A method for extracting standard proofs based on statistical data
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High-quality prints are an important guarantee for printing companies to maintain competitive advantage. In the printing production process, how to obtain prints with high consistency with standard proofs has always been the focus of printing companies. Therefore, the selection of the standard proofs for judging whether the printed products are qualified or not, the quality and quality standards, and the subsequent control of the printing process are highlighted.
1. Current standard proofs extraction methods and existing problems
In the traditional printing operation, there are usually three methods for selecting standard proofs: one is based on the original of the replica; the other is the proof sheet as the standard proof produced by the actual sample; the third is the printer in the trial printing process. The experience and habits are used to extract the proofs, and the quality samples of the extracted samples are checked (mainly by checking the color of the control strips, supplemented by the human eye to visually measure the overall picture), and the samples that meet the customer's standards are selected as standard proofs. However, these three methods have some unsatisfactory places.
When the original is a photo or a painting, the defect of the printing itself (separation, screening, printing color gamut, etc.) makes it difficult for the final printed product to reach the level of the original, and it is unreasonable to evaluate and adjust the printing according to the original. .
When the proof sheet is used as a standard proof, since the paper, ink, etc. used for proofing are different from the actual printing process, and there is a difference between the proof gamut and the printing gamut, then the color of the proof sheet is between the actual print and the actual print. The color deviation is difficult to avoid, and these deviations are generally not easy to eliminate. Therefore, referring to the proofing paper to judge the quality of the actual printed product, and the adjustment and control of the subsequent printing may be biased, resulting in waste of printing materials and working time, which is not conducive to the continuous operation of normal production.
Although the method of selecting standard proofs from the extracted sheets avoids the problem of different color gamut between the standard sheets and the actual printed products, there may still be other shortcomings due to the fact that it has strong randomness when selecting the sampling sheets: the extracted proofs It may not accurately reflect the actual printing process. If it happens to be in the printing process (such as loose printing parts, dynamic imbalance of ink at a certain stage, paper problems, etc.), when the printing quality changes, it will lead to a large judgment error. Because the sample sample extracted at this time may be the best, or it may be the worst, just a sudden change, not an accurate representation of the state of the machine.
2. Principle and implementation of standard sample extraction method based on statistical data
It can be seen from the above analysis that the extraction method of the commonly used standard proofs is not reasonable. Therefore, this paper proposes an analytical extraction method based on the standard proofs of actual print chromaticity statistics.
First, the printed sheets under normal stable printing conditions are selected as experimental proofs. Assuming that there are a total of n sheets (numbered No. 1, No. 2, No. 3, ... n), test points are set at the same position of each sheet, and the chromaticity values of the respective test points are measured under the same observation conditions. Based on the chromaticity data of the test points, standard proofs are selected by calculation and analysis. Specific steps are as follows:
① First, use the No. 1 printed sheet as the “standard” proof, calculate the color difference between the test points on the remaining sheets and the corresponding test points on the “standard” proof, and weight the sum of the color differences of each test point;
② Then successively use the 2nd, 3rd, 4th...n prints as the "standard" proofs, and calculate the color difference weighted sum in the first step;
③ Based on the weighted sum of the chromatic aberrations, find out that the sheet with the smallest chromatic aberration weight of each sheet is the standard proof.
The advantage of this is that, based on the objective data of the actual printed matter, through calculation and comparison, the influence of human factors on random sample extraction is avoided, and the selection result is more reasonable than the commonly used random extraction method; Proof of the wrong process of the printing process, reducing the chance of a larger error; overcome the traditional use of proofing as a reference, because the proof of paper, ink, etc. is different from the actual printing process, and can not truly reflect the printing system In the case, it is not possible to provide an accurate basis for subsequent printing control adjustments.
3. Experiment
200 continuous prints in the normal printing process were selected as experimental proofs. The printed sheets are partitioned corresponding to the actual printing ink areas, and test points are set at different longitudinal positions of each of the ink areas and the ink areas. The location of each test point is shown in Figure 1.
Figure 1 Schematic diagram of the test point position on the printed sheet
The chromaticity data of each test point on successive sheets was measured. Through calculation, the color difference weighting sum values of the remaining sheets are obtained when each sheet is used as a standard proof. As shown in Figure 2:
Figure 2: Separation of the color difference weights of the remaining sheets when each sheet is used as a standard proof
It can be seen from the calculation data that when the No. 96 printed sheet is used as a standard proof, the weight difference sum of each sheet is the smallest. Therefore, the standard proof of this test is the 96th printed sheet.
Now use the randomly selected sheets as standard proofs, go to the test points at different positions on the same sheet, compare the average values of the color differences of the same color block, and the values are as shown in Table 1:
Table 1 Color difference mean data table of test color block when some sheets are sampled
The data values in Table 1 are the standard proofs of the 96th printed sheet. Among the 7 test points selected, only the average of the color difference of the 15th and 35th test points is slightly larger than the average of the color difference when the other sheets are used as the standard proof. This shows that the method of this paper is more reasonable than the traditional random sample extraction.
4 Conclusion
This paper proposes a new method for selecting standard proofs through actual printing experiments. Compared with the extraction of standard proofs in random strips, it overcomes the influence of human factors in traditional random sampling methods to a certain extent, which is more consistent with the actual printing process, and can provide accurate basis for the evaluation of print quality and the control and adjustment of subsequent printing. .