Calibrating, Printing and Proofing by the G7™ Method

Calibrating, Printing and Proofing by the G7 Method Version 6, August 2006
2.5 HR, SC and HC aim values vs. dynamic range
The graph in Appendix A.5 shows that HR and HC are effectively constants for normal offset density
ranges, but SC (and to a less extent HR and HC) do vary as a function of high dynamic range.
2.6 Gray balance
GRACoL 7 breaks with tradition by raising gray balance to a more important status than TVI, but this
raises the question; how do you define 'gray balance'? Gray balance was traditionally defined as the
CMY percentages needed to match the color of a 50% black ink tint, or the color of paper, but these
definitions are too vague for today's ICC workflows. To avoid these ambiguities GRACoL 7 defines an
arbitrary table of CMY percentage 'triplets' based on the generic 50c, 40m, 40y ratio, and pre-defined a*
and b* values for each triplet.
In previous versions of this document gray balance was an arbitrarily constant of 0 a*, -2 b* throughout
the whole CMY gray scale, regardless of paper color. As of version 5, gray balance is now calculated
by a paper-dependent formula (see Appendix E) based on ideas from several public forums.
CAUTION: The new paper-dependent gray balance definition means that gray tones in a CMYK file will
shift in gray balance towards paper color. Separations and proofs made for one paper may therefore
not exactly match a proof created for another paper type.
2.6.1 Abbreviated Gray Balance chart
The following chart lists some CMY percentage values and approximate a* b* values for a paper white
of 0 a*, -2 b*. Note how the b* values converge towards zero at higher CMY percentages. (See
Appendix E for full gray balance formulae and chart of the P2P23 target gray balance.)
C%* M% Y% a* b*
0.0 0.0 0.0 0 -2
12.5 9.0 9.0 0 -1.7
25.1 18.8 18.8 0 -1.5
37.3 29.0 29.0 0 -1.2
49.8 40.0 40.0 0 -1
62.7 52.9 52.9 0 -0.70
75.3 66.3 66.3 0 -0.5
87.5 81.2 81.2 0 -0.2
100 100 100 0 0
Nominal gray balance percentages and a*,b* values for a paper white of 0 a* and -2 b*
*NOTE: For precision purposes GRACoL 7 expresses all percentage values as their nearest 8-bit
fractional value. (See Appendix D.)
2.6.2 Calibrating Gray Balance
Gray balance can be calibrated either by adjusting the device itself (for example with modified ink
densities) or by creating separate CMY RIP curves (see Section 8 'Achieving Gray Balance in RIP
Curves'.)
NOTE: Whenever possible, separate RIP curves should be AVOIDED when calibrating an offset press,
as most offset gray balance errors are unstable or inconsistent. The reverse is true when calibrating a
more stable device such as a proofing system or ink jet printer, where gray balance is more consistent.
2.6.3 Controlling Gray Balance in production
For process control purposes, measuring a single gray patch of (nominally) 50c, 40m, 40y is usually
enough to confirm a previously - calibrated device is still 'in balance'. Measuring gray balance also on
the SC patch (75c, 66m, 66y) and (optionally) on the HC patch (25c, 19m, 19y) may reveal run-specific
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