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Various chromaticity processing technologies for LED displays

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

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

We know that in the range of visible light, yellow and cyan are monochromatic light, and we already have high-saturation yellow and cyan LEDs. Purple is polychromatic light, and single-chip purple LEDs do not exist. Although we can’t realize red, green, blue plus yellow, cyan, purple 3+3 multi-color led display. However, it is feasible to study red, green, blue plus yellow, cyan 3+2 multi-primary LED display. Because there are a lot of high-saturation yellow and cyan in nature; therefore, this research is of certain value.

In the various current TV standards, the video source only has the three primary colors of red, green and blue, but not the two colors of yellow and cyan. Then, how to drive the yellow and cyan primary colors of the display terminal?

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

1. While increasing the color saturation, do not change the hue;

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

3. Take D65 as the center; take 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 the center of gravity, we can find a 3+2 multi-primary chromaticity processing method. However, in order to truly realize the 3+2 multi-primary color full-color screen, we still need to overcome difficulties such as insufficient brightness of yellow and cyan LEDs and a large increase in cost, 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 red, green and blue chromaticity coordinates of PAL TV (see Table 1), the color reproduction of LED full-color screens is poor. Especially when expressing human skin color, there are obvious deviations visually. As a result, color restoration processing technology came into being. Here I recommend two color restoration methods:

1. Carry out color coordinate space transformation on the red, green and blue three-primary color LED, so that the three-primary color coordinates between the LED and the PAL TV are as close as possible, thereby greatly improving the color reproduction of the LED display. However, this method greatly reduces the color gamut range of the LED display, which greatly reduces the color saturation of the picture.

2. Properly correct only the skin color gamut that is most sensitive to the human eye; and reduce the original color saturation as little as possible for the color gamut that is not sensitive to other human eyes. In this way, a balance can be achieved between color reproduction and color saturation.

3. Selection of base color wavelength:

LED display screens are becoming more and more demanding as people’s requirements for LED display screens are getting higher and higher. Only subdividing and screening LED color coordinates can no longer satisfy people’s critical eyes. Getting better is achievable.

We found that even the same grade of LED of the first international brand has large wavelength deviation and color saturation deviation, and the deviation range greatly exceeds the human eye’s threshold for distinguishing green color difference. Therefore, it is important to perform color uniformity correction. meaningful.

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

Although the chromaticity uniformity can be greatly improved. However, there is a noticeable drop in color saturation after correction. At the same time, another prerequisite for using red and blue to correct the uniformity of green chromaticity is that the red, green and blue LEDs in the same pixel should be distributed as intensively as possible so that the color mixing distance of red, green and blue can be as close as possible to achieve better results. Effect. At present, the 55mm Flexible LED Mesh Display uniform distribution method usually used in the industry will bring confusion to the chromaticity 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 gave hints about this.

4. Chromaticity uniformity treatment:

The chromaticity uniformity of LED display screens has always been a major problem for people in the industry. It is generally believed that the uneven brightness of LEDs can be corrected at a single point to improve the uniformity of brightness. The uneven chromaticity cannot be corrected, and can only be improved by subdividing and screening the LED color coordinates.

5. Adjustment of white field color coordinates:

White field color coordinate adjustment is one of the most basic technologies of full-color LED display. However, in the mid-1990s, due to the lack of industry standards and basic testing methods, the white point color coordinates were usually determined only by human eyes and feelings, resulting in serious color cast and randomness of white point color temperature. With the promulgation of industry standards and the completion of testing methods, many manufacturers have begun to standardize the color matching process of full-color screens. However, due to the lack of theoretical guidance for color matching, some manufacturers often adjust the color coordinates of hundreds of fields by sacrificing the gray level of some primary colors, and the overall performance cannot be improved.

To sum up, we mainly discussed three issues:

1. How to improve the chromaticity 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 chroma processing technologies are all interrelated in actual implementation, and in some aspects, it is even impossible to have both. The integrated LED display also needs to perform brightness uniformity correction, gray scale 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 the perspective of the system, 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.

All walks of life have a very wide range of applications, and in different application places have different requirements for the primary color wavelength of LEDs. Some of the choices for the primary color wavelength of LEDs are to achieve good visual effects, some are to meet people’s habits, and some It is the regulation of industry standard, national standard and even international standard.

For example, the choice of the wavelength of the primary color of the green tube in the full-color LED display; in the early days, people generally used yellow-green LEDs with a wavelength of 570nm. Although the cost is low, the color gamut of the display is small, the color reproduction is poor, and the brightness is low. After choosing a pure green tube with a wavelength of 525nm, the color gamut of the display is nearly doubled, and the degree of color reproduction is greatly improved, which greatly improves the visual effect of the display. For another example, on the securities market display screen, people are usually used to using red to indicate a stock price rise, green to indicate a stock price decline, and yellow to indicate a flat market. In the transportation industry, the national standard strictly stipulates that the blue and green bands are allowed to pass, and the red band is forbidden. Therefore, the selection of the primary color wavelength is one of the important links of the LED display.