Discussion on online automatic detection of LED full-color screen modules

　　introduction:

　　According to the inspection requirements of display screen modules, the original manual inspection was transformed, and the system was designed to integrate machine vision technology into the LED module inspection process. Without changing the hardware conditions of the LED module, the online, automatic and rapid quality inspection of the LED module was realized, which met the inspection requirements of customers and realized the standardization of module production. It will have a large application market in the field of rapid on-site inspection of LED unit boards. It is not only conducive to improving the economic benefits of the enterprise itself, but also can drive the technological progress of the industry, promote the development of China's optoelectronics industry, improve the technical level of our industry, reduce and replace imports, and has significant economic benefits.

　　A control system software was developed to control the display pattern of the LED display module. A high-precision color brightness sensor probe was used to detect the brightness and chromaticity of the module. Then, self-developed data acquisition and analysis software was used to determine whether the brightness and chromaticity uniformity of the display module was qualified, and statistics and records were performed, thus realizing the online automatic detection and industrialization of brightness and chromaticity consistency of large quantities of LED display modules.

　　The principle of online detection of LED full-color screen modules is that the camera (including brightness and color probes) collects the luminous signals of the LED full-color display module on site, and the software analyzes the results in real time. The camera brightness and color calibration is achieved through standard brightness and colorimeter, as shown in Figure 1 below.

　　The design flow chart is as follows:

　　(1) Automatic positioning detection of LED full-color screen modules

　　The LED full-color display module automatically enters the online detection system through the assembly line, and triggers the online detection system for detection after automatic positioning. The positioning system is also responsible for powering the module and realizing the color change of the module under several conditions such as positive and negative 45 degrees and 90 degrees. After the signal is stable, the detection system is triggered to perform brightness and chromaticity distribution tests.

　　(2) LED full-color screen brightness, chromaticity distribution detection and quality classification

　　The LED full-color screen brightness and chromaticity distribution detection system is composed of a camera and a high-precision CCD. The camera can change different focal length lenses as needed. The detection system collects image signals and realizes the positioning of LED pixels through image binarization processing, image threshold segmentation of histogram and other technologies; the RGB color model in the image is used to calculate the three stimulus values, and the chromaticity parameters such as color coordinates, color purity and correlated color temperature are calculated; the RGB color model used in the image is converted into XYZ and YUV color models (Y represents brightness, U and V are two components that constitute color), which can easily and quickly calculate the brightness value of each pixel. On this basis, the full-screen brightness and chromaticity distribution uniformity is statistically analyzed and processed, and the quality of the screen is graded according to production requirements to meet the requirements of controlling the consistency of screen quality.

　　(3) LED full color screen detection system calibration

　　The response of the detection system composed of the camera and high-precision CCD does not meet the requirements of the XYZ three-stimulus color matching function in the colorimetric system, so its response signal must be calibrated to meet the colorimetric and brightness test requirements. The calibration of the LED full-color screen detection system is completed using a calibration system composed of a standard lamp, a standard spectrum colorimeter and a high-precision spectrometer. Using the standard signal generated by the calibration system, a mathematical model is constructed to establish a response function corresponding to the detection system response and the XYZ three-stimulus color matching function. The model contains response functions under a series of different test conditions such as different lenses and different apertures, which can be automatically called in actual tests to achieve one-time calibration and continuous use. Simplify the complexity of use and greatly improve production efficiency.

　　Conclusion: Through the automatic positioning control of the display screen, on-site detection and analysis of brightness uniformity, chromaticity distribution, quality judgment and grading and other key technology research and development, the brightness consistency of the display screen can be improved, standardized detection can be achieved, production efficiency and output can be improved, and the goal of large-scale industrialization of LED full-color display screens can be achieved.