Calibrating, Printing and Proofing by the G7™ Method

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Calibrating, Printing and Proofing by the G7 Method Version 6, August 2006 Finding the New Aim for a 50% Curve Point by the ‘Up-Across-Down’ method 7.7 Correcting gray balance in RIP curves The G7 CMY graph sheet assumes the device is already gray-balanced via SID adjustments or some other means. In this case, one common RIP curve should be calculated for CMY, with a separate curve for black. If you could NOT achieve gray balance at the device level (e.g. on the press), follow the steps in Section 8, to produce separate RIP correction values for Cyan, Magenta, and Yellow. 7.8 Apply new aim values to RIP or CtP device. Apply the new aim values from your worksheet to the RIP or calibration utility. Note that some RIPs require you to enter 'Measured' values instead of 'Wanted' values, while other RIPs ask for delta values. This method assumes you know how to modify the New Aim values accordingly. When in doubt, remember the new aim value is what each curve point should be AFTER correction. 26 Copyright © International Digital Enterprise Alliance, Inc. [IDEAlliance] (2006). All Rights Reserved.
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.
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Calibrating, Printing and Proofing by the G7™ Method

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