The LED display cabinet calibration can greatly improve the display quality of the spliced ​​display screen, and the relative on-site calibration efficiency is higher, free from time and site constraints, and the cost is also low. Therefore, the box correction technology will become an integral part of the LED display manufacturing process and has a good application prospect.
Due to the discreteness, attenuation, and discreteness of LED components, LED displays have inconsistencies in brightness and chromaticity, which seriously affect display quality. In order to overcome the problem of LED display brightness and color non-uniformity, a point-by-point calibration technology has emerged and developed rapidly. It can significantly improve the uniformity of LED displays and improve display quality.
Depending on the application, point-by-point calibration techniques can be further divided into two types: one is box-by-box calibration (box calibration) on the production line; the other is on-site large-screen calibration (on-site calibration). The on-site calibration technology can select the appropriate viewing location for calibration to ensure that the LED display can achieve a satisfactory display effect in the field application environment. However, the complicated and changeable on-site environment and off-site technical support are the challenges that limit the on-site calibration. In particular, the cost and difficulty of field corrections for some foreign orders are relatively large.
In order to ensure the uniformity of the factory LED screen and reduce the technical support cost, the box correction technology reflects its unique value. The box correction can greatly improve the display quality of the spliced ​​display screen, and the relative site correction efficiency is higher, not limited by time and space, and the cost is also low. Therefore, the box correction technology will become an integral part of the LED display manufacturing process and has a good application prospect.
A, LED display cabinet calibration
Box calibration is a type of production line correction and requires LED display manufacturers to increase this link in the production line. Under normal circumstances, the box correction is arranged at the final stage before delivery, mainly to eliminate the difference in brightness and chromaticity between the inside of the box and the box, and to improve the uniformity of the LED display after splicing.
In addition to the addition of the corrective link in the production process, manufacturers generally need to follow the screen body factory correction effect. Commonly used methods are the following three: First, all the boxes are spliced ​​together to observe the display effect, but the splicing workload is relatively large, it is inconvenient to achieve; Second, some boxes are randomly selected for splicing, observe the correction effect; It is the use of calibration data recorded by the calibration system to perform a simulation evaluation of the correction effect of all cabinets.
The box calibration needs to be performed in a darkroom. It needs to be equipped with an area array imaging device and a colorimeter to measure the luminance and chromaticity information of each box. In order to ensure that the calibration process of all cabinets is performed under the influence of external environmental conditions and achieves the goal of uniform brightness and chromaticity, the darkroom is required to be completely sealed, and the temperature and humidity are constant values. 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 reflections from the ground.
Similar to on-site calibration, for each cabinet, the process of cabinet calibration includes data acquisition, data analysis, target value setting, calculation of correction coefficients, and coefficient upload. It also requires the cooperation of the control system.
Second, key technologies and difficulties
Case correction is an effective way to improve the image quality of LED display screens. The key technical aspects are mainly reflected in the following two aspects: one is the uniformity between the pixels inside the box, and the second is the consistency of the bright color between the boxes.
1.Inter-pixel homogeneity inside the box The homogeneity correction between the pixels inside the box and the on-site calibration are basically similar and relatively mature, including the correction of bright color uniformity and the correction of bright and dark lines:
(1) Brightness uniformity correction The measurement of the brightness and chromaticity information of each LED lamp in the LED cabinet is performed by a measuring device. The measurement method involves photometry, colorimetry, and digital image processing related knowledge; 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 cabinet; after the cabinet is lighted, the display screen control system adjusts the LED current according to the correction coefficient so that all the LEDs in the cabinet are The brightness and color are consistent.
Brightness correction is to adjust the luminance of the fluctuating LED to a consistent level, and it is necessary to properly reduce the maximum luminance value of most of the LEDs during the adjustment of the luminance. Chroma correction is based on the principle of RGB color matching, by changing the color coordinates of RGB three colors to solve the problem of color deviation, Figure 3 shows the comparison before and after the calibration of the color gamut, the large triangle is the gamut of the screen before the correction , RGB trichromatic color coordinates discrete distribution; small triangles for the corrected display color gamut, RGB three-color color coordinates better consistency.
(2) Due to the limitation of mechanical processing accuracy, assembling precision and other technological reasons, there is a slight inconsistency in the spacing of the splicing lamp panels. After passing through the low-pass filtering process of the human visual system, bright or dark lines appear on the display. . Due to the limitation of the existing mechanical process, the small pitch display generally requires bright and dark line corrections to significantly improve the uniformity of the box.
2. Consistency of bright color between different cabinets There is a significant difference between cabinet calibration and on-site calibration. When the cabinet is calibrated, it is not spliced. When it is calibrated, it lacks the surrounding area as a reference, and after calibration It is necessary to ensure that the cabinet is arbitrarily spliced ​​and that there is no difference in lightness. More importantly, the human-eye vision system is a band-pass filter that is not sensitive to slight differences in the brightness of flattened gradients or to the smallest details of angular resolution, but is extremely sensitive to edge step signals with low and medium frequency components. Applied to the LED display field, it is reflected that the human eye can only distinguish between 4-5% brightness difference between LED pixels, but can easily recognize the 1% difference in light color of the box. In other words, the human eye has lower requirements for the consistency of pixels inside the box, and the consistency between the boxes is higher. Therefore, the consistency of the bright color between the cabinets is a key technology unique to the cabinet calibration.
The inconsistency of the brightness between the boxes is mainly reflected in two aspects:
(1) There is a difference in average light chroma between cabinets. When splicing 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 is necessary to provide accuracy. A higher colorimeter performs auxiliary measurements.
(2) The distribution of bright chromaticity of the box presents a graded gradient distribution, which is due to the phenomenon of gradient distribution in the measurement data of the box. Since the vision system is not sensitive to the difference in brightness at low frequencies, ie, smooth gradation, this problem is difficult to find in a single box correction. However, when the cabinets are spliced ​​together, the brightness of the splicing place will undergo a large jump, forming a distinct splicing line. This requires the calibration system to be able to detect and resolve the gradient distribution of the measurement data.
Due to the discreteness, attenuation, and discreteness of LED components, LED displays have inconsistencies in brightness and chromaticity, which seriously affect display quality. In order to overcome the problem of LED display brightness and color non-uniformity, a point-by-point calibration technology has emerged and developed rapidly. It can significantly improve the uniformity of LED displays and improve display quality.
Depending on the application, point-by-point calibration techniques can be further divided into two types: one is box-by-box calibration (box calibration) on the production line; the other is on-site large-screen calibration (on-site calibration). The on-site calibration technology can select the appropriate viewing location for calibration to ensure that the LED display can achieve a satisfactory display effect in the field application environment. However, the complicated and changeable on-site environment and off-site technical support are the challenges that limit the on-site calibration. In particular, the cost and difficulty of field corrections for some foreign orders are relatively large.
In order to ensure the uniformity of the factory LED screen and reduce the technical support cost, the box correction technology reflects its unique value. The box correction can greatly improve the display quality of the spliced ​​display screen, and the relative site correction efficiency is higher, not limited by time and space, and the cost is also low. Therefore, the box correction technology will become an integral part of the LED display manufacturing process and has a good application prospect.
A, LED display cabinet calibration
Box calibration is a type of production line correction and requires LED display manufacturers to increase this link in the production line. Under normal circumstances, the box correction is arranged at the final stage before delivery, mainly to eliminate the difference in brightness and chromaticity between the inside of the box and the box, and to improve the uniformity of the LED display after splicing.
In addition to the addition of the corrective link in the production process, manufacturers generally need to follow the screen body factory correction effect. Commonly used methods are the following three: First, all the boxes are spliced ​​together to observe the display effect, but the splicing workload is relatively large, it is inconvenient to achieve; Second, some boxes are randomly selected for splicing, observe the correction effect; It is the use of calibration data recorded by the calibration system to perform a simulation evaluation of the correction effect of all cabinets.
The box calibration needs to be performed in a darkroom. It needs to be equipped with an area array imaging device and a colorimeter to measure the luminance and chromaticity information of each box. In order to ensure that the calibration process of all cabinets is performed under the influence of external environmental conditions and achieves the goal of uniform brightness and chromaticity, the darkroom is required to be completely sealed, and the temperature and humidity are constant values. 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 reflections from the ground.
Similar to on-site calibration, for each cabinet, the process of cabinet calibration includes data acquisition, data analysis, target value setting, calculation of correction coefficients, and coefficient upload. It also requires the cooperation of the control system.
Second, key technologies and difficulties
Case correction is an effective way to improve the image quality of LED display screens. The key technical aspects are mainly reflected in the following two aspects: one is the uniformity between the pixels inside the box, and the second is the consistency of the bright color between the boxes.
1.Inter-pixel homogeneity inside the box The homogeneity correction between the pixels inside the box and the on-site calibration are basically similar and relatively mature, including the correction of bright color uniformity and the correction of bright and dark lines:
(1) Brightness uniformity correction The measurement of the brightness and chromaticity information of each LED lamp in the LED cabinet is performed by a measuring device. The measurement method involves photometry, colorimetry, and digital image processing related knowledge; 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 cabinet; after the cabinet is lighted, the display screen control system adjusts the LED current according to the correction coefficient so that all the LEDs in the cabinet are The brightness and color are consistent.
Brightness correction is to adjust the luminance of the fluctuating LED to a consistent level, and it is necessary to properly reduce the maximum luminance value of most of the LEDs during the adjustment of the luminance. Chroma correction is based on the principle of RGB color matching, by changing the color coordinates of RGB three colors to solve the problem of color deviation, Figure 3 shows the comparison before and after the calibration of the color gamut, the large triangle is the gamut of the screen before the correction , RGB trichromatic color coordinates discrete distribution; small triangles for the corrected display color gamut, RGB three-color color coordinates better consistency.
(2) Due to the limitation of mechanical processing accuracy, assembling precision and other technological reasons, there is a slight inconsistency in the spacing of the splicing lamp panels. After passing through the low-pass filtering process of the human visual system, bright or dark lines appear on the display. . Due to the limitation of the existing mechanical process, the small pitch display generally requires bright and dark line corrections to significantly improve the uniformity of the box.
2. Consistency of bright color between different cabinets There is a significant difference between cabinet calibration and on-site calibration. When the cabinet is calibrated, it is not spliced. When it is calibrated, it lacks the surrounding area as a reference, and after calibration It is necessary to ensure that the cabinet is arbitrarily spliced ​​and that there is no difference in lightness. More importantly, the human-eye vision system is a band-pass filter that is not sensitive to slight differences in the brightness of flattened gradients or to the smallest details of angular resolution, but is extremely sensitive to edge step signals with low and medium frequency components. Applied to the LED display field, it is reflected that the human eye can only distinguish between 4-5% brightness difference between LED pixels, but can easily recognize the 1% difference in light color of the box. In other words, the human eye has lower requirements for the consistency of pixels inside the box, and the consistency between the boxes is higher. Therefore, the consistency of the bright color between the cabinets is a key technology unique to the cabinet calibration.
The inconsistency of the brightness between the boxes is mainly reflected in two aspects:
(1) There is a difference in average light chroma between cabinets. When splicing 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 is necessary to provide accuracy. A higher colorimeter performs auxiliary measurements.
(2) The distribution of bright chromaticity of the box presents a graded gradient distribution, which is due to the phenomenon of gradient distribution in the measurement data of the box. Since the vision system is not sensitive to the difference in brightness at low frequencies, ie, smooth gradation, this problem is difficult to find in a single box correction. However, when the cabinets are spliced ​​together, the brightness of the splicing place will undergo a large jump, forming a distinct splicing line. This requires the calibration system to be able to detect and resolve the gradient distribution of the measurement data.
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