Quality control GRACoL, GRACoL7 and G7
GRACoL (General Recommendations for Applications in Offset Lithography) is an application promotion agency for offset printing technology in the United States, which is affiliated to the International Digital Enterprise Alliance (IDEAlliance for short). GRACoL has developed a set of standard printing production data testing and implementation methods in accordance with the ISO 12647-2 offset printing standard, China Printing, and released the latest version of this implementation method, GRACoL7, in June 2006.
Based on extensive testing, GRACoL7 has extracted a series of new print detection concepts and corresponding parameters. Through measurement and calculation of these parameters, some data are finally obtained. Based on these results, the RIP output calibration curve of the CTP platesetter or imagesetter is adjusted. The printing house can further improve the tone and color reproduction quality of the printed matter on the basis of ensuring the repeatability of the system.
GRACoL7 test principle and implementation steps
1. Related concepts and parameters
GRACoL has proposed a lot of new concepts and parameters for printing test and judgment, mainly including: Neutral Print Density Curve (NPDC), Bright Range (Highlight Range), Dark Contrast (Shadow Contrast) and Concepts such as HC (Highlight Contrast). Among them, the neutral printing density curve NPDC and the bright tone range HR are the two most important concepts.
NPDC includes CMY superimposed colors and K monochromatic curve packaging printing, reflecting the relationship between the measured neutral density and the percentage of original dots on a neutral gray scale, because the neutral gray density is an absolute measurement Value, and the dot expansion value is a relative quantity related to the density of the field, so this concept can more ensure the consistency of the measurement results of the tone and density of the printing and the appearance.
Dot expansion is a very important parameter in the field of printing control printing China, but there are some shortcomings in operation comparison and other aspects. For this reason, GRACoL also proposed the parameter of bright tone range. The bright tone range replaces dot expansion in its proposed method to reflect The parameters of the overall tone and gray balance of the printed matter. HR needs to be measured 4 times (2 times for each of CMY and K) and calculated 2 times; while traditional dot expansion requires measuring at least 3 times for each color plate a total of 12 times, and because HR is an absolute measurement value and dot expansion is A relative value label exhibition, so the contrast and visual brightness of the printed product reflected by HR are more reliable than the dot expansion. among them:
HR_cmy = ND (50c, 40m, 40y) -ND (paper) HR_k = ND (50k) -ND (paper)
The dark contrast SC and the bright contrast HC are parameters for quickly detecting the correctness of the neutral printing density curve in the dark and light tones, among which:
SC_cmy = ND (75c, 66m, 66y) -ND (paper) SC_k = ND (75k) -ND (paper) The contrast of dark tone depends on the dynamic range of the printed product, which can be calculated by IDEAlink curve software. Bright contrast and dynamic The scope is irrelevant and 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 printing quality control:
(1) The calculation methods of these parameters and dot expansion rate are different from lithographers. They are independent of the field density, so they directly reflect the visual darkness and make the subjective and objective quality assessments more consistent.
(2) It has nothing to do with the screen parameters, so no matter what the shape of the dots 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 machine is greatly improved by adjusting the printing results of the measurement results.
2. Preparation
(1) According to the description of GRACoL7, about 6,000 to 10,000 sheets of paper are required for testing and calibration of the printing press. Both paper and ink must meet the GRACoL7 recommended test target of GRACoL7 neutral printing density coordinate chart as specified in ISO 12647-2. standard.
(2) Print layout design. The layout graphic is the main detection object of the test, and the necessary layout content includes:
a. Two sets of GRACoL7 P2P color-coded dek printing presses, reverse 180-degree typesetting respectively;
b. A set of GRACoL7 gray balance guide table;
c. Two IT8.7 / 4 characterization target boards, reverse 180 degree layout;
d. About 1cm wide three-color overprint gray balance color bars: 50c, 40m, 40y;
e. Monochromatic black-grey balanced color bar about 1cm wide: 50k;
f. Necessary printed measurement and control strips and subjective evaluation images.
(3) Clear all calibration curves on the phototypesetting or CTP platesetter (but make sure that the exposure system belongs to a normal and stable working state) and measure the dot size on the printing plates made in this state.
(4) Ensure that the performance of various color measuring equipment and observation equipment is stable, and then print the document under the normal working conditions of the printing press.
3. Printing
GRACoL recommends that the printing color sequence be k-cm-y; the solid density that should be achieved during the printing process is c = 1.45 ± 0.10, m = 1.45 ± 0.10, y = 1.0 ± 0.07, and k = 1.65 ～ 1.9.
4. Generate neutral printing density curve sticker printing
After the proofs are dried, measure the monochrome ladders on the P2P color scale and the gray scales on the three-color overprinted black. In order to eliminate the error, measure the two sets of color scales arranged 180 degrees from each other, and then average the measurement results. Plot the measurement results on the GRACoL self-made coordinate map, then find the closest point of the preset curve and the maximum density point of the drawn curve and some inflection points of the curve, and find the RIP correction curve in the figure according to its drawing rules Key point value, and input into the RIP calibration curve.
5. Generate gray balance curve
The gray balance curve is obtained from the gray balance guide table and the graph of GRACoL7. The process is as follows: First, use a spectrophotometer to measure the color value of the color block at the C50 of the gray balance guide table on the printed sample. The gray balance reduction of the output device is very accurate. It is printed in Shanghai. There is no need to establish a new gray balance curve. If the gray balance point is not the preset central color block, it means that the output device needs the correction of the gray balance curve.
If you need a gray balance correction curve, first make a CMY three-color curve on the GRACoL7 coordinate chart, use the reference line as the basis for the digital printing branch, find the difference between the three colors to achieve gray balance, and then find the gray balance correction in RIP Data, and then input the data into the RIP. Finally, use the newly added RIP calibration curve, re-plate printing the test target, check whether the corresponding parameters meet the standard.
6. Conclusion
This article briefly describes the steps of testing and calibration of the printing system regulated by GRACoL7. Beijing Screen Printing, the core mechanism of this method is to adjust the output calibration curve of CTP or CTF to achieve the purpose of improving the final printing quality; implementing this test and calibration process Two premises can not be ignored, one is that in the entire plate making and printing process, all output equipment must be stable and have repeatable output, otherwise the obtained parameters will be meaningless for actual production; the second is that the materials used must comply with ISO The relevant standards regulate the printing of anti-counterfeit labels, because this is the basis of measurement, calculation and judgment.
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