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 />
L_300 = L* value of 300% CMY triplet<br />
L_paper = L* value of paper<br />
NOTE: Fur<strong>the</strong>r research may change this formula, but <strong>the</strong> differences should be almost invisible.<br />
In practice <strong>the</strong> new gray balance definition produces almost no visible change compared to previous<br />
versions of this document, at least on nearly-neutral papers. The benefit of <strong>the</strong> new formula is that on<br />
less-neutral papers, for example 'warm' publication stocks, or 'cold' white papers, grays are closer to<br />
what <strong>the</strong> eye expects when <strong>the</strong> stock is used as a reference. The main disadvantage is that gray tones<br />
in a CMYK file will shift towards paper color, <strong>and</strong> a proof on one paper may not match a proof on<br />
ano<strong>the</strong>r paper type as well as under <strong>the</strong> old “arbitrary” gray balance definition.<br />
E.3 <strong>Printing</strong> on non-st<strong>and</strong>ard paper color<br />
When printing on paper that is not exactly <strong>the</strong> color of <strong>the</strong> reference paper, for example paper with<br />
excessive brighteners that create a strong blue bias, <strong>the</strong>re are three main options;<br />
E.3.1 No gray correction<br />
Let gray balance shift in <strong>the</strong> direction of paper color. No gray correction is common where paper color<br />
is <strong>the</strong> only reference <strong>the</strong> eye has for what is truly ‘gray’, <strong>and</strong> when <strong>the</strong> natural gray balance of <strong>the</strong><br />
printing device is unstable or hard to determine. This option is NOT recommended for proofing.<br />
E.3.2 Full gray correction<br />
Use an absolute colorimetric transform (usually in <strong>the</strong> pre-press workflow) to balance <strong>the</strong> whole gray<br />
scale, including paper color simulation. Full gray correction is mainly reserved for proofing applications<br />
where (a.) <strong>the</strong> proofing substrate is a different color from <strong>the</strong> press paper, (b.) <strong>the</strong> best possible color<br />
match is needed, <strong>and</strong> (c.) a fine dot pattern can be accepted in areas representing plain white paper.<br />
E.3.3 Partial gray correction<br />
Use RIP curves to balance darker grays, but let lighter grays shift towards paper color, Partial gray<br />
correction is best when matching a proof or print that was made on ano<strong>the</strong>r-colored paper, but when<br />
white areas must be free of dots or colorants.<br />
Custom target gray balance values for partial gray correction can be calculated using <strong>the</strong> formula in<br />
E.3.4, which maintains nominal gray balance when <strong>the</strong> Cyan value is higher than a threshold (usually<br />
30%), but o<strong>the</strong>rwise shifts gray balance towards paper a* b* in proportion to relative lightness.<br />
E.3.4 Partial correction formula for target a* <strong>and</strong> b* on non-st<strong>and</strong>ard paper<br />
If C_triplet > T (Threshold); /use nominal target a* <strong>and</strong> b* values (E.2.1)/<br />
Else<br />
ab*_triplet = ab*_paper x (L_triplet - L_300) / (L_paper - L_300) x<br />
(T - C) / T + nominal_ab* x C / T);<br />
Example (for 25% triplet, T = 30):<br />
St<strong>and</strong>ard paper = 95 L*, 0 a*, -2 b*<br />
Nominal a*, b* @25% = 0 a*, -1.46 b* (through formula in E.2.1)<br />
Non-st<strong>and</strong>ard paper = 2.5 a*, -7 b*<br />
L_300 = 23 L*<br />
Target a* = 2.5 x (75.4 - 23)/(95 - 23) x (30 - 25)/30 + 0 x 25/30;<br />
= 0.30;<br />
Target b* = -7 x (75.4 - 23)/(95 - 23) x (30 - 25)/30 + -1.46 x 25/30;<br />
= -2.07;<br />
NOTE: Results may depend on rounding<br />
48 Copyright © International Digital Enterprise Alliance, Inc. [IDEAlliance] (2006). All Rights Reserved.