Calibrating, Printing and Proofing by the G7™ Method

Calibrating, Printing and Proofing by the G7 Method Version 6, August 2006
8 Achieving Gray Balance in RIP Curves
The first part of this section explains how evaluate gray balance manually using the GrayFinder target
and graph paper. (If using IDEAlink Curve, go to step 8.5 'Correcting Gray Balance in IDEAlink Curve')
WARNING: This process requires considerable patience and attention to detail. If you do not feel
comfortable attempting it, just apply one common curve to C, M and Y or use the IDEAlink Curve
software to calculate gray balance corrections automatically.
8.1 Find the 'most neutral' patch in the C = 50% block
If you could not achieve gray balance on press (for example with altered SIDs) you can achieve gray
balance by creating separate RIP or CtP curves for C, M and Y. The following process describes one
method of doing this, using the GrayFinder target printed on the calibration run.
The GrayFinder target
With a spectrophotometer, measure the central patch in the nominal 50% (actual 49.8%*) Cyan block of
the lower half of the GrayFinder Target.
*NOTE: 49.8% is the nearest true percentage to 50% in an 8 bit image - (see Appendix D)
Also measure the neighboring patches and look for the one closest to
the 'target gray' value (e.g. 0 a*, -1 b*). Sometimes the best reading
will be found by straddling two patches with the measuring instrument.
If the center patch is nearest to target gray, the device is already gray
balanced (at 50% Cyan), and no corrections are needed.
If the patch nearest to the target
a* b* values is NOT the center patch, make a note of the Magenta
and Yellow percentages aligned with that patch. For example, if
the best measurement was found by straddling the +2 and +3
Magenta columns on the -3 Yellow row, good gray balance
requires +2.5 M and -3 Y.
Bad balance: Neutral gray requires + 2.5% M, - 3% Y
8.2 Repeat for other cyan percentages
Good balance: Center patch measures 'target gray'
If the gray balance error is not consistent throughout the CMY gray scale, you can make complex CMY
curves with different correction ratios at different density levels, by repeating step 8.1 for the 75%,
62.5%, 37.5%, 25% and 12.5% blocks.
NOTE: It is STRONGLY RECOMMENDED that only the 50% block is used to calculate gray balance
curves for offset presses, or any other device on which gray balance is inconsistent or hard to measure.
27 Copyright © International Digital Enterprise Alliance, Inc. [IDEAlliance] (2006). All Rights Reserved.