Briefly talk about various chromaticity processing technologies of LED display screens-EEWORLD

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In spring, everything comes back to life. Under the blue sky, the grass is green. In autumn, the wheat waves are rolling. Under the sunshine, it is golden. The colorful nature is so beautiful. Unfortunately, the existing LED display screen cannot fully reproduce this beautiful scenery. Although LED belongs to monochromatic light, each color LED still has a half-wave width of about 30~50nm, so its color saturation is limited.

　　1. 3+2 multi-primary color processing method:

　　In recent years, the field of flat panel display has been keen on discussing 3+3 multi-primary color display (red, green, blue plus yellow, cyan, and purple) to expand the color gamut and reproduce richer natural colors. So, can LED display screens achieve 3+3 multi-primary color display?

　　We know that in the visible light range, yellow and cyan are monochromatic light, and we already have highly saturated yellow and cyan LEDs. Purple is a complex light, and single-chip purple LEDs do not exist. Although we cannot achieve a 3+3 multi-primary color LED display screen of red, green, blue plus yellow, cyan, and purple. However, it is feasible to study the 3+2 multi-primary LED display screen of red, green, blue plus yellow and cyan. Since there are a large number of highly saturated yellow and cyan in nature, this research is of certain value.

　　In the current various television standards, the video source only has three primary colors of red, green and blue, but no yellow and cyan. So, how to drive the yellow and cyan primary colors of the display terminal?

　　In fact, when determining the driving intensity of the yellow and cyan primary colors, we should follow the following three principles:

　　1. While improving the color saturation, the hue must not be changed;

　　2. The purpose of adding the yellow and cyan primary colors is to expand the color gamut, thereby improving the color saturation. The overall brightness value cannot be changed;

　　3. With D65 as the center and the RYGCB color gamut boundary as the endpoint, linear expansion is made at each point within the color gamut.

　　Under the guidance of the above three principles, according to the law of the center of gravity, we can find a 3+2 multi-primary color processing method. However, in order to truly realize the 3+2 multi-primary full-color screen, we must also overcome the difficulties of insufficient brightness of yellow and cyan LEDs and large cost increases, which are currently limited to theoretical discussions.

　　2. Color restoration processing:

　　The birth of pure blue and pure green LEDs has made full-color LED displays popular in the industry for their wide color gamut and high brightness. However, due to the large deviation between the chromaticity coordinates of red, green and blue LEDs and the chromaticity coordinates of red, green and blue PAL TVs (see Table 1), the color restoration of LED full-color screens is poor. Especially when expressing human skin color, there is a more obvious visual deviation. As a result, color restoration processing technology came into being. Here, the author recommends two methods of color restoration processing:

　　1. Transform the color coordinate space of the red, green and blue primary color LEDs to make the three primary color coordinates between LED and PAL TV as close as possible, thereby greatly improving the color restoration of the LED display. However, this method greatly reduces the color gamut of the LED display and greatly reduces the color saturation of the picture.

　　2. Only the skin color gamut that the human eye is most sensitive to is properly corrected; and the original color saturation of other color gamuts that the human eye is not sensitive to is reduced as little as possible. In this way, a balance can be achieved between color restoration and color saturation.

　　3. Selection of base color wavelength:

　　As people's requirements for LED display screens become higher and higher, only subdividing and screening LED color coordinates can no longer meet people's discerning eyes. It is feasible to perform comprehensive correction on the display screen to improve the chromaticity uniformity.

　　We found that even the same-grade LED of the first-class international brand has large wavelength deviation and color saturation deviation, and the deviation range greatly exceeds the threshold of human eye's identification of green color difference. Therefore, it is of great significance to correct the chromaticity uniformity.

　　In the CIE1931 chromaticity diagram, according to the law of gravity center, we found that: in the G range (□abcd), any point of green mixed with a certain proportion of red and blue can adjust the color coordinates of the mixed color to the intersection of the straight line cR and the straight line dB. O.

　　Although it can greatly improve the chromaticity uniformity. However, the color saturation after correction is significantly reduced. At the same time, another prerequisite for using red and blue to correct the green chromaticity uniformity is that the three LEDs of red, green and blue in the same pixel are distributed as centrally as possible to make the mixing distance of red, green and blue as close as possible, so as to achieve better results. At present, the industry usually adopts the LED uniform distribution method, which will bring confusion to the chromaticity uniformity correction. In addition, how to carry out the measurement of tens of thousands of red, green and blue LED color coordinates is also an extremely difficult problem. We have given tips for this.

4. Chromaticity uniformity processing:

　　The chromaticity uniformity problem of LED display screens has always been a major problem that troubles industry insiders. It is generally believed that the uneven brightness of LEDs can be corrected at a single point to improve the brightness uniformity. However, chromaticity unevenness cannot be corrected and can only be improved by subdividing and screening the LED color coordinates. 　5. Allocation of white field color coordinates: 　　Allocation of white field color coordinates is one of the most basic technologies for full-color LED display screens. However, in the mid-1990s, due to the lack of industry standards and basic testing methods, the white field color coordinates were usually determined by human eyes and feelings, resulting in serious color cast and randomness of white field color temperature. With the promulgation of industry standards and the completion of testing methods, many manufacturers began to standardize the full-color screen color matching process. However, some manufacturers still lack the theoretical guidance of color matching, and often sacrifice the grayscale level of some primary colors to adjust the color coordinates of the hundred fields, and the comprehensive performance cannot be improved. In summary, we mainly discussed three aspects of the problem: 　　1. How to improve the color uniformity of the LED display; 　　2. How to expand the color gamut and restore more natural colors; 　　3. How to improve the color reproduction of the LED display; 　　The above-mentioned color processing technologies are interrelated in specific implementation, and some aspects are even not available. The comprehensive LED display must also perform brightness uniformity correction, grayscale nonlinear transformation, noise reduction processing, image enhancement processing, dynamic pixel processing, etc. The entire signal processing process is very complicated. Therefore, we must comprehensively weigh the various performances from a system perspective, grasp the order of various processing, and increase the depth of signal processing, so that the LED full-color display can show a colorful and colorful world. 　　LED has a very wide range of applications in various industries, and different application sites have different requirements for the primary color wavelength of LED. Some of the selections of LED primary color wavelengths are to achieve good visual effects, some are to conform to people's habits, and some are stipulated by industry standards, national standards, and even international standards. For example 　　, the selection of the green tube primary color wavelength in the full-color LED display; in the early days, people generally chose a yellow-green LED with a wavelength of 570nm. Although the cost was low, the color gamut of the display was small, the color reproduction was poor, and the brightness was low. After selecting a pure green tube with a wavelength of 525nm, the color gamut of the display was nearly doubled, and the color reproduction was greatly improved, which greatly improved the visual effect of the display. For another example, in the stock market display, people are usually accustomed to using red to indicate rising stock prices, green to indicate falling stock prices, and yellow to indicate flat trading. In the transportation industry, the national standard strictly stipulates that the blue-green band indicates passage and the red band is prohibited. Therefore, the selection of the primary color wavelength is one of the important links of the LED display.

　

　　

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