Various color processing techniques for LED high-definition LCD screens

　　Although LED high-definition display screens are monochromatic, each color LED still has a half-wave width of about 30~50nm, so its color saturation is limited. The colorful nature is so beautiful, but unfortunately the existing LED display screens cannot fully reproduce this beautiful scenery.

　　1. Adjustment of white field color coordinates:

　　White field color coordinate matching 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, 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 color matching process of full-color screens. However, some manufacturers still lack theoretical guidance on color matching, and often sacrifice the grayscale levels of certain primary colors to match the hundred-field color coordinates, and the overall performance cannot be improved.

　　In summary, we have mainly discussed three aspects:

　　1. How to improve the color uniformity of LED high-definition display screen;

　　2. How to expand the color gamut and restore more natural colors;

　　3. How to improve the color reproduction of LED display screen;

　　It has a very wide range of applications in various industries, and different application scenarios have different requirements for the primary color wavelength of LED. Some of the choices of LED primary color wavelengths are to achieve good visual effects, some are to conform to people's habits, and some are even stipulated by industry standards, national standards and even international standards.

　　The above-mentioned chromaticity processing technologies are interrelated in their specific implementation, and some aspects are even mutually exclusive. The integrated LED display screen also needs to 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 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 screen can show a colorful and colorful wonderful world.

　　For example, the choice of wavelength of the green tube primary color in the full-color LED display; in the early days, people generally chose yellow-green LEDs 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 choosing 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.

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

　　In recent years, the flat panel display field has been keen on discussing 3+3 multi-primary color display (red, green, blue plus yellow, cyan, 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 lights, and we already have high-saturation yellow and cyan LEDs. Purple is a complex light, and single-chip purple LEDs do not exist. Although we cannot realize a 3+3 multi-primary color LED display screen of red, green, blue plus yellow, cyan, and purple. However, it is feasible to study a 3+2 multi-primary color LED display screen of red, green, blue plus yellow and cyan. Since there are a lot of highly saturated yellow and cyan in nature; therefore, this research is of certain value.

　　In the current TV standards, the video source only has three primary colors: red, green and blue, but not yellow and cyan. So how do the display terminals drive the two primary colors of yellow and cyan?

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

　　1. The purpose of adding the two primary colors of yellow and cyan is to expand the color gamut and thus improve the color saturation. The overall brightness value cannot be changed;

　　2. When increasing the color saturation, the color tone must not be changed;

　　3. Take D65 as the center and the RYGCB color gamut boundary as the endpoint, and make linear expansion at each point within the color gamut.

　　Under the guidance of the above three principles: According to the law of gravity center, we can find the 3+2 multi-primary color processing method. However, to truly realize the 3+2 multi-primary full-color screen, we still have to overcome the difficulties such as insufficient brightness of yellow and cyan LEDs and large cost increase, which is currently limited to theoretical discussion.

　　3. Chromaticity uniformity processing:

　　The color uniformity of full-color LED display screens has always been a major problem that has troubled 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, the uneven color cannot be corrected and can only be improved by subdividing and screening the LED color coordinates.

　　4. 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 those of PAL televisions (see Table 1), the color reproduction of full-color LED screens is poor. Especially when expressing human skin color, there is a more obvious visual deviation. As a result, color reproduction processing technology came into being. Here, the author recommends two color reproduction processing methods:

　　1. Only the skin color gamut that the human eye is most sensitive to is properly corrected, while the original color saturation of other color gamuts that the human eye is not sensitive to is reduced as little as possible. This process can achieve a balance between color reproduction and color saturation.

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

　　5. Choice of primary color wavelength:

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

　　We found that even the same grade of LED from the world's top brand has large wavelength deviation and color saturation deviation, and the deviation range far exceeds the threshold of the human eye to distinguish green color difference. Therefore, it is of great significance to perform color uniformity correction.

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

　　Although the color uniformity can be greatly improved, the color saturation after correction is significantly reduced. At the same time, another prerequisite for using red and blue to correct the green color uniformity is that the three LEDs of red, green and blue in the same pixel are distributed as concentratedly as possible so that the mixing distance of red, green and blue is as close as possible to achieve better results.

　　The current LED uniform distribution method commonly used in the industry will cause confusion in color uniformity correction. In addition, how to measure the color coordinates of tens of thousands of red, green and blue LEDs is also an extremely difficult problem. We have given some tips for this.