Application and advantages of LED full-color display box calibration technology
LED display cabinet calibration can greatly improve the display quality of the display after splicing, and it is more efficient than on-site calibration, 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, the LED display screen has inconsistent brightness and chromaticity, which seriously affects the display quality. In order to overcome the problem of non-uniformity of brightness and chromaticity of LED display screen, point-by-point correction technology came into being and developed rapidly. It can significantly improve the uniformity of LED display screen and improve the display quality.
According to different applications, point-by-point correction technology 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 (on-site correction). On-site calibration technology can select a suitable viewing location for calibration to ensure that the LED display can achieve satisfactory display effects in the on-site application environment, but the complex and changeable on-site environment and off-site technical support are the difficulties 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 factory LED screen and reduce the cost of technical support, the cabinet calibration technology reflects its own unique value. Cabinet calibration can greatly improve the display quality of the display screen after splicing, and it is more efficient than on-site calibration, 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.
1. Introduction to LED display cabinet calibration
Cabinet calibration is a type of production line calibration, which requires LED display manufacturers to add this link to the production line. Under normal circumstances, the cabinet calibration is the last link before leaving the factory, which is mainly used to eliminate the brightness and chromaticity difference between the cabinet and the cabinet, and improve the uniformity of the LED display after splicing.
In addition to adding calibration links in the production process, manufacturers generally need to follow up on the calibration effect of the screen body when it leaves the factory. There are three commonly used methods: one is to splicing all the cabinets together to observe the display effect, but the workload of splicing is relatively large, which is inconvenient to realize; the second is to randomly select some cabinets for splicing and observe the correction effect; three It is to use the measurement data recorded by the calibration system to simulate and evaluate the calibration effect of all cabinets.
Cabinet calibration needs to be carried out in a dark room, and an area array imaging device and a colorimeter are required 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 brightness and chromaticity consistency, the darkroom is required to be completely sealed, and the temperature and humidity are constant values. During the calibration process, it must be fixed. The position of the box and the calibration instrument, the box must be placed on the base to avoid the influence of reflection on the ground.
Similar to on-site calibration, for each cabinet, the process of cabinet calibration includes data acquisition, data analysis, target value setting, calibration coefficient calculation and coefficient upload, and also requires the cooperation of the control system.
2. Key technologies and difficulties
Cabinet calibration is an effective way to improve the image quality of LED displays. Its key technical aspects are mainly reflected in the following two aspects: one is the uniformity of pixels inside the cabinet, and the other is the consistency of brightness and chromaticity between cabinets.
1. Inter-pixel uniformity inside the box
The pixel-to-pixel uniformity correction inside the cabinet is basically similar to the 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 is measured by measuring equipment. The measurement method involves the knowledge of photometry, chromaticity and digital image processing; Then, according to the corresponding calibration standard, the corresponding correction coefficient is calculated and sent to the receiving card of the corresponding cabinet; after the cabinet is lit, the display control system will adjust the current of the LED according to the correction coefficient, so that all LEDs in the cabinet are lit. 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, and solves the problem of chromaticity deviation by changing the color coordinates of RGB three colors. Figure 3 shows the color gamut comparison diagram before and after correction, and the large triangle is the color gamut of the display screen 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 color coordinates of the RGB three colors are consistent.
(2) Due to the limitations of machining accuracy, assembly accuracy and other technological reasons, there is a slight inconsistency in the spacing of the spliced light panels. After the low-pass filtering process of the human visual system, bright lines or dark lines will appear in the display. . Due to the limitations of the existing mechanical process, the small-pitch display screen generally requires the correction of bright and dark lines to significantly improve the uniformity of the cabinet.
2. Brightness and chromaticity consistency between different cabinets
There is a significant difference between cabinet calibration and on-site calibration, that is, the cabinets are not spliced during calibration, and the surrounding area is lacking as a reference during calibration. After calibration, it is necessary to ensure that the cabinets are randomly spliced and there is no difference in brightness and chromaticity. . More importantly, the human visual system, as a band-pass filter, is not sensitive to the gradual brightness difference or the detail difference of extremely small angular resolution, but it is extremely sensitive to the edge step signal with medium and low frequency components. Applied to the field of LED display, the human eye can only distinguish the brightness difference of more than 4-5% between LED pixels, but can easily identify the brightness and chromaticity difference of 1% of the cabinet. That is to say, the human eye has lower requirements on the consistency of the pixels inside the box, but higher requirements on the consistency between the boxes. Therefore, the consistency of brightness and chromaticity between cabinets is a key technology unique to cabinet calibration.
The inconsistency of brightness and chromaticity between 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, a clear boundary line will appear. This can be achieved by adjusting the color gamut and setting the appropriate target value; if necessary, it is necessary to equip the precision A higher colorimeter for auxiliary measurements.
(2) The brightness and chromaticity distribution of the cabinet presents a gradient distribution, which is caused by the gradient distribution phenomenon in the measurement data of the cabinet. Since the visual system is not sensitive to low frequency, i.e. smooth gradient brightness differences, this problem is difficult to detect in single-cabinet calibration. However, when the cabinets are spliced together, the brightness of the splicing place will undergo a large jump, forming an obvious splicing line.