LED display cabinet calibration can greatly improve the display quality of the spliced display, and it is more efficient than on-site calibration, not limited by time and place, 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 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 application occasions, the point-by-point calibration technology can be divided into two types: one is box-by-box calibration on the production line (cabinet calibration); the other is 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 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 problems that limit on-site calibration. In particular, the cost and difficulty of on-site calibration for 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 spliced display, and it is more efficient than on-site calibration, 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, which requires LED display manufacturers to add this link in the production line. Under normal circumstances, the cabinet calibration is the last link before leaving the factory. It is mainly used to eliminate the brightness and chromaticity differences inside and between the cabinets, 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 results of the screens when they leave the factory. There are three commonly used methods: one is to splice all the cabinets together and observe the display effect, but the workload of splicing is relatively large and it is inconvenient to implement; the other is to randomly select some cabinets for splicing and observe the correction effect; three It uses the measurement data recorded by the calibration system to simulate and evaluate the calibration effects of all cabinets. Added LED production lines for cabinet calibration and simulation evaluation/sampling inspection.

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 consistent brightness and chromaticity, 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 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, 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 display screens. 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. Uniformity between pixels inside the cabinet

The inter-pixel uniformity correction inside the cabinet is basically similar to the on-site correction, and is relatively mature, including brightness and chroma uniformity correction and bright and dark line correction:

(1) Calibration of brightness and chromaticity uniformity The brightness and chromaticity information of each LED lamp in the LED box is measured by measuring equipment, and the measurement method involves knowledge of photometry, chromaticity and digital image processing; Then according to the corresponding calibration standard, calculate the corresponding correction coefficient and send it 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. the

Brightness correction is to adjust the brightness of fluctuating LEDs to a consistent level. In the process of adjusting brightness, it is necessary to properly reduce the maximum brightness value of most 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. The color coordinates are discretely distributed; the small triangle is the corrected display color gamut, and the RGB three-color color coordinates have better consistency.

(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 boards. After the low-pass filtering process of the human visual system, bright or dark lines will appear in the display . Due to the limitations of the existing mechanical process, small-pitch display screens generally require correction of bright and dark lines to significantly improve the uniformity of the cabinet.

2. Consistency of brightness and chromaticity among different cabinets

There is a significant difference between cabinet calibration and on-site calibration, that is, the cabinets are not spliced during calibration, and there is no surrounding area as a reference during calibration, but after calibration, it is necessary to ensure that the cabinets are spliced arbitrarily and there is no difference in brightness and chromaticity . More importantly, the human visual system, as a band-pass filter, is not sensitive to gently gradual brightness differences or small angular resolution detail differences, but is extremely sensitive to edge step signals with mid- and low-frequency components. Applied to the field of outdoor led screen display display, it is reflected that the human eye can only distinguish more than 4-5% of the brightness difference between 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 lower requirements for the consistency of the pixels inside the cabinet, but higher requirements for the consistency between the cabinets. 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 are differences in the average brightness and chromaticity between the cabinets. When splicing the cabinets, there will be obvious boundary lines. This can be achieved by adjusting the color gamut and setting a suitable target value; if necessary, it needs to be equipped with precision A higher colorimeter is used 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, smooth gradient brightness differences, this problem is difficult to detect when correcting for a single box. But when the cabinets are spliced together, the brightness at the splicing point will jump greatly, forming an obvious splicing line. This requires a calibration system capable of detecting and resolving gradient distributions in the measured data.

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