Seminar on

Parameters Affecting Gray Balance in G7 Caibration

By

Mr. Aditya Deepak Hugay

 

Under the Guidance of

Prof. Dr. Akshay V. Joshi

 

Pune Vidyarthi Griha’s College of Engineering and Technology & G. K. Pate (Wani) Institute of Management, Pune, India 

Academic Year 2020-21

Digital printing is a printing process in which digital-based images directly printed onto a variety of substrates. Unlike offset printing there is no need for plate in the case of digital printing. Digital files such as PDFs files can be sent directly to the digital printing press to print on paper and other substrates. One of the problems faced by the industry is that of correct color reproduction on different machines and on various printing processes. To solve this problem Idealliance® developed a specification called the G7 which defined a universal appearance of B&W imagery or a printed grey scale and also how to calibrate and control any printing and proofing system to achieve that appearance. G7 defines a* and b* target values for each step of the combined three process colors grey scale. G7 stands for greyscale plus seven colors (RGBCMYK). This method is used in many printing processes such as gravure, offset and flexo. It’s been recently introduced into digital printing (Fig. 1).

Fig.1 G7 Calibration

     Image copywrite © Idealliance. All Rights Reserved.

G7 defined grey balance as a standardized scale of CMY percentages that should appear neutral to the eye and defined the a* and b* for each scale step. G7grey balance is “paper-relative” meaning that images printed on different-colored substrates appear slightly different when viewed side byside [1]. Unlike offset printing there is no need for plate in the case of digital printing. Digital printing is certainly more expensive per page than other conventional however digital printing does not need manufacturing of plates, so the overall cost comes down(Fig.2) [2].

            Fig.2 Ink-jet printer

               Image copywrite © Encyclopedia Britannica. All Rights Reserved.

The G7 method uses colorimetric concepts for press calibration and control. Instead of controlling solid ink density (SID) and tone value increase (TVI), this method controls the press by colorimetric values of gray to achieve a better visual match. G7 can be applied to any color reproduction irrespective of printing methods. With the results from the press calibration, the G7 method demonstrates its ability in calibrating a printing press other than offset printing. The improvement in gray balance proved that colorimetric control can achieve a better visual match. The G7 method was developed from offset printing using standard inks and paper defined in ISO 12647-2 (1997). Compared to commercial offset printing, the difference in ink colorants resulted in different densities at different tone values and different densities of solid process colors when printed with liquid inks. Using the curves created by offset data, the calibration may not be as definite as for offset printing. There is less control in digital press in adjusting solid ink density during printing, which means control the press by colorimetric value alone may not be sufficient. It affects the accuracy of using gray balance control in controlling press calibration and proofing. The concept of gray balance control is valid for different ink and substrates combination [3].

            G7 technology is a new calibration method developed to dominate the printing qualities, which uses colorimetric values to control the zone of neutral gray balance areas on printing sheets. It matches visual effects between originals and printings in process of the ink transfer, with the new three parameters introduced to, G7 technology can achieve the ideal same appearances with multiple devices to each other at least in neutral gray tones. But in lack of an appropriate tolerance among manufacturers, there still are some confines in subsequent productions. It is known that, G7 technology parameters are identical with the traditional ways of control from the research of G7 technology standard parameters. They are both have the concentrate on light, middle and shadow tones, G7 technology control the printing quality by commanding three parameters of HC, HR, SC as to make the tone control more accurate and easier. G7 technology relies more on the integrated effects of CMY ink and the neutral gray balance. G7 technology can predigest the complexity of the process largely, and improves the operations compared with the domination of four-color dot increase curves. How to achieve the neutral gray balance stability is the key, and is the most prominent feature of G7 technology. G7 technology is still relatively of new, there is no generally consensus of the tolerance. The results show G7 technology has a positive effect on controlling and can apply to the realistic production for better [4].

        Along with more and more attention focus on the digital printing, and ink-jet technology’s own superiority, ink-jet printing industry has gradually developed into a single flower. Therefore, it has important significance to realize the color accurate representation and get high quality print of ink-jet printing. However, the most effective way to achieve this goal is to control the gray balance of the printing process. G7 is a new method of control gray balance. It is based on GRACoL 7 standard to formulate a set of new calibration method, and it is established on the image of CTP (Computer to Plate) digital publishing and spectrophotometer detection. Generally, speak, controlling the gray balance in traditional printing is first to monitor the density of monochromatic, then based on the determined density of each monochromatic to chase the gray balance of CMY overprinting. However, the introduction of G7 has broken the traditional control method. G7 control the gray balance first, then, adjusting the density of monochromatic based on getting good representation of gray balance.G7 also introduced the parameters of HC (Highlight Contrast), HR (Highlight Range), SC (Shade Contrast) and the curve of NPDC to replace traditional TVI (Tone Value Increase), K value, etc. This method can greatly strengthen the accuracy of the color transfer processing of the digital ink jet printing. Therefore, we can say G7 puts the detection of gray balance onto a significant position [5].

  Gray balance is the critical control parameter in color reproduction which balance that correlates material characteristics and process control parameters. The paper chromaticity is measured using CIE L* a* b* three-dimensional space coordinate system to determine the characteristics of paper in ISO12647 standard. A slight deviation in chromaticity can have an impact on gray balance in process color printing. The experiment conducted for three paper grades viz. #1 (91.3L* 0.5a* 3.61b*), #2 (91.09L* 0.52a* -1.8b*) and #3 (91.21L* 2.24a* -5b*) showed chromaticity of paper #2 close to ISO 1 (95L* 0a* -2b*) standard followed by paper #1 and paper #3. The gray balance achieved by Cyan, Magenta and Yellow ideally has maximum amount of Cyan and equal amount of Magenta and Yellow. All the paper grades had higher proportion of Cyan than Magenta and Yellow. The paper #1 having slope towards yellow resulted in higher proportion of Yellow than Magenta while paper #3 with slope towards cyan resulted in higher proportion of Magenta than Yellow to achieve the gray balance. The proportion of Magenta and Yellow will differ according the deviation in chromaticity of paper. The gray balance curve will coincide with theoretical ideal value if the ink and paper property coincide with ISO standard. The gray balance varies due to different color characteristics of papers and will not be identical even under the same printing conditions. Therefore to achieve consistent color reproduction for a particular paper, the printer should re-calibrated for gray balance [6-7].

References

1. Science of G7. Retrieved from- https://connect.idealliance.org/g7/about/scienceofg7
2. A Step-by-Step Guide to the Digital Printing Process. Retrieved from- https://cgsasp.com/digital-printing-process/.
3. Xlaoylng R. (2008) G7 method for Indigo Press Calibration and Proofing. NIP24and Digital Fabrication (2008). Retrieved from- https://www.researchgate.net/publication/48909446
4. Li K., Sun H., Jiang W, (2012) The Standard Parameters Research in Digital Printing by G7 Technology. Applied Mechanics and Materials Vols 182-183 pp 348-351 Retrieved from- https://www.scientific.net/AMM.182-183.348
5. Zhenrong W. and Wanyou T. (2013) Research on the grey balance of ink-jet printing based on G7. Applied Mechanics and Materials Vol. 329 pp 429-433 Retrieved from- https://www.scientific.net/AMM.329.429
6. Wang H., Chen G.,Cui X. (2012)  Effect on Grey Balance caused by Slight Chromaticity Deviation of Paper in Image Reproduction.Applied Mechanics and Materials Vols. 236-237 pp 459-463Retrieved from- https://www.scientific.net/AMM.236-237.459
7. Li M., Chen G.(2015) Research on Grey Balance in Ink-Jet printing based on Paper Color Characteristics. Applied Mechanics and Materials Vol 731 pp 308-311 Retrieved from-https://www.scientific.net/AMM.731.308