Gray balance

GRACOL 7 breaks the tradition and places gray balance in a more important position than TVI, but this will cause some problems. How do you define "gray balance"? Gray balance used to think that CMY percentage was used to match 50% neutral black or paper color, but these definitions are too general for today's ICC process. In order to solve these general problems, GRACOL 7 has developed a special CMY percentage table. In the chart, the CMY percentage values ​​are arranged in groups of three on the basis of 50C, 40M, 40Y, and a * b * is set for each group in advance. value.

In the previous version, the definition of gray balance is that the gray balance of the entire CMY gray scale is a constant 0a *,-2b *, without considering the paper. In the fifth version, the measurement of gray balance has begun to use the calculation formula based on paper, these methods were obtained in some seminars.

Note: The definition of gray balance based on paper means that the gray tone in the CMYK file will change with the change of paper color. A proof is only useful for one type of paper, but it cannot be used for tracking and matching for Other paper types.

Simplified gray balance chart

The following table lists the CMY dot area and approximate a * b * values ​​of paper with whiteness a * 0, b * -2. Notice how the b * value changes towards 0 in the CMY bright tone.

The standard gray balance value and a * b * value of paper with whiteness a * 0, b * -2 Note: For accuracy, GRACOL 7 uses 8 decimal places to represent the percentage value.

Calibration gray balance

There are two adjustment methods for gray balance, one is to adjust the device itself (such as adjusting the amount of ink), and the other is to create a CMY RIP color separation curve.

Note: When performing offset press calibration, it is best not to use RIP color separation curves, because the gray balance error of most offset printing equipment is unstable or not uniform. On the contrary, for more stable devices, such as proofing systems or inkjet printers, they can be used during calibration because the gray balance of these devices is relatively stable.

Control gray balance in production

In the production process control, by measuring the color blocks 50C, 40M, 40Y (standard) to ensure that the corrected equipment is still in gray balance. Measurement of gray balance also needs to find small errors in the reproduction of the image table in the running process through SC color blocks (75c, 66m, 66y) and HC (25c, 19m, 19y) color blocks. These are difficult or impossible to correct because of the equipment itself Intrinsic instability, such as offset presses. Therefore, in actual production, the standard Lab value of HR, SC and HC color patches should not be too harsh.

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