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 />
Appendix A: Production Press Control<br />
NOTE: This process assumes all separations, plates <strong>and</strong> proofs have been made in an ICC workflow<br />
based on st<strong>and</strong>ard Characterization Data <strong>and</strong> G7 calibration.<br />
The G7 press control method is similar to conventional press control, with CMYK solid-ink readings (in<br />
CIELab or equivalent densities) as a starting point, followed <strong>by</strong> mid-tone readings to monitor tone<br />
shape. The difference is that traditional TVI readings are replaced with absolute ND (or Lab) 'HR'<br />
readings, <strong>and</strong> traditional Print Contrast calculations are replaced with absolute ND (or Lab) 'SC'<br />
readings. This approach monitors gray balance <strong>and</strong> lightness more reliably than TVI readings, <strong>and</strong> with<br />
less chance of mid-tone density or gray balance variations, however because gray balance is typically<br />
less stable than TVI values, be prepared for wider deviations throughout <strong>the</strong> run.<br />
Note also that this method of press control is quite new, with no well-established history of acceptable<br />
tolerances. Be prepared to experiment over time to establish <strong>the</strong> optimum tolerances for your<br />
equipment, conditions <strong>and</strong> expectations (see Appendix G).<br />
A.1 Run press up to nominal ink levels<br />
Begin <strong>by</strong> running <strong>the</strong> press to specified ink colorimetric values (see Appendix H) or SIDs. Remember<br />
that CIELab values take priority over density values. Once make-ready is complete, RELATIVE<br />
densitometry (based on reading a good sheet with your densitometer) remains a valid <strong>and</strong> efficient<br />
basis for run-time press control, <strong>and</strong> <strong>the</strong> same density goals can be used to make-ready future runs – at<br />
least for that specific paper/ ink/ densitometer combination.<br />
A.2 Adjust gray balance, HR_cmy <strong>and</strong> SC_cmy<br />
Adjust solid ink densities or o<strong>the</strong>r press variables within permitted tolerances till <strong>the</strong> HR_cmy patch (50c,<br />
40m, 40y) is as close as possible to <strong>the</strong> a* <strong>and</strong> b* gray balance aim values, <strong>and</strong> ND or L* aim value.<br />
Also check that <strong>the</strong> CMY SC patch (75c, 66m, 66y) is as close as possible to <strong>the</strong> SC_cmy aim value.<br />
Some compromise will usually be necessary due to normal press variations, but try <strong>and</strong> share any error<br />
evenly between HR <strong>and</strong> SC patches.<br />
A.3 Adjust HR_k <strong>and</strong> SC_k<br />
Adjust black solid ink density or o<strong>the</strong>r press variables till <strong>the</strong> K HR patch (50k) is as close as possible to<br />
<strong>the</strong> nominal HR_k aim. Also check that <strong>the</strong> SC_k patch (75c, 66m, 66y) is as close as possible to <strong>the</strong><br />
nominal SC_k aim value. Some compromise will usually be necessary due to normal press variations,<br />
but try <strong>and</strong> split any error evenly between HR <strong>and</strong> SC patches.<br />
A.4 Nominal HR, SC <strong>and</strong> HC aim values<br />
The chart below shows nominal aim values for HR, SC <strong>and</strong> HC for paper with a 95 L* value. These<br />
values assume <strong>the</strong> new G7 50k < 50,40,40cmy convention introduced in February 2006.<br />
Gray Minus Paper (ND) With Paper (ND) With Paper (L*)<br />
Patch CMY K CMY K CMY K<br />
25% (HC) 0.25 0.22 0.31 0.28 75.7 77.7<br />
50% (HR) 0.54 0.50 0.59 0.55 57.5 59.9<br />
75% (SC) 0.90 0.90 0.96 0.96 39.5 39.7<br />
Nominal gray patch aim densities <strong>and</strong> L* values for GRACoL #1 grade (ISO Type 1 & 2) paper<br />
NOTE: If you prefer <strong>the</strong> 2005 G7 specification with same NPDC for K <strong>and</strong> CMY, use <strong>the</strong> CMY aim<br />
values for black, <strong>and</strong>/or <strong>the</strong> CMY graphs in A.5, but remember this is not acceptable in official GRACoL<br />
7 or SWOP implementations, <strong>and</strong> will darken <strong>the</strong> black plate slightly more than ‘normal’.<br />
38 Copyright © International Digital Enterprise Alliance, Inc. [IDEAlliance] (2006). All Rights Reserved.