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