High-quality prints are an important guarantee for printing companies to maintain a competitive advantage. In the printing production process, how to obtain printed products with high consistency with standard proofs has always been the focus of printing companies. Therefore, the selection of standard proofs as the basis for judging whether the printed products are qualified, the quality standards and the adjustment control of the subsequent printing process highlights its importance.
1. The current standard sample extraction method and existing problems
In traditional printing operations, there are usually three methods for selecting standard proofs: one is based on the original copy of the copy; the second is based on the proofs as the actual production of standard proofs; the third is by the printer in the trial printing process Experience and habits to extract proofs, quality inspection of the extracted proofs (mainly to check the color of the control strip, supplemented by human eyes to visually inspect the overall picture), and select the proofs that meet customer standards as standard proofs. However, these three methods are not satisfactory.
When the manuscript is a photo or a drawing, due to the defects of the printing itself (color separation, screening, printing color gamut, etc.), it is difficult for the final printed product to reach the level of the manuscript. Evaluating and adjusting the printing based on the manuscript shows its irrationality. .
When the proof sheet is used as the standard proof sheet, because the paper, ink, etc. used for proofing are different from the actual printing process, there is a difference between the proof color gamut and the printing color gamut. Then, between the color of the proof sheet and the actual printed product It is difficult to avoid color deviations, and these deviations are generally not easy to eliminate. Therefore, referring to the proofing manuscript to judge the quality of the actual printed product, and the adjustment and control of the subsequent printing may cause deviations, resulting in the waste of printing raw materials and working time, which is not conducive to the continuous production of normal production.
The method of selecting standard proofs from the extracted prints avoids the problem of different color gamut between the standard and actual prints, but due to its strong randomness when selecting the proofs, there may still be other deficiencies: the proofs drawn It may not accurately reflect the actual printing process. If it happens to be in the printing process (such as the looseness of the printing press parts, the dynamic imbalance of ink at a certain stage, the paper problem, etc.), and the printing quality changes suddenly, it will cause a large judgment error. Because the sample drawn at this time may be the best, or it may be the worst, but the sudden change is not an accurate reflection of the state of the machine.
2. The principle and implementation of standard sample extraction method based on statistical data
It can be seen from the above analysis that the current standard sample extraction method is not very reasonable. Therefore, this paper proposes a method for analyzing and extracting standard proofs based on the actual print color statistics.
First, select the printed sheet under normal and stable printing conditions as the experimental sample. Suppose there are a total of n printed sheets (No. 1, No. 2, No. 3 ... n), set test points at the same position of each printed sheet, and measure the chromaticity value of each test point under the same observation conditions. Based on the chromaticity data of the test point, standard samples are selected through calculation and analysis. Specific steps are as follows:
â‘ Firstly, take the No. 1 printing sheet as the "standard" sample, calculate the color difference between the test points on the remaining sheets and the corresponding test points on the "standard" sample, and weight and sum the color differences of the test points;
â‘¡Then take the No. 2, 3, 4 ... n printing sheets as the "standard" sample respectively and follow the first step to calculate the weighted sum of color difference;
â‘¢ Based on the weighted sum of color differences as the basis for judgment, when using it as a sample, the one with the smallest weighted sum of color differences of each printed sheet is the standard sample.
The advantage of this is that, based on the objective data of the actual print, through calculation and comparison, the influence of human factors when randomly drawing samples is avoided, and the selection result is more reasonable compared with the commonly used random extraction method; it can reduce the random extraction Samples and misjudgment of the printing process to reduce the chance of greater errors; overcomes the traditional use of proofs as a reference, because the proofing paper, ink, etc. are different from the real printing process and cannot truly reflect the printing system ’s Situation, it can not provide an accurate basis for the subsequent printing control adjustment.
3. Experiment
Select 200 continuous prints in the normal printing process as the experimental proofs. Corresponding to the actual printing ink area, the printed sheets are partitioned, and test points are set at different longitudinal positions of each ink area and ink area.
Measure the chromaticity data of each test point on the continuous printing sheet. Through calculation, the weighted sum of the color difference of the remaining sheets is obtained when each printed sheet is used as the standard sample.
It can be seen from the calculation data that when the 96th printed sheet is used as the standard sample sheet, the weighted sum of the color difference of each printed sheet is the smallest. Therefore, the standard sample for this test is the No. 96 printed sheet.
At present, randomly selected printed sheets are used as standard samples, and the test points at different positions on the same printed sheet are used to compare the average color difference of the same color block.
According to the measured values, when the 96th printed sheet is used as the standard sample, only the average color difference of the 15 test points and the 35 test points among the 7 test points selected is slightly larger than that when the other sheets are used as the standard sample. This shows that the method in this paper is more reasonable than the traditional random sampling.
4 Conclusion
This paper proposes a new method for selecting standard proofs through actual printing experiments. Compared with extracting standard samples from randomly selected printed sheets, it has overcome the influence of human factors of traditional random sampling to a certain extent, and is more in line with the actual printing process. It can provide an accurate basis for the evaluation of print quality and the control and adjustment of subsequent printing .
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