Determining G7 Solid Ink Densities for Coated/Uncoated Papers

[Justin M]

I've been reading through all of the available G7 how to documentation available and there's still two things that I can't seem to find definitive answers about. So this is a two part questions:

1) During the calibration press run, I know that LAB values are used to measure the individual solids to get within the G7 specifications. Those solid ink densities can then be recorded for baseline color. The LAB values provided for G7 are wet measured values or dry measured values? If dry, how can I adjust during a press run to get to the right LAB value for each ink color? Or is the difference between wet/dry ink LAB measurements negligible?

2) There are LAB values provided for G7 for coated papers. What about coated papers? How can the G7 calibration process be used for an uncoated sheet?

Thank you in advance for your help and responses!

 

[arossetti]

The simple answer to all your questions is that G7 has no standard ink densities or LAB values. The beauty of G7 is that it will work on any size gamut.

Now if you are trying to acheive G7 gray and tonality while achieving a certain colorspace you can find CRPC3 for GRACoL Uncoated and CRPC6 for GRACoL Coated or use any of the other 5 CRPC's: http://www.npes.org/programs/standardsworkroom/toolsbestpractices/characterizationdatasets.aspx

 

[ddonevski]

It's been a while since I read the G7 Specification, so I'll write what I remember.

It defines:
1) Tonality
2) Gray balance

As arossetti said, the Spec has no standard densities or Lab. It is not intended to align different devices to some standard behavior (like ISO12647), but to achieve similar appearance across different devices while taking advantage of their dynamic ranges. That's why it provides sets of NPDC (Neutral Print Density Curves), one for CMY and one for K. So in order to calibrate a given device to G7 Spec, you first determine the maximum density achieved on that device, and choose the NPDC from Spec. graph with that maximum density.

If You want to align your process to some standard, I can only tell You about ISO12647 because that's what I know. ISO12647 defines aimed Lab and dE tolerances for CMYK, RGB (secondary colors, trapping), and aims and tolerances for dot gain (curves). Aligning the press to standard starts by printing a test form with low inking, and gradually increasing it until too much inking is reached. That's because we don't know in advance which wet density will result in aimed dry Lab on a given press. In the process, samples are taken and wet DENSITIES are measured. When prints dry, corresponding LAB values are measured. The logic is, you want a certain appearance (CIE Lab) of dry prints, but you cannot control it directly during the process. The useful process control variable is DENSITY. So among those prints, you find the one where the dry Lab closest to standard Lab is achieved and if it's within dE tolerance, it becomes your process aim. You check its corresponding wet DENSITY, and that's your process control aim. This is done for all four (CMYK) inks. In addition, the standard requires that the ductor roller rotation is adjusted so that it provides uniform inking on the sheet in direction perpendicular to rollers. The densities in that direction should not vary by more than ± 10%.

 

[Manprint]

So spectrophotometry is useful, after the job is printed?

 

[ddonevski]

So spectrophotometry is useful, after the job is printed?

As far as I know.. I remember reading some article on this topic some 8 years ago. It was about spectrophotometry used in process control. I don't remember the details, but since some machine builders were equipping their machines with spectrophotometric devices and advertised it as an advantage over classic densitometric measurements, one machine building company released an article which argued that there are no advantages. They explained that one cannot control Lab directly, wet Lab is different from dry Lab so it only makes sense to keep densitometry for process control and determine wet density - dry Lab pairs. Makes sense to me.
Even ISO12647 family of standards do not attempt to prescribe some wet (process control) Lab. They prescribe dry Lab aimed values (and tolerances) and the process of determining wet SID (solid ink density) which corresponds to dry Lab ahieved on a given press which is closest to Lab prescribed by the norm.

 

[arossetti]

As far as I know.. I remember reading some article on this topic some 8 years ago. It was about spectrophotometry used in process control. I don't remember the details, but since some machine builders were equipping their machines with spectrophotometric devices and advertised it as an advantage over classic densitometric measurements, one machine building company released an article which argued that there are no advantages. They explained that one cannot control Lab directly, wet Lab is different from dry Lab so it only makes sense to keep densitometry for process control and determine wet density - dry Lab pairs. Makes sense to me.
Even ISO12647 family of standards do not attempt to prescribe some wet (process control) Lab. They prescribe dry Lab aimed values (and tolerances) and the process of determining wet SID (solid ink density) which corresponds to dry Lab ahieved on a given press which is closest to Lab prescribed by the norm.

What if your output is instantly dry?

 

[ddonevski]

What if your output is instantly dry?

In that case spectrophotomrty for process control does make sense.

 

[pauly92]

https://printplanet.com/forum/prepress-and-workflow/color-management/18975-g-seven-calc-v2-1

If you're trying to get G7 curves with out a program. use this excel spreadsheet. I've used it a few times. Worked pretty well. You just need somewhere you're able to enter in the numbers on your RIP after restricting the ink.