Calibrating, Printing and Proofing by the G7™ Method

Calibrating, Printing and Proofing by the G7 Method Version 6, August 2006
L_300 = L* value of 300% CMY triplet
L_paper = L* value of paper
NOTE: Further research may change this formula, but the differences should be almost invisible.
In practice the new gray balance definition produces almost no visible change compared to previous
versions of this document, at least on nearly-neutral papers. The benefit of the new formula is that on
less-neutral papers, for example 'warm' publication stocks, or 'cold' white papers, grays are closer to
what the eye expects when the stock is used as a reference. The main disadvantage is that gray tones
in a CMYK file will shift towards paper color, and a proof on one paper may not match a proof on
another paper type as well as under the old “arbitrary” gray balance definition.
E.3 Printing on non-standard paper color
When printing on paper that is not exactly the color of the reference paper, for example paper with
excessive brighteners that create a strong blue bias, there are three main options;
E.3.1 No gray correction
Let gray balance shift in the direction of paper color. No gray correction is common where paper color
is the only reference the eye has for what is truly ‘gray’, and when the natural gray balance of the
printing device is unstable or hard to determine. This option is NOT recommended for proofing.
E.3.2 Full gray correction
Use an absolute colorimetric transform (usually in the pre-press workflow) to balance the whole gray
scale, including paper color simulation. Full gray correction is mainly reserved for proofing applications
where (a.) the proofing substrate is a different color from the press paper, (b.) the best possible color
match is needed, and (c.) a fine dot pattern can be accepted in areas representing plain white paper.
E.3.3 Partial gray correction
Use RIP curves to balance darker grays, but let lighter grays shift towards paper color, Partial gray
correction is best when matching a proof or print that was made on another-colored paper, but when
white areas must be free of dots or colorants.
Custom target gray balance values for partial gray correction can be calculated using the formula in
E.3.4, which maintains nominal gray balance when the Cyan value is higher than a threshold (usually
30%), but otherwise shifts gray balance towards paper a* b* in proportion to relative lightness.
E.3.4 Partial correction formula for target a* and b* on non-standard paper
If C_triplet > T (Threshold); /use nominal target a* and b* values (E.2.1)/
Else
ab*_triplet = ab*_paper x (L_triplet - L_300) / (L_paper - L_300) x
(T - C) / T + nominal_ab* x C / T);
Example (for 25% triplet, T = 30):
Standard paper = 95 L*, 0 a*, -2 b*
Nominal a*, b* @25% = 0 a*, -1.46 b* (through formula in E.2.1)
Non-standard paper = 2.5 a*, -7 b*
L_300 = 23 L*
Target a* = 2.5 x (75.4 - 23)/(95 - 23) x (30 - 25)/30 + 0 x 25/30;
= 0.30;
Target b* = -7 x (75.4 - 23)/(95 - 23) x (30 - 25)/30 + -1.46 x 25/30;
= -2.07;
NOTE: Results may depend on rounding
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