[snip]
As for am screening chroma hump in the 1/4s, I think Gordo has an illustration on this forum illustrating the phenomenon in ColorThinkPro. With FM compared to AM, a greater percentage of reflected light is filtered through the inks, making the light more colorful in lighter tones than with FM screening. If by full you mean too dark, then you have an NPDC problem --undoubtedly the press is printing different after the initial calibration run.
There also FM G7 threads on this forum (I think). So I hear, G7 gray balance is more difficult to achieve. To print FM and have the best possible match to proof, you may need to consider making a private characterization data set that is "based on" Gracol but with FM idiosyncrasies. On one hand I am jealous you get to print FM but I do not envy your proofing problem.
A bit of clarification...20 micron FM screening has a larger gamut than AM/XM screening up to about 400lpi.
The GRACoL data set is based on 175 lpi AM/XM screening which has a smaller gamut. Therefore the color delivered by FM/Segundo screening will not match the GRACoL color. The difference will be seen in one and two color screen tint builds. Vissually the difference is most obvious in screen values from about 60%-10%.
Here's a link to gamut projections and a movie of the two gamuts compared:
Quality In Print: AM and FM gamuts compared - part 1 of 2
An explanation as to why this happens is here:
Quality In Print: AM and FM gamuts compared - part 2 of 2
So, presswork using FM screening that tonally matches the GRACoL data set and that is grey balanced and is using the proper base process colors will not match a GRACoL 7 proof. At least not in one and two color screen tint build colors.
One option is to not worry about it (what many printers do). The other is to re-separate incoming files to "dirty" them up to eliminate the extra gamut (don't know anybody that does that). Another is to fine tume your proof based on the FM print characteristic rather than the generic GRACoL 7 (what some printers do).
Grey balance should not be any harder with FM screening - it's just that the screen percentages that deliver it may not be the same as they are for AM/XM.
To Todd:
Yes, too many data points can create bumpy curves. But, it is a good idea to first check the uncalibrated response curve of Segundo. This is to help ensure that there are no inherent ink transfer issues with your ink/screening combination. If the inherent response curve is "bumpy" - then it is dangerous to try and build plate curves. Better to work with your ink vendor on fixing any issues first. You probably should NOT go to a higher ink tac as this can lead to ink transfer and other problems.
I don't know what you mean by quarter tones appearing "fuller" than the proof. Usually the extra gamut with FM makes the quarter tones appear lighter (because of the extra chroma) even if there is a tonal match.
You might also check the OBA content of your proofing paper vs your press paper. Typically press paper has a high OBA content while proofing paper does not. Since ink acts to block UV, the difference in ink distribution between AM and FM screens might exaggerate a visual difference between AM/XM and FM in the quarter tones.
Go here:
Quality In Print: Optical Brightners
for a 5 part explanation on the issue of OBAs (skip the first posting)
best, gordon p