The difference between GRACoL, GRACoL7 and G7
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It is one of the dreams of many printing companies to establish a standardized printing production environment to improve the overall quality of printed products. The development of various printing standards and the development of digital testing and control technology are all aimed at achieving this goal. The standardized and standardized production environment (system) and the corresponding detection methods are a hot spot for many research and production organizations. In this respect, GRACoL in the United States has made useful attempts and achieved good results.
GRACoL (General Recommendations for Applications in Offset Lithography) is an offset printing technology application promotion organization in the United States, which is affiliated to the International Digital Enterprise Alliance (IDE International Alliance). GRACoL developed a standard set of test production and implementation methods for printing production data in accordance with ISO 12647-2 offset printing standards. In June 2006, the latest version of the implementation method, GRACoL7, was released.
Based on extensive testing, GRACoL7 has extracted a series of new print inspection concepts and corresponding parameters. After measuring and calculating these parameters, some data are finally obtained. Based on these results, the RIP output calibration curve of the CTP platesetter or imagesetter is adjusted. Dongfeng Printing further improves the quality of prints and color reproduction based on the system's reproducible work.
GRACoL7 test principle and implementation steps
1. Related concepts and parameters
GRACoL has proposed a number of new concepts and parameters for printing test and judgment, including: Neutral Print Density Curve (NPDC), HR (HighlightRange), Shadow Contrast (SC) and Concepts such as highlight contrast HC (Highlight Contrast). Among them, the neutral printing density curve NPDC and the bright range HR are the two most important concepts.
NPDC contains both CMY overprint and K-monochrome print translations, reflecting the relationship between the measured neutral density and the original dot percentage on a neutral gray scale, since neutral gray density is an absolute measure The value, and the dot gain value is a relative amount related to the density of the field, so this concept can ensure the consistency of the measurement of the tone and density of the print with the apparent feeling.
The dot enlargement is a very important parameter printing quotation system in the field of printing control, but there are some defects in the operation comparison. For this reason, GRACoL also proposes the parameter of the bright adjustment range, and the bright adjustment range replaces the dot expansion in its proposed method. Parameters that reflect the overall tone and gray balance of the print. HR needs to be measured 4 times (CMY and K 2 times each) and calculated 2 times; while traditional dot enlargement requires at least 12 times for each color plate measurement, and because HR is an absolute measurement, the dot gain is A relative value of the printing color, so the contrast and visual darkness of the printed product reflected by the HR is more reliable than the dot gain. among them:
HR_cmy=ND(50c, 40m, 40y)-ND(paper)HR_k=ND(50k)-ND(paper)
Dark Contrast SC and Bright Contrast H C are parameters that quickly detect the correctness of the neutral printing density curve in dark and light, where:
SC_cmy=ND(75c, 66m, 66y)-ND(paper)SC_k=ND(75k)-ND(paper) The dark contrast depends on the dynamic range of the print and can be calculated by the IDEAlink curve software. Bright contrast and dynamics Range-independent, is a constant:
HC_cmy=ND(25c,19m,19y)-ND(paper)=0.25HC_k=ND(25k)-ND(paper)=0.22 The introduction of these parameters brings many benefits to print quality control: (1) these parameters and outlets The expansion rate is calculated in different ways. They are independent of the density in the field, so they directly reflect the visual shading, making the subjective and objective quality evaluation more consistent; (2) independent of the screening parameters, so no matter what kind of dot shape and The number of screen lines can be compared under the same standard; (3) Due to the reduction of the necessary measurement data, the efficiency of the printing press is greatly improved by the adjustment of the measurement results.
2. Preparation
(1) According to GRACoL7, approximately 6,000 to 10,000 sheets of paper are required for testing and calibration of the printing press. Paper and ink must meet the GRACoL7 neutral printing density map of the GRACoL7 recommended test mark specified in ISO 12647-2. standard.
(2) Print layout design. The layout image is the main test object of the test, and the necessary layout contents include:
a. Two groups of GRACoL7 P2P color code Kunming printing, respectively reverse 180 degrees typesetting;
b. A set of GRACoL7 gray balance guides;
c. Two IT8.7/4 characterization targets, reverse 180 degree layout;
d. Three-color overprint gray balance color strip of about 1 cm width: 50c, 40m, 40y;
e. Monochrome black and gray balance color strip of about 1cm width: 50k;
f. Necessary printed control strips and subjective evaluation images.
(3) Remove all calibration curves on the phototypesetting or CTP platesetter (but ensure that the exposure system is in a normal stable operating state) and measure the dot size on the plates produced in this state.
(4) Ensure that the performance of various color measuring equipment and observation equipment is stable, and then print the document under normal working conditions of the printing press.
3. Printing
GRACoL recommends a printing color sequence of k-cm-y; the solid density to be achieved during printing is c = 1.45 ± 0.10, m = 1.45 ± 0.10, y = 1.0 ± 0.07, and k = 1.65 to 1.9.
4. Generate neutral printing density curve word printing
After the sample is dried, the monochromatic black and the three-color overprinted black scales on the P2P color scale are respectively measured. In order to eliminate the error, two sets of color scales which are mutually formatted by 180 degrees are measured, and then the measurement results are averaged. The measurement results are drawn on the GRACoL self-made coordinate map for offset printing, and then the closest point of the preset curve to the maximum density point of the drawn curve and some inflection points of the curve are found, and the RIP calibration curve is found in the figure according to the drawing rule. Key point values are entered into the RIP calibration curve.
5. Generate gray balance curve
The gray balance curve is obtained by the gray balance guide and the coordinates of GRACoL7. The process is: firstly, the color value of the color patch at the gray balance guide portion C50 on the printed sample is measured by a spectrophotometer, and if the gray level of the color patch at the central position reaches the standard (a=0, b=-2), The gray balance of the output device is very accurate. Zhengzhou printing does not need to establish a new gray balance curve. If the gray balance point is not the preset central color block, then the output device needs to be corrected by the gray balance curve.
If you need a gray balance calibration curve, first make a CMY tri-color curve on the GRACoL7 coordinate chart, and use the reference line as the basis to print the material to find the difference between the three colors to achieve the gray balance, and then find the data of the gray balance correction in RIP. And then enter the data into the RIP. Finally, using the newly added RIP calibration curve, reprint the test target and check whether the corresponding parameters are up to standard.
6. Conclusion
This paper briefly describes the steps of printing and calibration of the printing system specified by GRACoL7. The core mechanism of this method is to adjust the output calibration curve of CTP or CTF to improve the quality of the final print. There are two implementations of this test and calibration process. The premise can not be ignored. First, all output devices must be stable and output repeatable during the entire plate making and printing process. Otherwise, the parameters obtained will be meaningless for actual production. Second, the materials used must comply with ISO standards. Regulate the printing company because it is the basis for measurement, calculation, and judgment.