Statistical analysis of color rendering characteristics of printed sheets based on color shift detection
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Offset printing is a relatively complicated production process. Ink, dampening solution, printing plate, blanket, substrate, printing pressure and other factors in the offset printing process will directly affect the printing process and printing effect. The consistency and stability of print quality has always been the focus of the industry. In actual production, in order to obtain the best printing quality, the printing process is first adjusted to the best working conditions, then the quality control threshold is set, and finally the measurement and adjustment are carried out during the printing process to maintain the quality and consistency of the printing.
1. Research status of evaluation of printed product quality
The methods for evaluating the quality of printed products include subjective visual inspection, density detection and colorimetric detection.
Subjective visual inspection is a completely subjective and at the same time the simplest method. The printed product and the standard sample are directly compared artificially to determine the quality of the printed sheet. However, it is greatly affected by the experience and psychological factors of the observers. There are too many variable factors, which can not be quantitatively described, which affects the accuracy and reliability of the evaluation.
Density measurement is actually a measure of the amount of reflected or transmitted light and is a reflection of the relative value of the same ink saturation. The instrument used in this method is inexpensive and widely used. However, when measuring the color surface, only the relative amount of a certain primary color ink in printing can be obtained, and the hue of the measured color cannot be indicated, and the color system cannot be contacted, and the color is described, and its use is limited.
The chromaticity detection method first determines the spectral reflectance in the debugging range, and then realizes the adjustment and control of the ink amount according to the spectral reflectance or some chromaticity characteristic parameters derived therefrom, and the measurement result is directly related to the visual sensation of the human eye. More intuitive to show the color of the print. It is generally accepted that the printing machine operator or the quality inspector extracts some printed sheets, compares the printed sheets with the printed proofs by visual comparison, and measures the quality control strip printed at the same time as the printed images to judge the quality of the printed sheets. These control strips are generally placed on the printed sheet tip, although the parameters such as the solid density, overprinting rate, dot enlargement, contrast, etc. of the inks of the respective colors can be obtained, but the overall printing of the printed image cannot be completely displayed. Their judgment on the quality of printed matter is subjective, and the requirements for customers are often overcompensated, which is not conducive to the control and management of print quality.
Recently, the online inspection of printed images can detect the overall printing of printed sheets. However, it is limited by the speed of computer image processing and printing format. It is mainly used for small-sized ticket printing. Based on this, the color measurement of the printed sheets by the spectrophotometer is carried out, and the color development of the detection points set on the printed sheets is obtained, which is more feasible and effective as the basis for the printing control.
Most of the printing industry uses the CIE1976L*a*b* uniform color space and its corresponding color difference formula. The advantages of this color space are: L*, a*, b* are consistent with the vision of the human eye. When the color difference of the color is larger than the visual recognition threshold and less than the adjacent two levels of color difference in the Munsell system, it is better. The ground reflects the psychological feeling effect of the object color. The color difference is used to evaluate the similarity of the printed matter and the visual characteristics of the human eye are consistent, and a good evaluation effect can be obtained. The color difference can comprehensively reflect the color condition and can effectively control the color quality of the printed product. In general, >5 can clearly detect the color difference between the test sample and the standard proof.
2. Experimental design and data acquisition
In this paper, samples are obtained by normal printing, and 200 consecutive sheets (extracted from the 18th sheet after paper printing) are taken as test objects. Considering that the color of the printed screen is greatly affected by the ink area of the printing machine, several test points are set in different ink areas and positions of the printed screen, and the chromaticity values thereof are measured, and then statistical analysis is performed. In this paper, nine test points with representative different positions are selected for comparative analysis.
2.1 experimental instrument.
175g coated paper, Tianjin Coswood quick-drying four-color offset sheetfed ink, Aierfa laser photo, Huaguangyang PS version, Japan Screen FT-R3050 Phototyper, Shanghai Nanren Xingda W- PS-880 automatic developing machine, Shanghai Xinxing SBK1150-GM printing machine, Heidelberg M04K printing machine, printing color sequence is KMCY, round dot, screen line number 200L/In, screen angle: C15°, M45°, Y90° , K75 °, X-Rite 528 spectrodensitometer.
2.2 data collection environment.
The chromaticity values of the selected test patches on the sheets were measured using an X-Rite 528 spectrodensitometer under D65 source illumination with a 2° field of view, CIE 1976 L*a*b* chromaticity space.
2.3 statistical analysis of the quality of printed products.
Through comparative analysis, after selecting the standard proofs, the chromaticity values of the selected test patches on the 200 consecutive sheets are measured, and the color difference values are calculated one by one, and finally the point map (dynamic) analysis and the probability distribution (static) statistics are performed. Analysis (limited to space, only the test results of 3 points are listed here), the results are shown in Figure 1 to Figure 3.
3. According to the above data and analysis chart, the following analysis results can be obtained:
3.1 Overall, in this study, from the 17th official printing, the printing process reached a steady state (the number 1 in this paper is the 18th after the official printing). Except for a very small number of singular points (8 out of 600 points), the color difference ΔE of the corresponding points of the three test points and the standard proofs fluctuate within a small range, less than 5, to meet the general printing needs. In the 95% probability range, the color difference distribution range of test point No. 1 is [0,3.3942], the color difference distribution range of test point No. 2 is [0,3.1799], and the color difference distribution range of test point No.3 is [0,1.9070]. ]. The color fluctuation of the same test spot on the continuous sheet may be caused by factors such as the dynamic balance of the ink, the printing pressure, the moisture absorption of the paper, the vibration of the printing machine, and the like.
3.2 The test results on 2200 continuous prints showed that the average color difference of test point No.1 was 1.8180 and the distribution variance was 0.9553; the average color difference of test point No.2 was 1.3222, the distribution variance was 1.1259; the average color difference of test point No.3 was 1.1355, and the distribution variance was 0.4676; Test point No. 1, within the range of ±3σ, the statistical probability is 86.37%; ±2.58σ, the statistical probability is 75.08%; ±2σ, the statistical probability is 53.86%; the test point No. 2, within the range of ±3σ The statistical probability is 96.60%; ±2.58σ, the statistical probability is 92.00%; ±2σ, the statistical probability is 79.48%; the third test point, within the ±3σ distribution range, the statistical probability is 71.62%; ±2.58σ, the statistical probability is 56.02%; ±2σ, the statistical probability is 33.42%; it can be seen that the chromatic aberration of each test point is bell-shaped and does not conform to the normal distribution law.
3.3 There are two singular points (color difference > 5) at test point No. 31, which occur on No. 26 (color difference is 5.1595) and No. 148 sheet (color difference is 5.1187); the color difference can be clearly seen in the human eye. As can be seen from the analysis chart, the test block is darker and redder than the standard proof. |5.1187-| =3.3007 > 3=30.9553, which is a coarse error, should be deleted; there are 6 singular points in the 2nd test point, which occur in No. 26 (color difference is 6.6116) and No. 27 (color difference is 5.4212), No. 28 (color difference is 5.0997), No. 29 (color difference is 6.6607) and No. 149 printed sheet (color difference is 5.5939); from 26th to 29th, the test points on these sheets are greener than the standard, indicating that the amount of cyan ink is too large. . Compared with the standard proofs on the 149th test, the color difference is 5.5939, and there is a large fluctuation in the burst, |5.5939-| = 4.2717 > 3=31.1259, and it basically corresponds to the test point No. 1, and the fluctuation quickly disappears. It is a gross error and should be deleted; no test point on the 3rd test point occurs.
3.4 Comprehensive analysis of the coloration of the three test points, it is not difficult to find: the average color difference and distribution variance of the test point No. 3 are the smallest, indicating that the color consistency and stability are better than the other two points; this may be The position on the printed sheet is related. The first and second test points are in different ink areas in the direction of the mouth of the printed sheet, and the ink is greatly affected by the ink adjustment. Correspondingly, the color difference from the corresponding point on the standard sample is relatively large. The third test point and the first test point are in the same ink zone and are located in the middle of the printed sheet. In theory, the color of the printing is greatly affected by the ink area, and the color development in the same ink area should be consistent. However, as can be seen from the above distribution map, although the third test point and the first test point are in the same ink zone, its color rendering effect is better than that of the first test point, and is stable, indicating that the printing is Under the system, the coloration of different positions in the same ink zone is different, and the printing effect in the middle of the printed sheet is better than that in the mouth part.
4 Conclusion
In this paper, through the actual printing experiment, the paper analyzes the coloration on different positions on the printed matter and the different positions on the printed sheet. It is found that the color patches of different areas on the printed matter are different, and they are different in the printing. The location is related. They all have a bell-shaped distribution, but they do not conform to the normal distribution law. The distribution characteristics of each test point are different, and the printing quality of the middle portion of the printed sheet is better than the position of the cornice.