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john@megacolour
04-20-2015, 05:21 PM
We've recently had our KMC8000 audited by our ISO guy(Mellow) and have scored 86% after setting up custom profiles and custom device links through CPS.
Our dot gain and K density values are higher by 3 to 4 points as shown on the audit sheet.
Is there a way to adjust these values as I've tried to edit through CPS but can't find where to adjust?
:confused:

Lou_P
04-21-2015, 10:26 AM
Hi John.

Whats the audit sheet request? That you match the dot gain and density of another press? Are you trying to match the dot gain (TVI) of another digital press? In general we don't track dot gain on digital presses since there we can't adjust it as we go like on a conventional press. So instead we linearize to get the per-channel color response linear in the human visual system (there is some "dot gain" purposely introduced here in the sense that print devices need to use up the majority of their tonal range in the highlight to midtone where the eye is sensitive). Then we make an ICC output profile to characterize not just the dot gain/TVI but also the gray balance and the rendering of solid colors. Were you also audited on your colorimetric match to ISO Coated, GRACoL, or another appearance matching standard? Did you meet that compliance?

Pay no attention to the above paragraph if it is confusing you further :)

Just let us know what the audit targets are and we can suggest how to get closer to them.

~Lou


We've recently had our KMC8000 audited by our ISO guy(Mellow) and have scored 86% after setting up custom profiles and custom device links through CPS.
Our dot gain and K density values are higher by 3 to 4 points as shown on the audit sheet.
Is there a way to adjust these values as I've tried to edit through CPS but can't find where to adjust?
:confused:

john@megacolour
04-21-2015, 05:45 PM
Thanks for the reply Lou.
Here's the current audit sheet

http://i.imgur.com/a6dCQi6.png

We've already created a custom .icc profile for our KM8000 and also profiled our paper types : Gloss , Matt, Uncoated.
Was quite surprised that we got a pass after calibrating the press with paper types to get a good and consistent density and colour density control definitions set.
Is it possible Lou to tweak the current custom profiles that we have created to get closer to the values shown? And where and what do i adjust through CPS?

Lou_P
04-22-2015, 10:20 AM
Hi John.

If you really want to adjust the TVI you can do this with CPS Editor. Open your output profile, go to global edits -> curves, and adjust there. From your report I would really only pull down M at the midtone by maybe 3 and at the 75% by maybe 4%. I think you wil find your precision to ISO match will be reduced if you do this.

I'd caution you against trying to adjust all the curves to get the target TVI's -you will almost certainly mess up your gray balance and get worse precision on matching ISO.

TVI has no real meaning on digital. We can match the TVIs of the conventional press (what they are asking you to do) but we won't get a color match to an appearnance matching standard like GRACoL, right? The toners are different colors and have different trap characteristics than conventional inks, right?

Consider this example:

You are matching the OSO 12647 solids within 5 DE except for K. So there is no way you would change your solid ink densities of 1.49, 1.44, 0.98. 2.14 for CMYK respectively, right? All but the C are off by more than 5 points (0.05) and showing red though, right? That's because those solids only work for a specific conventional ink set. The densities of the KM toner and therefore the TVI are both expected to be different. The goal is not to match these numbers but to match the reference colormetrically using a tolerance like 12747-8.

Let me know if the precision gets better or worse when you try the curve edits and please send the same report on the results so we can see what the TVIs did.

A couple of other thoughts:

1. Did you use the "Optimize Colorimetric Precision" option in CPS Printer Profiler when you made the profile? If not you should try it, I can give instructions if needed.

2. What about the MaxGCR option? if i were auditing you I would be much more concerned with the high delta in the K compared to ISO. We need to do something with your shadow point. MaxGCR may help this. Or if you tell me the LABs for the solid K and the 4 color K and maybe I can make a suggestion. Remember to get the black right for compliance testing you _must_ set black and gray processing on the Fiery to off. If you have it set to and of the options where K only is preserved in images you will fail as we are seeing in this report since the toner is a very different color than conventional black ink.

john@megacolour
04-22-2015, 06:08 PM
We've noticed that KM(digital press) tends to always lean to Magenta but will try your method as mentioned Lou. In the past, the magenta has been slightly altered but since we've profiled the KM and paper stocks, the test prints have been more neutral compared to previous non profiled KM and paper stocks.
I understand that toner used varies compared to offset inks due to different pigments and other characteristics of both toner and soy based inks but some of the prints produced so far come pretty close.
We'll give it another go as out ISO guy is with us this month but he's still impressed by the initial results compared to what used to printed.

Not sure if i did apply Optimize Colorimetric Precision but will check when i get a chance.

Please instruct me where to and how to tweak the MaxGCR also where do i find the info your after Lou in regards to LaB and solid k? Is it via CPS or CW?

adam1991
04-23-2015, 05:59 PM
In CPS there's actually a simple button for max GCR.

But going back to the basics, keep in mind that TVI--dot gain--is an old offset mechanism used to validate the "quality" of the image on paper, used simply because that was the only thing we were able to measure.

And as Lou says, digital presses don't play in that TVI world directly the same way an offset press does. You're trying to compare apples and space shuttles.

And now that we have much better tools and methodologies to measure the actual visual image on page, starting with gray balance, I don't really understand why anyone is focusing on dot gain as a measure of anything. Sure, you can use it secondarily as a quick measure once you've gray balanced your press--find the TVI on a proven gray balanced press sheet, then measure TVI of the same patches on press runs and compare. It's an indicator. But it's not THE measure of the quality of the manufacturing process anymore.

And it's most certainly useless in evaluating the quality of the digital press manufacturing process.

Maybe your ISO guy is stuck in the 80s? Or maybe he sees you as job security?