Christian,
I suspect it's some kind of conflict between your plate linearization curves and your G7 calibration curves, or more likely, the fact that you're even linearizing the plates in the first place.
I stopped linearizing offset plates 25 years ago when I discovered how unstable and pointless that process can be – especially when you're only going to replace those curves with G7 calibration curves a few minutes later.
First of all, it's extremely difficult to get a reliable reading of plate dot size with a true plate reader (with microscope and camera) and even harder with an area-integrating spectrophotometer like the otherwise excellent Techkon Spectrodens.
Secondly, I have been told by plate technologists from Agfa and Kodak that any visible plate emulsion left over after processing is an accidental by-product of exposure and development and was never designed to be a true and faithful representation of how that dot will print.
For example, the emulsion thickness on small dots is typically thinner than on larger dots, which confuses an integrating device like the Spectrodens into thinking those dots are smaller.
The same experts showed me under a microscope that the perimeter of a visible dot's emulsion "cap" is often damaged in processing and therefore slightly smaller or larger than the ink-holding dot beneath it, depending on whether the plates are positive-working plates or negative-working.
These emulsion- and processing- dependent characteristics can vary from plate to plate throughout the day even when there is no variation in the resulting printing characteristics of the final plates.
This information explains why the so-called "dot size" reported by any plate-measuring instrument will often be significantly different from the effective dot size printed on paper, especially in light highlights areas, where a 5% or 2% dot is often invisible on plate but will actually hold more ink than predicted. In these highlight areas, the plate linearization process compensates by increasing small dot values in the RIP, which could well account for that nasty highlight bump in your graphs.
The result of all the above is that the so-called "plate linearization" curves you've struggled so diligently to create are probably causing a very non-linear "press linearization", which is not only a waste of time and materials, but actually sets the G7 calibration process up for failure.
Please try setting your plate curves to NULL and just do a simple G7 calibration. Check the predicted curves in your G7 software and, if necessary, include several light control points (e.g. 2, 4, 6, 8, 10, etc.) to ensure the best possible "press linearization" in critical light tones. As long as your platemaking and press system are stable and repeatable, the G7 calibration should work beautifully first time and I suspect that terrible bump in the light tones will disappear.
The only real argument against my wicked, heretical, anti-establishment advice, is "how do I maintain my plate process?"
There are two simple answers:
1. For base-level plate control, measure a few patches (e.g. 25, 50, 75) on the first NULL plate, prior to going on press. Don't panic if they are different from the nominal values, (e.g. 23, 55, 78) – just use those measured values when checking an uncalibrated plate of a new batch.
2. A more useful approach is to measure the same 25, 50, 75 patches on a the first G7-calibrated plate. Those values can then be used in production to measure any plate on its way to press.
In both cases, either avoid measuring very small dots (e.g. 2%), or pay no attention if they seem to vary from plate to plate. And don't over-react if you see slight changes throughout the day in the 25, 50 and 75 patches. No CTP system I know of can have that much variation – it's almost certainly just normal variability in how the emulsion is washed off during processing.
If you keep editing the plate curves to try and cancel these small variations, you will just create instability on press where none had really existed.
So to summarize, it looks like plate linearization may be a big part of your problem.
In fact in my opinion, plate linearization is a complete waste of time and causes more problems than it's worth, especially when you follow it with G7 calibration. But then, who cares about my opinion?
Don
**************************
Don Hutcheson
President
HutchColor, LLC
908-500-0341
**************************
We calibrated to new inks last week and I would like someone to explain to what causes a spike shown in the dot gain graph but, first I would like to explain my process.
After multiple iterations and manual moves, the spike never goes away.
What causes this spike? Are my plates truly not linear? I put screenshots of what our Image Control console shows when reading a sheet as attachments.