The LED display cabinet calibration can greatly improve the display quality of the spliced display screen, and it is more efficient than on-site calibration, and is not limited by time and venue, and the cost is also low. Therefore, the cabinet calibration technology will become an indispensable part of the LED display manufacturing process and has a good application prospect.
Due to the discreteness and attenuation of LED light-emitting tubes and the discreteness of circuit components, LED display screens have inconsistencies in brightness and chromaticity, which seriously affects the display quality. In order to overcome the problem of LED display brightness and chromaticity non-uniformity, point-by-point correction technology came into being and developed rapidly. It can significantly improve the uniformity of the LED display and improve the display quality.
According to different applications, the point-by-point calibration technology can be divided into two types: one is the box-by-box calibration on the production line (cabinet calibration); the other is the on-site large-screen calibration (on-site calibration). On-site calibration technology can select a suitable viewing location for calibration to ensure that the LED display achieves a satisfactory display effect in the on-site application environment. However, the complex and changeable on-site environment and remote technical support are difficult problems that limit the on-site calibration. In particular, the cost and difficulty of on-site correction of some foreign orders are relatively high.
In order to ensure the uniformity of the ex-factory LED screen and reduce the cost of technical support, the cabinet calibration technology has its own special value. The cabinet calibration can greatly improve the display quality of the display screen after splicing, and it is more efficient than on-site calibration, and is not limited by time and space, and the cost is also low. Therefore, the cabinet calibration technology will become an indispensable part of the LED display manufacturing process and has a good application prospect.
1. Introduction to LED display cabinet calibration
Cabinet calibration is a kind of production line calibration, and LED display manufacturers are required to add this link in the production line. Under normal circumstances, the cabinet calibration is the last link arranged before leaving the factory. It is mainly used to eliminate the brightness and chromaticity differences between the inside of the cabinet and the cabinet, and to improve the uniformity of the LED display after splicing.
In addition to increasing the calibration process in the production process, manufacturers generally also need to follow up on the calibration effect of the screen when it leaves the factory. There are three commonly used methods: one is to splice all the cabinets together to observe the display effect, but the work load of splicing is relatively large and it is inconvenient to implement; the second is to randomly select part of the cabinets for splicing to observe the correction effect; It uses the measurement data recorded by the calibration system to simulate and evaluate the calibration effects of all cabinets. The schematic diagram of the LED production line with the addition of cabinet calibration and simulation evaluation/sampling inspection is shown in Figure 1.
The cabinet calibration needs to be carried out in a dark room, and 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 not affected by external environmental conditions and achieve the goal of consistency of brightness and chromaticity, the dark room is required to be completely sealed, and the temperature and humidity are constant values. During the calibration process, it must be fixed. For the position of the cabinet and the calibration instrument, the cabinet 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 collection, data analysis, target value setting, correction coefficient calculation and coefficient upload. It also requires the cooperation of the control system.
2. Key technologies and difficulties
Cabinet correction is an effective way to improve the image quality of the LED display. Its key technical aspects are mainly reflected in the following two aspects: one is the uniformity between pixels inside the cabinet, and the other is the consistency of brightness and chromaticity between the cabinets.
1. Uniformity between pixels inside the cabinet
The uniformity correction between pixels inside the cabinet is basically similar to on-site correction, and is relatively mature, including brightness and chromaticity uniformity correction and bright and dark line correction:
(1) Brightness and chromaticity uniformity correction The brightness and chromaticity information of each LED tube in the LED box are measured by measuring equipment. The measurement method involves knowledge of photometry, chromaticity and digital image processing; According to the corresponding calibration standard, calculate the corresponding correction coefficient and send it to the receiving card of the corresponding cabinet. After the cabinet is lit, the display control system will adjust the LED current according to the correction coefficient to make all the LEDs in the cabinet. The brightness and chromaticity are consistent.
Brightness correction is to adjust the brightness of the fluctuating LEDs to a consistent level. In the process of adjusting the brightness, it is necessary to appropriately reduce the maximum brightness value of most of the LEDs. Chromaticity correction is based on the principle of RGB color matching. The problem of chromaticity deviation is solved by changing the color coordinates of the RGB three colors. Figure 3 shows the color gamut comparison before and after correction. The big triangle is the color gamut of the display before correction. , The color coordinates of the RGB three colors are discretely distributed; the small triangle is the corrected color gamut of the display screen, and the RGB color coordinates are consistent.
(2) Due to the limitation of machining accuracy, assembly accuracy and other technological reasons, there is a slight inconsistency in the spacing of the splicing lamp panels. After the low-pass filtering process of the human visual system, bright or dark lines will appear during display. . Due to the limitations of the existing mechanical technology, the small-pitch display screen generally requires bright and dark line corrections to significantly improve the uniformity of the cabinet.
2. The consistency of brightness and color between different cabinets
There is a significant difference between the cabinet calibration and on-site calibration, that is, the cabinet is not spliced during the correction, and the surrounding area is not used as a reference during the correction. After the correction, it is necessary to ensure that the cabinet is randomly spliced and there is no difference in brightness and chromaticity. . More importantly, as a band-pass filter, the human visual system is not sensitive to gradual brightness differences or small angular resolution details, but it is extremely sensitive to edge step signals with mid- and low-frequency components. Applied to the field of LED display screens, the human eye can only distinguish the brightness difference of more than 4-5% between the LED pixels, but can easily recognize the 1% difference in the brightness and chromaticity of the cabinet. That is to say, the human eye has low requirements for the consistency of the pixels inside the cabinets, but higher requirements for the consistency between the cabinets. Therefore, the consistency of brightness and chromaticity between cabinets is a unique key technology for cabinet calibration.
The inconsistency of brightness and chromaticity between the cabinets is mainly reflected in two aspects:
(1) There is a difference in the average brightness and chromaticity between the cabinets. When the cabinets are spliced, there will be obvious boundary lines. This can be achieved by adjusting the color gamut and setting appropriate target values; when necessary, precision is required A higher colorimeter is used to perform auxiliary measurements.
(2) The brightness and chromaticity distribution of the cabinet presents a gradient distribution, which is caused by the phenomenon of gradient distribution in the measurement data of the cabinet. Since the vision system is not sensitive to low-frequency, that is, smoothly gradual brightness differences, this problem is difficult to find during single-box calibration. But when the cabinets are spliced together, the brightness of the splicing place will undergo a large jump, forming an obvious splicing line. This requires the calibration system to be able to detect and solve the problem of gradient distribution of the measured data.
