Calibrating, Printing and Proofing by the G7™ Method

Calibrating, Printing and Proofing by the G7 Method Version 6, August 2006
G.5 Characterization vs. production tolerances
Characterization tolerances should be stricter than production tolerances because a characterization
print must be as near to 'perfection' as the device is capable of. This accuracy will usually not be
achieved in a single press run, but may be achieved by averaging several prints made on different days.
A good general rule is that characterization tolerances should be roughly half as large as production
tolerances, or at least as strict as the most stringent production aims. For example, if gray balance
tolerances for production work are +/- 1 a* and +/- 2 b*, characterization tolerances for that device
should be +/- 0.5 a* and +/- 1 b*.
If gray balance (and other parameters) cannot be controlled to these tolerances on the characterization
print, the resulting characterization will probably NOT represent the average capability of that press.
G.6 Printing vs. proofing tolerances
Most pre-press proofing systems are much more stable and often have a higher color gamut than the
typical offset press, partly by avoiding wet-trapping issues (see G.6.1). This alone makes it reasonable
to apply higher tolerances to proofing systems than to production offset presses, but another more
important reason is that in most pressrooms the proof is still the ultimate 'target' (regardless of
measured values), and a 'moving target' will obviously lead to less consistent printing.
GRACoL 7 recommends that proofing tolerances be as fine as possible, regardless of pressroom
capabilities, to minimize proof-to-proof variations. Anything less would make the proofing system a
moving target. Actual local proofing tolerances should be determined by trial-and-error or statistical
analysis, along with an understanding of local process, control capabilities, and limitations. Wider
tolerances may be necessary on coarser grades of paper.
G.6.1 Wet-trapping - the bane of offset lithography
Modern offset presses print all four (or more) inks in rapid succession, 'wet-on-wet'. The first inks down
usually adhere to the paper better than later inks, which are partially repelled by the still-wet earlier inks.
But in some cases the later inks actually remove some of the earlier inks, depending on the relative tack
(stickiness) of each ink in the sequence. Either way, the amount of one or more inks remaining on
paper is usually less than would be achieved on a 'dry trapping' press, (printing one color at a time, or
with a drying system between each unit), and the visual effects are typically unpredictable and unstable
- often varying as the press warms up over the course of a run.
'Wet-trapping' introduces two problems that are largely absent in slower, single-color presses - namely
reduced color gamut and unstable performance in darker tones - in each case due to the loss of some
wanted ink. In fact the relative instability of wet trapping in 3-color and 4-color areas is one of the main
problems in matching multiple presses to a standard characterization data set, even when each press
uses the same paper and ink.
Another trapping-related problem with many offset presses is that gray balance is often seriously
affected by micro register changes from sheet to sheet, which create microscopic area wet trapping
differences from sheet to sheet within the halftone pattern itself, and consequent short-term gray
balance errors. This makes it dangerous to demand tight gray balance tolerances on an offset press -
at least on any single sheet. To measure gray balance reliably on an offset press, or to calculate
separate RIP correction curves for CM and Y, be sure to average several sheets in succession.
G.6.2 Solving or minimizing wet-trapping issues
Most trapping solutions work on the concept of drying each ink (at least partially) before subsequent
inks are laid down. This is usually accomplished by some form of inter-station drying system, such as
IR (Infra-Red) heat for conventional inks, or UV (Ultra-Violet) light for UV-curable inks. A less practical
(but highly effective) solution is to print one ink at a time in 4 passes through the press. By the time the
paper is ready for each new ink, the earlier inks have dried naturally, ensuring much better trapping.
One-color-at-a-time printing is obviously slower, and REALLY needs tight tolerances!
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