Using Device Link Profiles to improve color consistency

[meddington]

The surprise, though, was that while they hit the same color targets for the solids on each, the dot gain characteristics of FM screening caused plugging and hue shifting in saturated colors. The measurements from the AM screened target exhibited none of this - the measurements were beautiful. In essence, the FM screening seemed to actually limit the gamut because the saturated colors became uncontrollable. The data indicated that the operator was over-inking.

overinking, but yet, not overinking on the same plates. Two different beasts am & fm. Separations/color builds and screening all make a difference on consistency on press. I mentioned in another post how heavily GCR'd images print visually more consistently, but lighter coverage channels (cyan for example) may be harder to control as far as maintaining consistent ink film thickness. Makes it harder to achieve a pass/fail based on measured data and makes a pressrun look more fluctuating, but since the channel in question is light coverage, often doesn't make a huge difference visually.

 

[Deleted member 16349]

Couple of odd-duck thoughts to go with this thread.

1) I've recently been going over some measurement data from a press test that someone was generous enough to share. They ran AM and FM screening on the same sheet. Now, the ink and water requirements are vastly different for AM and FM screening, so with them being on the same sheet the operator can only optimize the conditions for one of the two. The surprise, though, was that while they hit the same color targets for the solids on each, the dot gain characteristics of FM screening caused plugging and hue shifting in saturated colors. The measurements from the AM screened target exhibited none of this - the measurements were beautiful. In essence, the FM screening seemed to actually limit the gamut because the saturated colors became uncontrollable. The data indicated that the operator was over-inking.

.

Offset presses do not have positive control of ink feed, therefore there will be variations in the amount of ink being fed into the press for a particular image.

With AM, the variation in ink feed gets out of the press all across the screen range from >0% to 100%.

With FM screens the lower end of the screen range is very much less affected by the ink films on the form rollers and therefore there is not much ink variation going out of the press in that range. This means that any variation in ink feed must go out of the press in the higher screen range and could show up as more variation ie. less control of the density in the higher end of the screen range, which includes the solids.

From earlier analysis, this is what I would expect would be a problem running FM. Maybe it is related to what you have seen in your measurements.

 

[gordo]

G7 is a specification, please see 'The G7 Specification 2008' final working draft.

Regards,
Todd

It appears that you are right - but I'm not wrong:

From The G7® Specification 2008 Working Draft B
"The G7 Specification was initially developed for commercial printing by the GRACoL Committee and first published within GRACoL "

Then a year later in 2009 from idealliance's own definitions: (http://www.idealliance.org/sites/default/files/Definitions Final_09_09_2009-1.pdf)

"G7® is a method of attaining a desired grayscale and tonal curve used for calibrating a proofing and/or printing system."

"A Specification...is a detailed description of design criteria for a piece of work. GRACoL1, SWOP3 and SWOP5 are specifications for printing using a precise characterization data set for a specific print process and substrate." (note that G7 is not included as an example because GRACoL1, SWOP3 and SWOP5 are specifications and not methods).

I have a feeling that in the 2008 document they were using a different meaning for the term "specification".
I think that they were referring to it in the same way as a recipe contains the specification of how to prepare a meal.

Maybe someone who is closer to idealliance could clarify what G7 is.

best, gordo

 

[gordo]

1) I've recently been going over some measurement data from a press test that someone was generous enough to share. They ran AM and FM screening on the same sheet. Now, the ink and water requirements are vastly different for AM and FM screening, so with them being on the same sheet the operator can only optimize the conditions for one of the two. The surprise, though, was that while they hit the same color targets for the solids on each, the dot gain characteristics of FM screening caused plugging and hue shifting in saturated colors. The measurements from the AM screened target exhibited none of this - the measurements were beautiful. In essence, the FM screening seemed to actually limit the gamut because the saturated colors became uncontrollable. The data indicated that the operator was over-inking.

That goes completely against my experience.
Were dot gain compensation curves applied to the FM screening so that it would have the same tone response on press as the AM screening at the same targets for the solids?

best, gordo

 

[Deleted member 16349]

Some problems

Some problems

Offset.

One will not have consistency because there is no positive control of ink feed into the press.

One will not have consistency because the inking of the plate is not done consistently around the plate cylinder in line with the solid patch. This also means that there is not a specific characteristic profile for how the press prints with a specific ink and paper combination.

One will not have consistency from one impression to another due to different inking conditions of the plate by the form rollers.

Without consistency, one can not have predictability.

Even if the above conditions are corrected, the CMYK channels in the printing process are nonlinear and nonindependent. Therefore curves such as TVI can not define the outcome. That means that standards have problems.

Also the tone curves TVI for CMY produce data which you can not see. Measuring Magenta uses a green filter. Can you see the green tone curve in Magenta screens? If you can, you are really good.

With all of these systemic problems in the process, it is amazing that some reasonable results are obtained. That implies to me that the actual problems of consistency and predictability of colour reproduction are not so difficult as one might think and if the hardware and software technologies are changes, things would work quite well.

Until then, it seems that there will continue to be a lot of frustration.

 

[gordo]

[snip]
Even if the above conditions are corrected, the CMYK channels in the printing process are nonlinear and nonindependent. Therefore curves such as TVI can not define the outcome. That means that standards have problems.[snip]

One thing at a time.

I hope that Rich Apollo answers my question about whether dot gain compensation curves were applied to the FM screening so that it would have the same tone response on press as the AM screening at the same targets for the solids.

If the tone reproduction of the two screens is normalized then, in my experience, FM will exhibit a larger gamut than AM/XM screening at the same solid ink targets. You have a sample that demonstrates this, and it has been verified in tests by many print organizations (PIRA, GATF, TAGA, RIT, etc.).

There are problems with running tonally normalized AM/XM screens on the same sheet as FM but they have more to do with water than with ink.

best, gordo

 

[gordo]

[snip]
Even if the above conditions are corrected, the CMYK channels in the printing process are nonlinear and nonindependent. Therefore curves such as TVI can not define the outcome. That means that standards have problems.[snip]

One thing at a time.
I hope that Rich Apollo answers my question about whether dot gain compensation curves were applied to the FM screening so that it would have the same tone response on press as the AM screening at the same targets for the solids.

If the tone reproduction of the two screens is normalized then, in my experience, FM will exhibit a larger gamut than AM/XM screening at the same solid ink targets. You have a print sample in hand that demonstrates this, and it has been verified in tests by many print organizations (PIRA, GATF, TAGA, RIT, etc.).

There are problems with running AM/XM screens on the same sheet as FM but they have more to do with water than with ink.

best, gordo

 

[Deleted member 16349]

One thing at a time.
I hope that Rich Apollo answers my question about whether dot gain compensation curves were applied to the FM screening so that it would have the same tone response on press as the AM screening at the same targets for the solids.

If the tone reproduction of the two screens is normalized then, in my experience, FM will exhibit a larger gamut than AM/XM screening at the same solid ink targets. You have a print sample in hand that demonstrates this, and it has been verified in tests by many print organizations (PIRA, GATF, TAGA, RIT, etc.).

There are problems with running AM/XM screens on the same sheet as FM but they have more to do with water than with ink.

best, gordo

I added to my earlier post about the problem of not being able to see the tone measured values of CMY screens due to the filters used.

Also if just printing one ink on the paper, the colour (hue) changes as one goes down the screen range The a/b ratio changes.

When comparing the two screens AM/FM as on those great samples I got from you, it also shows that even if the TVI is the same, the colour is not. One can not match always an AM print with an FM print with only one channel. One needs to colour manage the match. So one screen printed with one ink in one of the systems, might need to be printed with all CMY channels to match it. Is that what you also think?

Your comment about water is probably very true but I think that water control is related to ink control. Operators might not want to adjust water for fear of throwing the density off.

 

[rich apollo]

That goes completely against my experience.
Were dot gain compensation curves applied to the FM screening so that it would have the same tone response on press as the AM screening at the same targets for the solids?

Hey, Gordo. No, these were uncompensated tests. I was as surprised as anyone by what I saw.

There are problems with running tonally normalized AM/XM screens on the same sheet as FM but they have more to do with water than with ink.

I understand, and agree. With both types of screening on the same plate(s) you will only be able to optimize the conditions for one of them.

 

[rich apollo]

Here's a look at the data plotted in ColorThink.

The FM screened data is in white, the AM screening is in color.

 

[Deleted member 16349]

Here's a look at the data plotted in ColorThink.

The FM screened data is in white, the AM screening is in color.

Nice plot. The AM screening data is in colour but it seems that the colour is based on its value at each point and tends to look white as it goes to the centre where one can not tell what is what.

Is it possible to just keep the AM colour constant for clarity. Thanks.

 

[tmiller_iluvprinting]

It appears that you are right - but I'm not wrong:

From The G7® Specification 2008 Working Draft B
"The G7 Specification was initially developed for commercial printing by the GRACoL Committee and first published within GRACoL "

Then a year later in 2009 from idealliance's own definitions: (http://www.idealliance.org/sites/default/files/Definitions Final_09_09_2009-1.pdf)

"G7® is a method of attaining a desired grayscale and tonal curve used for calibrating a proofing and/or printing system."

"A Specification...is a detailed description of design criteria for a piece of work. GRACoL1, SWOP3 and SWOP5 are specifications for printing using a precise characterization data set for a specific print process and substrate." (note that G7 is not included as an example because GRACoL1, SWOP3 and SWOP5 are specifications and not methods).

I have a feeling that in the 2008 document they were using a different meaning for the term "specification".
I think that they were referring to it in the same way as a recipe contains the specification of how to prepare a meal.

Maybe someone who is closer to idealliance could clarify what G7 is.

best, gordo

Gordo,

I agree, I think 'specification' is being used in a different context regarding the 2008 document. It has created some confusion and a clarification from IDEAlliance would be helpful. As a matter of fact, my original post was meant more to shed some light on this confusion than a rebuttal of your statement that G7 was not a 'specification.'

Regards,
Todd

 

[gordo]

Hey, Gordo. No, these were uncompensated tests. I was as surprised as anyone by what I saw..

You're killing me.

I'm surprised that you were surprised that "while they hit the same color targets for the solids on each, the dot gain characteristics of FM screening caused plugging and hue shifting in saturated colors. The measurements from the AM screened target exhibited none of this - the measurements were beautiful. In essence, the FM screening seemed to actually limit the gamut because the saturated colors became uncontrollable."

Of course the higher dot gain of the FM is going to cause plugging.

If you want to compare FM to AM/XM you need to eliminate any differences between the two screens other than the actual screen pattern itself. That means that you have to apply a tone reproduction compensation curve to the FM to align it to the AM/XM. Ideally you would also run them separately to eliminate the difference in ink/water characteristics. Then you need to plot the profiles of the two screens. If you do that you'll be comparing "apples to apples" - a fair comparison. The results look like this:

In the above image the 175 lpi AM screen's gamut is the solid shape inside the larger translucent 20 micron FM gamut.

It's more clearly shown in the short movie here: Quality In Print: AM and FM gamuts compared - part 1 of 2 I can provide a high res version of the movie - just PM me.

Comparing a 175 lpi AM and 20 micron FM screen clearly shows that the FM has a larger gamut. I.e. some colors that are produced by FM screens cannot be reproduced with an AM/XM screen (until you're over about 400 lpi).

best, gordo

PS This test was done by first normalizing the dot gains/area. That means, for example, that if you measure the dot area of the AM and FM screens through the tone range they would measure the same values. e.g. at the 50% tone both screens measured 64%.
Next two ICC profiling targets were placed in line with one another on the same sheet so that the targets were in the same ink and blanket zones of the press.
Then the press was run up to the the target SIDs.
Then the water was balanced for the AM screen and once the press had stabilized sheets were pulled to measure for the AM.
Then the water was balanced for the FM screen while keeping the SIDs the same. Then sheets were pulled to measure for the FM
Then profiles were made and compared using color think.
Later, I produced a printed marketing piece for Creo/Kodak that made the same comparison using one color and two color step wedges on #1 gloss coated and on newsprint. This clearly showed in physical form how much more vibrant the FM was in comparison to the AM.

 

[Deleted member 16349]

You're killing me.

I'm surprised that you were surprised that "while they hit the same color targets for the solids on each, the dot gain characteristics of FM screening caused plugging and hue shifting in saturated colors. The measurements from the AM screened target exhibited none of this - the measurements were beautiful. In essence, the FM screening seemed to actually limit the gamut because the saturated colors became uncontrollable."

Of course the higher dot gain of the FM is going to cause plugging.

If you want to compare FM to AM/XM you need to eliminate any differences between the two screens other than the actual screen pattern itself. That means that you have to apply a tone reproduction compensation curve to the FM to align it to the AM/XM. Ideally you would also run them separately to eliminate the difference in ink/water characteristics. Then you need to plot the profiles of the two screens. If you do that you'll be comparing "apples to apples" - a fair comparison. The results look like this:

In the above image the 175 lpi AM screen's gamut is the solid shape inside the larger translucent 20 micron FM gamut.

It's more clearly shown in the short movie here: Quality In Print: AM and FM gamuts compared - part 1 of 2 I can provide a high res version of the movie - just PM me.

Comparing a 175 lpi AM and 20 micron FM screen clearly shows that the FM has a larger gamut. I.e. some colors that are produced by FM screens cannot be reproduced with an AM/XM screen (until you're over about 400 lpi).

best, gordo

Gordon,

Have you ever plotted a vector to show the direction of change for an AM screen point and the FM screen point at the same TVI. I think that would be interesting to see if the move is vertical, up and/or down, or outwards or a combination of both.

 

[gordo]

Gordo,

I agree, I think 'specification' is being used in a different context regarding the 2008 document. It has created some confusion and a clarification from IDEAlliance would be helpful. As a matter of fact, my original post was meant more to shed some light on this confusion than a rebuttal of your statement that G7 was not a 'specification.'

Regards,
Todd

In the early days Idealliance was very adamant about the difference between a specification, standard, and method which is why they published their definitions with examples on their web site. This misuse of terminology had also gotten them into a severe conflict with the ISO committee in Europe.

It's incumbent on all of us to always take the time to make sure that we are using terms properly and clearly.
Until I hear different - G7 is a method to grey balance a color output device. Period. It's neither a standard nor a specification.

I quit the Ideallance committee years ago so don't have any contacts there. But many posters in this forum are either in the committee itself or have contacts within the committee. If you're one of those I implore you to make contact and clarify/update documents to properly reflect the definitions.


best, gordo

 

[Lukas Engqvist]

I added to my earlier post about the problem of not being able to see the tone measured values of CMY screens due to the filters used.

I am a little surprised at the comment…yes I think it would be ideal to look at spectral curves, but I don't think that the filters are as limiting as you seem to imply. A green filter appears green because it blocks magenta, or in fact the true is even more correct it is the Magenta ink that is designed to cut out the green from the reflected light, and so a green filter would see how much green light is let through by the magenta. I do admit that I have not seen a spectral curve of a green filter, but after studying the spectral curves of monitors and inks the relationship between CMY and RGB are quite clear. (the colour of the light emitted by the measuring device that is reflected I think is a bigger problem than the filter anyway: Also a spectrophotometer can be used, and that uses a prism rather than a filter.)

Also I wonder when you are talking of inconsistencies what levels are we talking about? Is this perceived inconsistencies or measured inconsistencies? Language can be very crude since the word inconsistencies can be a mathematical entity that requires very high end equipment to measure but the same word is used to distinguish differences that a colour blind person can discriminate between in a poorly lit kitchen. (In plain language are we comparing apples to oranges or peas to water melons?)

 

[Deleted member 16349]

I am a little surprised at the comment…yes I think it would be ideal to look at spectral curves, but I don't think that the filters are as limiting as you seem to imply.

Also I wonder when you are talking of inconsistencies what levels are we talking about? Is this perceived inconsistencies or measured inconsistencies? )

Lukas,

TVI or dot gain, was first used with black screens and it made sense to state that the resulting curve was a tone curve since one could see that in the print. When the dot gain measurement method was applied to CMY screens, the practice of calling it a tone curve was continued.

The difference between the tone curve of black and CMY is that with black what is measured is very much related to what you see. On the other hand, the tone values that are measured for CMY are indirect values. One measures the tone values of CMY with filters that measure a part of the spectral curve that you can't see directly. In this indirect way, there is not guarantee that the tone that you see is the same as what is measured.

Will it be a large difference? Maybe or maybe not. My point was that something people are calling a tone value is something they can not see. I think that is funny.

On the question about inconsistency. Is it large or small or is it a problem? The printing industry sees no problem with inconsistency. They accept it as a part of the process. I view it differently but that is only me.

Inconsistency of the printed image in an offset press is a result of how ink films are managed on the form rollers of the press. As I understand, when taking print data from the test forms, the data is averaged as if averaging will result in a predictable value. There seems to be no concern for the total range of variation of data points. If there is a lot of variation in the range of each data point, the chances of producing a predictable profile is not as good.

Statistics also tells us that the variation values one samples is smaller than the whole variation range that the process will see. So you might take a few samples for an average but those samples do not show the true situation of variation.

With a press that does not manage the ink films on the form rollers well, the variation will be introduced by the printed image itself. This means that the variation in the values of the points in the image is affected by the image itself.

Heidelberg and others have some press designs with a single form roller that prints in register to the plate and is inked with an anilox roller. These types of presses have consistent inking of the plate that is independent of the image. The fact that Heidelberg has stated that the Anicolor press does this better than their existing presses is an indirect statement that the inconsistency that exists is not such a good thing.

What gets me is that normally in this industry, any inconsistency in the process is brushed off as being minor and not important at the same time as the industry does not have a reliable method for reproduction unless great effort is applied and waste paper is produced.

Maybe things are better than they were before but there are inconsistencies that still need to be driven out of the process.

 

[fiatlux]

[snip] One measures the tone values of CMY with filters that measure a part of the spectral curve that you can't see directly. In this indirect way, there is not guarantee that the tone that you see is the same as what is measured.

Will it be a large difference? Maybe or maybe not. My point was that something people are calling a tone value is something they can not see. I think that is funny.

It doesn't matter. As long as you get consistent tone values from your instrument you will get the information needed to build tone curves to achieve the results that you want.

FL

 

[gordo]

Gordon,

Have you ever plotted a vector to show the direction of change for an AM screen point and the FM screen point at the same TVI. I think that would be interesting to see if the move is vertical, up and/or down, or outwards or a combination of both.

Yes, but unfortunately I no longer have the data. If you have access to a spectrophotometer you could measure the Lab values of the sample I gave you. Send them to me and I'll plot them.
If the dot areas are the same then the L* (the vertical direction) points should be the same. If you plot the Lab values as a spider diagram then with FM there is a move outwards (chroma shift) and a different curve shape (hue shift) than with AM. Depending on what paper you're using and which screens you are comparing you can easily get a ∆E of five between the two colors.

It looks like this:

The blue dots are 133 lpi and 200 lpi AM. The other dots are 10 and 20 micron FM and 20 micron first order FM.

This raises an interesting question. Since halftone screens affect hue - what is the correct color of a 50% tone? And therefore which halftone screen best maintains the fidelity of that color.

best, gordo

 

[Deleted member 16349]

It doesn't matter. As long as you get consistent tone values from your instrument you will get the information needed to build tone curves to achieve the results that you want.

FL

Yes, you are right. In that sense it does not matter that much.

But on the other hand, the idea that adjusting plate curves to obtain the desired tone curve will result in predictable printed colour all the time is not founded. That is shown with the AM FM example. Same paper, same ink, same tone curve, different appearance when printing AM and FM screens.