Color Print Quality Inspection Technology Introduction

Traditional print inspection techniques include subjective visual inspection, density inspection, and colorimetric detection.

Subjective visual inspection refers to the human comparison of prints and standard prints, either directly or with the aid of a magnifying glass, to subjectively assess the quality of prints. Although this method is simple and flexible, it is susceptible to the operator's experience and psychological, physiological and other factors, and the accuracy and stability are poor.

The density detection method and the colorimetric detection method use the instrument to measure the density and the color of the printed product, and use the relevant theory to analyze and process the data, and then evaluate the quality of the printed product. Both of these methods can provide a theoretical basis for the objective evaluation of the quality of printed materials, overcome the instabilities of subjective visual inspection methods, and other defects. Shanghai printing has a wide range of applications in the printing field. However, the instruments used for density and colorimetric detection can only detect areas around 10mm2. Therefore, it is necessary to add measurement and control strips to obtain the information we require from the print screen completely and directly. Moreover, related research shows that there are certain inaccuracies and limitations in the theory based on density and colorimetry.

The traditional printing inspection technology also has a common defect: they are all after the completion of the printing of the sample inspection, once the print quality problems, it is often several hundred, may even reach thousands, can not be found in the printing process Problems, resulting in waste of printed materials and working hours, increase production costs.

Due to many deficiencies of the traditional detection technology, on-line printing inspection technology came into being.

2 Printing Online Inspection Technology

1) Advantages of Online Detection Technology for Printed Products

The on-line detection of printed matter is a kind of automatic detection method suitable for high-speed printing formed by combining the image processing technology with the printing principle. Its advantages are mainly manifested in the following two aspects:

First, on-line detection is more accurate and can detect very small defects. For example, if we concentrate on inspecting a certain printed product, only when the contrasting color of the printed product is relatively strong, the human eye can find defects larger than 0.3 mm, and the quality of the obtained test is difficult to maintain continuous stability. In the same color system, the Shanghai printing factory detects the quality of printed matter, especially in light-colored printed matter, and it is difficult for the human eye to find defects. On the other hand, on-line inspection can easily detect 0.1mm defects, and only one gray level difference is required.

Second, on-line inspection improves production efficiency. The quality assessment of the on-line inspection system is independent of the operator's subjective judgment. In the running process, regulation is implemented more quickly and easily, and adjustment is completed in one step. The detection speed is greatly improved. At the same time, in the event of a quality defect, the operator can promptly resolve problems that occur during the work according to the real-time reports on the site, reduce scrap rates, and managers can also track the production process based on analysis reports of test results. Management benefits.

2) principle of online detection technology of printed products

On-line detection system principle: The system uses CCD camera lens to collect non-defective prints as standard images, and then collects the image to be checked on the printing production line, and transmits each frame of the collected image to the scene for comparison and analysis with the standard image. Find out the image with quality as the subject, and find the quality problem of the print product corresponding to the frame image; finally adjust the corresponding printing part automatically to realize the online control of the quality of the printed matter.

In the specific implementation of on-line detection of printed products, it is generally divided into two steps: First, test preparation, that is, through the image acquisition of qualified products, to obtain standard print image, also called the template is followed by the actual detection, the image and standard of the printed matter to be examined. The template is compared to determine the presence or absence of a defect and the location of the defect based on the comparison result, and the defect information is recorded

Printing defects mainly include shape defects and color defects. Two kinds of shape defects mainly refer to foreign material stains, such as knife wire. It expresses the difference between the gray value of the defect image and the standard image. The value is compared with the standard image, and it is judged whether the difference has exceeded the preset threshold range, it can be judged whether the image has a shape defect. The color defect mainly means that there is a deviation in the color of the printed product from the standard print. The color space can be used. The conversion model obtains the colorimetric value CIE L*a*b* of the print image, and uses the color difference to evaluate the color cast of the printed product.