Обзор сертификации цветопередачи G7 в полиграфической компании

Keyi Printing (hereinafter referred to as Keyi) passed FOGRA color certification back in 1998 under the guidance of the Printing Research Center. Afterwards Keyi applied the FOGRA standard process to derive adjustment curves for the TVI values of the Y, M, C, K four colors, but this adjustment could not solve the accuracy of color reproduction, especially since it is difficult to achieve color accuracy in printed matter in metallic product colors (overprinting of neutral color series) and the theoretical gray-balance dot values do not easily produce printed matter with a correct gray balance.

Because G7 color calibration controls color including the characteristics of three-color gray, Keyi specially added a G7 color calibration plan this year; in mid-May, guided by this Center, it began G7 color certification and, after many rounds of education, training and simulation drills, completed certification in early July.

After obtaining full-process printing color certification from the German FOGRA Graphic Technology Research Association (Fogra Forschungsgesellschaft Druck e.V.), Keyi Information Technology Co., Ltd. passed G7 Master certification under the guidance of this Center, becoming the only company in Taiwan, China to obtain dual certification for its printing color process.

In the printing process, dot gain is the main object of control in print reproduction. A printing plant's prepress production department must, according to the plate curve of the plant's printing department, reduce the plate dot output so as to achieve the correct dot values in the printed matter. The figure below is an example of a typical TVI value versus dot-gain adjustment curve:

In the left figure, the vertical axis is the dot size that print reproduction desires to express, and the horizontal axis is the corresponding plate dot output after reduction to compensate for dot gain; the 45-degree diagonal line in the center represents the ideal printed dot value that the output plate dots can return to after printing dot gain.

Both SWOP standard and FOGRA control put forward standard specifications to ensure the stability and accuracy of printing color; this control is achieved by controlling the changes in single-color dot values of the Y, M, C, K four-color inks.

During this certification process for Keyi, a problem of discrepancies in measured values was encountered. After comparison and analysis, this Center found that the consistency of the measuring instruments and the accuracy of instrument measurement are very important. When certifying for G7, the preparation of hardware and software is quite important, and in particular the calibration accuracy of the measuring colorimeter has a great influence on the measured values. Complete preparation of hardware and software before certification can speed up the completion of certification and avoid unnecessary calibration runs.

With a system that has undergone G7 color calibration, the colors reproduced from originals after color management are more accurate and the benefits are more lasting. The G7 authorities suggest that printing plants perform another G7 calibration every two weeks or every month. Normally, the main G7 control patches can be placed on the printing control strip to ensure the system operates normally; the patches to be checked include:

Y, M, C, K solid patches (first-color printing)

R, G, B solid patches (second-color printing)

25% three-color gray (C25%, M19%, Y19%)

50% three-color gray (C50%, M40%, Y40%)

75% three-color gray (C75%, M66%, Y66%)

100% three-color gray (C100%, M100%, Y100%)

This Center's analysis, comparison and summary of FOGRA and G7 color certification and control are set out in the following explanation:

G7 control method based on NPDC

Control the highlight tone range HR (50C, 40M, 40Y), highlight tone contrast HC (25C, 19M, 19Y) and shadow tone contrast SC (75C, 66M, 66Y), and keep the gray balance values at a=0±1.0, b=-1±2.0

1. Printing preparation

CTP plate maker: check whether the difference in plate dot output after linearization compensation is within ±0.5%; the plate-making screen angles are chosen as K45°, C15°, M75°, Y90°.

Press: the parameters of the fountain solution are pH 4.8-5.5, liquid temperature 10-12℃ and conductivity 800-1200μS/cm; the color sequence is K, C, M, Y.

Paper and ink: use ISO 1# paper and ink conforming to the ISO2846-1 standard.

1. Printing screening standard: screen ruling 175 lpi; screen angles chosen as K45°, C15°, M75°, Y90°; round-square dot plate-making.
2. Density standard without loading the printing compensation curve (note: the color difference tolerances are L*±3, a*±2, b*±2).

After the standard is reached, the NPDC curve (the CMY three-color and K curves) is drawn through software and the RIP curve is calculated (for printing without applying the compensation curve).

1. Verification of the standard after using the compensation curve: through NPDC calibration.
2. Making an ICC Profile

TVI-based Fogra control method

Match the TVI curve of the printed matter with the original

1. Printing preparation: [CTP plate maker] check whether the difference in plate dot output after linearization compensation is within ±0.5%; the plate-making screen angles are chosen as K45°, C15°, M75°, Y90°. [Press] the parameters of the fountain solution are pH 4.8-5.5, liquid temperature 10-12℃ and conductivity 800-1200μS/cm; the color sequence is K, C, M, Y.

Paper and ink: paper conforming to the ISO2846-2 standard and ink conforming to the ISO2846-1 standard.

1. Printing screening standard: screen ruling 175 lpi; screen angles chosen as K45°, C15°, M75°, Y90°; round-square dot plate-making
2. Density standard without loading the printing compensation curve
3. Verification of the standard after using the compensation curve: with the CTP dot-gain compensation curve loaded for printing, check that the 50% dot gain value is within the allowable range
4. Making an ICC Profile and comparison
5. Gamut comparison (on the same art paper; the white line is the gamut obtained by the G7 control method and the red line is the gamut obtained by the Fogra control method)

● In the shadow tones, the gamut using the G7 method is much smaller than that of Fogra, but smoother, which means that G7 sacrifices color in the shadow areas and focuses instead on ensuring the reproduction of gray balance in the mid and highlight tones

● In the midtones, the gamut using the G7 method is larger than that of Fogra

● In the highlight tones, the gamut using the G7 method is about the same as that of Fogra; only in the red and yellow areas is the gamut using the G7 method larger than that of Fogra

● Overall, the G7 gamut is smoother than the Fogra gamut, with more natural tonal transitions, and better meets the human eye's requirements for image reproduction. 2. Differences in operating methods

● Fogra TVI curve: the four colors can individually reach the ideal, but the gray balance produced by overprinting the four colors cannot be controlled by TVI

● G7 control method

▓ Use the NPDC curve to replace Fogra's complex CMYK four-color TVI control

▓ Use HR instead of TVI values to reflect the tone of the printed matter

▓ Use SC instead of printing contrast to reflect gray balance in the shadow tones

▓ Use HC to check gray balance in the highlight tones

● Adopting G7's NPDC method is not only simpler in process, but also ensures that the mid and highlight tones can achieve reproduction capability across different materials and equipment

After completing G7 color certification, Keyi captured the essence of FOGRA and G7 printing color control and completed Keyi's dedicated color control guide in coordination with Keyi's CPC ink control system, so that in the future the printed matter produced by Keyi will greatly improve color accuracy and stability and increase customer trust and the level of quality. Keyi Information Technology Co., Ltd. has always taken 'Quality, Service, Efficiency' as its business philosophy.

In addition to upholding consistent professional prepress and printing production technology, a high-quality service spirit and a rigorous work attitude, it has also continuously upgraded its introduction of new technologies and new equipment. To ensure even more complete and professionally stable quality, under the guidance of the Printing Industry Research Center (a foundation) it passed G7 Master certification in July and was awarded a certificate; in the future it will continue to implement printing color quality control in the production process, and its dedication to printing quality is enough to demonstrate the outstanding strength of the printing industry in Taiwan, China. General Manager Shi of Keyi made a special trip in October to visit Chairman Chen of this Center and presented a commemorative plaque to thank this Center for its assistance with color certification.

—This article is reprinted from China Packaging Network (pack.cn); original link: http://news.pack.cn/show-361962.html