Gray Balance in Press Calibration

What's so important about gray balance at press? The gray balance concept is essential for excellent color reproduction in scanning, proofing, and in the pressroom. There are two steps in press calibration. The very first step is to align the corner points of the target color gamut. Gray balance is the first important step undertaken by the color separator in determining the color content and contrast that the final printed piece will have. The second step is to adjust press tonality which is by gray balance method. In scanning images that are not in gray are said to be casted while casted images require color correction to remove the unwanted colors. Images in gray balance only need to be adjusted for specific areas of color enhancement, e.g., greener grass or bluer skies. The little secret of the process color printing at press is that you can only print two ways on press in gray balance or casted that's it.

The majority of printing plants have the platemaking problem. It is not possible for the pressroom to control gray balance and color with the wrong size dots on the plates. The first procedure is to print a test form with complete tone scales at the required density. Comparing the scales against a standard and adjust the plate values accordingly. Every color bar should include a three-color gray patch which is represented by 50C-40M-40Y. The patch, when printed at the correct density and dot gain, will appear neutral without any casts.

Mainly Graycolor separation method is selected to control the amounts of CMYK colors because it increase the quality of printing. The single printer used for printing the same image on different paper also results in different printed images. To remove this problem a different ICC profile based on gray level control is developed and the sheet offset printer is calibrated using the profile and a subjective evaluation shows satisfactory results for different quality paper. The gray balance print run ICC profiles has been created. Printing using those profile is same on dull, gloss and dull silk paper whereas for coated paper it is just satisfactory. Tests made using the currently available software (Adobe Photoshop - image processing, Profile Maker - profile creation, Print-Open - profile creationfor separation of color images GCR color separation is more preferred rather than UCR. The overall evaluation results showed that image separated using gray balance profile is more preferred than density control profile. Instead of controlling solid ink density (SID) and tone value increase (TVI), the G7 method introduce a gray balance control as the key to achieve consistent color reproduction. G7 method controls the press by colorimetric values of gray to achieve a better visual match. The gray patches quantify CIE L*a*b* values. Based on the gray balance, the G7 method is believed to be valid for calibrating digital printing and other conventional printing processes. The test target printed with RIP curve adjustment showed better result in gray balance control, but the color gamut of the target printed by this method was smaller than the one printed with profile embedded. It implies the gray balance valid on different substrates and ink combination.

Figure 1. (G7 Shared Neutral Appearance)

The print parameters (Calibration, Screening, Resolution, Source and Destination These parameters rely on the 4-C’s (Calibration, Characterization, Conversion and Control) of Color Managed Workflow. As the implementation of Color Management Digital Printing Workflow is costly, time consuming and a tedious process. It does, however, benefit those who implement this workflow to get consistent color from device to device. The L*a*b* Greyness measurement accuracy is compared with new system called GATF colour circle. The GATF Colour Hexagon and the GATF Colour Triangle are adaptations from other colour notational systems using cyan, magenta, yellow solid ink density units as their basic input values. The reliance of the GATF Colour Circle on optical density as its basic input value is advantageous for controlling the ink density.

Taking this technology a step further, G7 allows not only for common files to be shared among the offset printers but also opens the possibility of the same files being repurposed to the wide variety of CMYK (non-offset) print processes—all aimed at near neutral gray balance targets—with the expectation of reasonable color match. All devices and systems are calibrated not to each other but to a measureable near neutral balance of colors throughout the tonal scale. If one component of the system slips out of calibration it can be identified and realigned.

Name-Sapkal Sarang Dadasaheb

TE Printing

PVG COET & GKPWIM

REFERENCES

1. Brede, M. H. (2008). Grey Balance Control with a Re-Purposed GATF Colour Circle. TAGA Journal, 35.

2.Steve Upton & Pat Herold http://www.colorwiki.com/wiki/Why_and_How_of_G7_Calibration

3.Standardization in Offset Printing. (2016, June 12). Standardization in Offset Printing, “Heidelberger Druckmaschinen AG”, www.Heidelberger.Com. https://www.heidelberger.com

4.Developing ICC Profile Using Gray Level Control in Offset Printing Process. (2012). Dilawari, J. S., & Khanna, R. (2012). Developing ICC Profile Using Gray Level Control In Offset Printing Process. ArXiv Preprint ArXiv:1210.5732. https://www.preprint.com.

5. Dharavath, H. N. (2020). Effect of Color Output Modification Approach (COMA) on the gray balance: A mismatch of device calibration, destination and source profiles, and halftone screening. Journal of Graphic Engineering and Design, 11(1), 13.

6.Gaurav Sharma, Robert P. Loce, Steven J. Harrington, Y. (Juliet) Zhang,”Illuminant Multiplexed Imaging: Special Effects using GCR” IS&T/SID Eleventh Color Imaging Conference.