs.
 

rich apollo

Well-known member
Can a press operator control dot gain?

Not directly, but there a number of actions they can take will effect dot gain:

Squeeze the sheet
Under-, or over-, pack the blanket(s)
Change the fountain solution dosing, pH, conductivity
Adjust the ink and water balance
change the viscosity of the ink
Change the temperature of the ink train

A press operator isn't really looking to effect dot gain, but rather they are looking to run the press in a way that keeps dot gain consistent. If your dot gain changes dramatically, it indicates that something mechanical, or chemical, has changed in your system.
 

Prepper

Well-known member
A press operator isn't really looking to effect dot gain, but rather they are looking to run the press in a way that keeps dot gain consistent. If your dot gain changes dramatically, it indicates that something mechanical, or chemical, has changed in your system.

Which is why I think dot gain, or TVI, is good for a pressman to know, if they are having trouble with a job on press, say it's too red, and it has to be pulled, if he then goes onto the next job with the same curves, AND the reason it was too red was that the magenta unit TVI had increased for some reason say 6-8%, then that next job would still be just as red wouldn't it? (Murphy's Law would be that the next few jobs had no reds and look great, then the next job with red looks bad again and it's, "hey, the last jobs were okay and now this one's off so your curves must be changing around,") But if he looked at the TVI and saw that magenta was up higher than normal, would there be something for him to do at that point to remedy the situation? Or is it up to prepress to just adjust the curves accordingly and carry on until the next change comes along? I realize I'm looking at this from an inplant environment and not a production one.

Also, yes, there is a lot a pressman can do to affect TVI, if not control it, so someone needs to know what neighborhood it should be in at least. I would applaud a pressman that's looking at other things besides just density, which of course is his primary concern, I think he will have a much better chance of correcting something even if it's minor before any pulling of jobs has to be done. I think a lot of what Gordo was saying is predicated on that the press is set up correctly to start with, and that isn't always the case, same as anything else, depends on the person, some are proactive and some aren't, some want to head off problems and keep things going good, others want to cruise along and have someone else fix things if they go bad. Just part of it.

Thanks
 

alibryan

Well-known member
Or is it up to prepress to just adjust the curves accordingly and carry on until the next change comes along? I realize I'm looking at this from an inplant environment and not a production one.

Okay I know I said I wouldn't but I wanted to say one more thing about this; Prepress is digital so theoretically there are no variables. Presswork is mechanical with environment, ink, paper, and chemistry involved so there are always going to be variables. With this in mind, the curves you set for press shouldn't change from job to job. You may want to increase or decrease one or more of the colors on a job to match expectations but that would be because of the design of the file and not the output of the file, as in the plate curve. You can change the curve to accomplish the change but you're still trying to compensate for something else that isn't correct.

One of the nice things about today's prepress is that it's all digital so a press operator knows that if his image is good and his numbers are correct, color should be correct, job after job. If it isn't, there's almost always something happening on press because the plates are generated from a digital source so they should be the same, job after job.
 
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gordo

Well-known member
@prepper

I think a lot of what Gordo was saying is predicated on that the press is set up correctly to start with, and that isn't always the case, same as anything else, depends on the person, some are proactive and some aren't, some want to head off problems and keep things going good, others want to cruise along and have someone else fix things if they go bad.

If the press is not set up correctly then it won't be stable enough to allow for prepress to provide the pressroom with correctly set up plates and proofs. You cannot fix a press problem by fiddling with things prepress - although I have seen it attempted (a heatset web printer in Australia comes to mind that was constantly getting prepress to tweak plate curves from form to form and pressrun to pressrun).

Pulling a job from the press is a big deal and it typically triggers some kind of investigation as to what went wrong and where and why it went wrong.
If the press operator has to move SIDs of a particular color consistently in a specific direction beyond his targets then that raises a red flag. It does not matter what the specific TVIs are. Measured TVIs are a ratio - you can, for example, raise SIDs and TVIs may go up or they may go down (I know that's weird but it can happen).

The press operator doesn't have to know if the TVIs are high or low relative to a TVI target. Here comes the bombshells and the paradigm shift. He is not making color on the press. He is not making images.

He is laying down films of ink. And the correctness and integrity of those films of ink are what he is concerned with. The result may be looked at as a printed image by everyone else but that is not the press operator's primary job.

Gordo must have lost it. Too much sun or vino for sure!

You think he should know the TVI targets? The majority of press operators that I've met don't even understand what a halftone screen is nor how it affects their presswork - even though they spend the majority of their time looking at the dots through a 10x loupe.

If you think about the process this way - the press is a device to lay down films of ink and the press operator is the control agent for making sure that the press is doing its function optimally - then you start to see the process and its potential points of failure much more clearly. You can then more easily chart a path towards systematic process control of prepress and pressroom.

alibryan also had a good insight - looking at the press as a digital to analog conversion device. You could look at solutions such as ink optimization software (max GCR separations), FM screening, etc. as attempts to make the digital to analog conversion more binary - effectively maintaining the digital aspect.
Just as a simple example, I did some work with my local newspaper publisher. The press operators do everything by eye and fiddle with the SIDs like most press folk do. But I ran a test job with them that had been ink optimized and used FM screening and had them run the magenta up and down some 25 points - way more than any standard SID target. They'd never done this before in a controlled way.
They were shocked at how little effect the extreme SID moves had on the presswork. Their typical SID moves had been more of a psychological move than a color move. The press was acting more like a digital device.
 

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