Calibrating, Printing and Proofing by the G7™ Method

Calibrating, Printing and Proofing by the G7 Method Version 6, August 2006
6.4 Adjust device-level gray balance (optional)
With a spectrophotometer set to D50, 2°, measure gray balance at several points across the sheet on
patches of 50c, 40m, 40y. The nominal gray balance for a # 1 sheet (ISO Grade 1&2 paper) is;
50c, 40m, 40y = a* 0.0 (+/- 1.0), b* -1.0 (+/- 2.0).
NOTE: At this stage the L* value is relatively unimportant.
If the 50c, 40m, 40y gray balance patch is already close to these numbers, try adjusting CMY solid ink
densities (SID) within permitted tolerances to achieve the ideal CIELab gray value(s). The chart below
shows how to adjust CMY ink SIDs in response to Lab error combinations. To use this chart, look for
the row with the appropriate a* and b* signs, (+, -) then look in the appropriate L* column to find the
suggested CMY moves, shown as up or down arrows. No arrow means no move.
Chart showing how to move C, M and Y inks in response to +/- Lab errors.
If gray balance is NOT close to the target a* and b* values, or cannot be corrected with slight SID
adjustments, check for excessive TVI on one or more units, improper ink hue, improper trapping
(possibly due to incorrect ink tack or emulsification), or non-standard ink sequence.
If you don't want to, or cannot, adjust gray balance on press, record how the actual a* and b* values
differ from the goal values (so you can check un-calibrated performance in future), and adjust gray
balance via separate CMY RIP curves. (See Section 8 'Achieving Gray Balance in RIP Curves)
6.5 Adjust cross-sheet evenness
NOTE: This is probably the hardest part of press calibration.
Adjust the press keys until maximum solid ink density deviation at ink-key intervals across the sheet is
as small as is possible, ideally no greater than +/- 0.05 deviation per solid color across the working area,
and so that gray balance deviation is as small as possible, ideally no more than +/- 1.0 a* or +/- 2.0 b*
across or around the working area.
Once the 50c, 40m, 40y patch measures within tolerance, use the 'Compare' feature on your
Spectrophotometer (if available) or manual calculations to determine the ∆E*ab (Delta-E) difference on
similar patches across the sheet. If the ∆E*ab value varies beyond 2, aim for the correct value at the
50c, 40m, 40y patch on the P2P target. If the two P2P targets on the page are more than 2 ∆E*ab
different, either adjust the press to get both in tolerance or, if this is not possible, just focus on the P2P
target closest to gray aims and ignore the other.
6.6 Run stabilizing speed-cycle
After achieving goal conditions, run at least 1000 sheets at typical production speed (to warm-up the
press) and re-check SID, gray balance and evenness. If ink SIDs or gray balance metrics or crosssheet
evenness have decayed beyond specs, adjust the press until desired conditions are achieved,
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