LED display box correction technology

Due to the discreteness, attenuation and discreteness of the circuit components of the LED light-emitting tube, the LED display screen has inconsistent brightness and chromaticity, which seriously affects the display quality. In order to overcome the problem of brightness and chromaticity non-uniformity of LED display screen, point-by-point correction technology emerges as the times require, and it develops rapidly. It can significantly improve the uniformity of LED display and improve display quality.

Depending on the application, point-by-point calibration techniques can be divided into two types: one is box-by-box correction on the production line (box correction); the other is on-site large-screen correction (field correction). The on-site calibration technology can select the appropriate viewing location for calibration to ensure that the LED display achieves satisfactory display performance in the field application environment, but the complex and varied on-site environment and off-site technical support are the problems that limit the field calibration. In particular, the cost and difficulty of on-site calibration of some foreign orders are relatively large. In order to ensure the uniformity of the factory LED screen body and reduce the technical support cost, the cabinet correction technology reflects its unique value. The cabinet calibration can greatly improve the display quality of the spliced ​​display, and the relative field correction efficiency is higher, not limited by time and space, and the cost is also low.

Therefore, with the development of LED display manufacturers, the cabinet calibration technology will become an indispensable part of the LED display manufacturing process, and has a good application prospect.

Second, the introduction of cabinet correction

Box calibration is a type of line correction that requires LED display manufacturers to add this link to the production line. Under normal circumstances, the cabinet calibration is arranged in the last link before leaving the factory, mainly to eliminate the difference in brightness and chromaticity between the interior of the cabinet and the cabinet, and improve the uniformity of the LED display after splicing.

In addition to adding corrections in the production process, manufacturers generally need to follow the correction effect of the factory. There are three commonly used methods: one is to splicing all the boxes together to observe the display effect, but the splicing workload is relatively large, which is inconvenient to implement; the second is to randomly extract some boxes for splicing and observe the correction effect; It is the simulation evaluation of the correction effect of all the cabinets by using the measurement data recorded by the calibration system. A schematic diagram of the LED production pipeline with increased cabinet calibration and simulation evaluation/sampling is shown in Figure 1.

Figure 1 LED production line to increase the cabinet calibration

The cabinet calibration needs to be performed in the darkroom. It needs to be equipped with an area array imaging device and a colorimeter to measure the brightness and chromaticity information of each cabinet. In order to ensure that the calibration process of all cabinets is carried out under the influence of external environmental conditions, the goal of brightness and chromaticity consistency is required, and the darkroom is completely sealed, and the temperature and humidity are constant. In the calibration process, it must be fixed. The position of the cabinet and the calibration instrument must be placed on the base to avoid the influence of ground reflection.

Similar to on-site calibration, for each cabinet, the process of cabinet calibration includes data acquisition, data analysis, target value setting, correction factor calculation, and coefficient uploading, as well as control system coordination.

Third, key technologies and difficulties

The cabinet calibration is an effective way to improve the image quality of the LED display. The key technical aspects are mainly reflected in the following two aspects: one is the uniformity between the pixels inside the cabinet, and the other is the consistency of the bright color between the cabinets.

1. Pixel uniformity inside the cabinet

The inter-pixel uniformity correction and field correction inside the cabinet are basically similar, and are mature, including bright chromaticity uniformity correction and bright dark line correction:

(1) Brightness uniformity correction The brightness and chromaticity information of each LED tube in the LED box are measured by the measuring device, and the measuring methods involve knowledge of photometry, colorimetry and digital image processing; After the degree information is calculated according to the corresponding calibration standard, the corresponding correction coefficient is calculated and sent to the receiving card of the corresponding box; after the box is lit, the display control system will adjust the current of the LED according to the correction coefficient, so that all the LEDs in the box The brightness and chromaticity are consistent.

Brightness correction is to adjust the brightness of the undulating LED to a consistent level. In the process of adjusting the brightness, it is necessary to appropriately reduce the maximum brightness value of most LEDs. Chroma correction is based on the RGB color matching principle, which solves the problem of chromaticity deviation by changing the color coordinates of RGB three colors. Figure 3 shows the gamut contrast chart before and after correction. The large triangle is the gamut of the display before correction. The color coordinates of RGB three colors are discretely distributed; the small triangle is the corrected display color gamut, and the RGB three-color color coordinates are consistent.

Figure 3 color gamut comparison chart before and after correction

(2) Due to the limitation of machining precision and assembly precision, there is a slight inconsistency in the spacing of the splicing light panels. After the low-pass filtering process of the human visual system, bright or dark lines appear during display. . Due to the limitations of existing mechanical processes, the small-pitch display generally requires bright and dark line correction to significantly improve the uniformity of the cabinet.

2. Bright color consistency between different cabinets

There is a significant difference between the cabinet calibration and the on-site calibration, that is, the cabinet is unspliced ​​during calibration, and the surrounding area is lacking as a reference during calibration. After the calibration, it is necessary to ensure that the cabinet is spliced ​​arbitrarily and there is no difference in brightness and chromaticity. . More importantly, the human visual system, as a bandpass filter, is not sensitive to light gradient differences or minimal angular resolution differences, but is extremely sensitive to edge step signals with medium and low frequency components. Applied to the field of LED display, it can be seen that the human eye can only distinguish the brightness difference of more than 4-5% between LED pixels, but can easily identify the difference in brightness of 1% of the cabinet. That is to say, the human eye has lower requirements on the consistency of the pixels inside the box, and the consistency between the boxes is higher. Therefore, the consistency of bright chromaticity between the cabinets is a key technology unique to the cabinet calibration.

The inconsistency of the bright color between the cabinets is mainly reflected in two aspects:

(1) There is a difference in the average bright chromaticity between the boxes. When the box is spliced, obvious boundary lines appear. This can be achieved by adjusting the gamut and setting the appropriate target value; if necessary, it needs to be equipped with precision. A higher colorimeter is used for auxiliary measurements.

(2) The bright chromaticity distribution of the box appears as a gradient gradation distribution, which is caused by the gradient distribution phenomenon of the box measurement data. Since the vision system is not sensitive to the difference in brightness between low frequencies, ie smooth gradations, this problem is difficult to detect in single box calibration. However, when the boxes are spliced ​​together, the brightness of the splicing will be greatly changed, forming a clear splicing line. This requires the calibration system to detect and resolve the gradient distribution of the measured data.

Fourth, new challenges

With the rapid development of the LED display industry, the application of display screens is more and more extensive. At the same time, people's display quality requirements are getting higher and higher, and the cabinet correction will face some new severe challenges:

(1) The development of small-pitch display screens is fast, and the cabinet will also develop toward higher luminous density, which requires the cabinet calibration to support the correction of large-resolution cabinets. Under the existing process conditions, it is more difficult to increase the CCD or CMOS resolution of the camera, and the cost is too expensive. This requires a new method to ensure the accuracy and measurement efficiency of the LED bright color measurement data.

(2) The brightness of the in-line LED tube is consistent at all angles. The light intensity of a certain type of LED light-emitting diode in the horizontal direction and the vertical direction is distributed, and the light intensity in the normal direction reaches a maximum value. The larger the angle in the direction away from the normal direction, the more severe the light intensity is attenuated. If there is a difference in the light distribution curve of the lamp, the lamp, or the LED light distribution curve, the display screen that has been corrected by the cabinet on the production line may have a poor effect at the scene due to the different viewing angles. Existing box correction techniques generally support a certain degree of elevation correction, but the main limitation is the level of the straight-line lamp.

(3) In actual production, there are cases where multiple batches of boxes are spliced ​​to form an LED display screen, which requires that the cabinet calibration supports multi-batch cabinet calibration, which is extremely accurate for the bright color measurement of existing calibration equipment. High requirements. What is more serious is that the existing measurement equipment on the market, including the high-color colorimeter, provides limited brightness measurement data, and the human eye is extremely sensitive to the bright color accuracy between the boxes. How to better eliminate the difference of bright color between different batches is a big problem in the industry.

All in all, the cabinet calibration technology can be used as a core tool to improve the display quality of the LED industry, but it is not a panacea. For LED display manufacturers who are committed to improving the quality of display images, on the one hand, it is necessary to improve the production process of the display screen, on the other hand, it is necessary to use the box correction technology to improve the uniformity of the display.