LED display technology has ranged from monochromatic displays in the early 1980s, to dual-primary color displays in the late 1980s, to three-primary (full-color) displays in the mid-1990s, and we have been widely used in flat panel displays today. Discussed multi-primary color (greater than three primary colors) processing technology. The chromaticity processing technology of the led display screen ranges from the most basic wavelength selection of the primary color, to the adjustment of the white field color temperature, to the color space conversion processing to improve the color reproduction and the color uniformity processing to improve the image quality. Today we have adopted multi-primary color (greater than three primary colors) processing in order to expand the color gamut and reproduce more natural colors. Various chromaticity processing technologies run through the development history of LED display screens and become one of the most core technologies in the comprehensive discipline of LED display screens.
LED display
Various chroma processing technologies:
1. The choice of primary color wavelength
LED display screens are widely used in various industries, and different application places have different requirements for the wavelength of the primary color of the LED. Some of the wavelengths of the primary color of the LED are selected to achieve a good visual effect, and some are to meet people's requirements. Habits, and some are more regulated by industry standards, national standards and even international standards. For example, the choice of the wavelength of the primary color of the green tube in a full-color LED display; in the early days, people generally chose a yellow-green LED with a wavelength of 570nm. Although the cost is lower, 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 screen has nearly doubled, and the color reproduction has been greatly improved, which greatly improves the visual effect of the display screen. For another example, on the stock market display screen, people are usually accustomed to using red to indicate rising stock prices, green to indicate falling stock prices, and yellow to indicate flat prices. In the transportation industry, national standards strictly stipulate that the blue and green bands are traffic, and the red bands are forbidden. Therefore, the choice of the wavelength of the primary color is one of the important aspects of the LED display.
2. Adjustment of white field color coordinates
White field color coordinate adjustment is one of the most basic technologies for full-color LED displays. 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 sensations, 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 have begun to standardize the color matching process for full-color screens. However, due to the lack of theoretical guidance for color matching, some manufacturers often sacrifice the gray levels of certain primary colors to adjust the 100-field color coordinates, and the overall performance cannot be improved.
3. Chromaticity uniformity treatment.
The chromaticity uniformity of LED display screens has always been a major problem that plagues the industry. It is generally believed that the uneven brightness of LEDs can be corrected by a single point to improve the brightness uniformity. The chromaticity unevenness cannot be corrected, and can only be improved by subdividing and filtering the LED color coordinates.
As people's requirements for LED displays are getting higher and higher, only subdividing and filtering the color coordinates of LEDs can no longer satisfy people's critical gaze. Comprehensive correction of the display can be achieved to improve the chromaticity uniformity. of.
We found that even the same grade of LED of the world's first brand has a large wavelength deviation and color saturation deviation, and the deviation range greatly exceeds the threshold value of the human eye for discrimination of 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 found that: at any point in the G file range (Square;abcd), mixing a certain proportion of red and blue with a certain proportion of red and blue can adjust the color coordinates of the mixed color to the straight line cR The intersection point O with the straight line dB.
Although the chromaticity uniformity can be greatly improved. However, the color saturation after correction is significantly reduced. At the same time, another prerequisite for the use of red and blue to correct the uniformity of green chromaticity is that the three LEDs of red, green and blue in the same pixel should be distributed as closely as possible to make the mixing distance of red, green and blue as close as possible to achieve better results. Effect. At present, the LED uniform distribution method is commonly used in the industry, which 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 have given a hint about this.
4. Color reduction processing
With the birth of pure blue and pure green LEDs, full-color LED displays are sought after by the industry for their wide color gamut and high brightness. However, because the chromaticity coordinates of the red, green, and blue LEDs are quite different from the chromaticity coordinates of the red, green, and blue of the PAL TV (see Table 1), the color reproduction of the LED full-color screen is poor. Especially when expressing human skin color, there are obvious deviations in vision. As a result, the color reduction processing technology came into being. Here the author recommends two color reproduction methods:
One: to transform the color coordinate space of the red, green and blue 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 screen, and greatly reduces the color saturation of the screen.
Second: Only the skin color gamut that is most sensitive to the human eye is appropriately corrected; and the color gamut that is not sensitive to other human eyes is as little as possible to reduce the original color saturation. In this way, a balance can be obtained between color reproduction and color saturation.
LED display
5. 3 plus 2 multi-primary chromaticity processing method
In spring, everything recovers. Under the blue sky, the green grass is green; in autumn, the waves of wheat are rolling; under the sun, it is golden. The colorful nature is so beautiful, unfortunately, the existing LED display can not fully reproduce this beautiful scenery. Although LEDs belong to monochromatic light, each color LED still has a half-wave width of about 30-50nm, so its color saturation is limited. It can be seen from Figure 3 that the color saturation of the LED full-color screen is seriously insufficient in the yellow and cyan areas where the colors are extremely rich in nature.
In recent years, in the field of flat panel displays, there has been a keen discussion of 3 plus 3 multi-primary color displays (red, green, blue plus yellow, cyan, and purple) in order to expand the color gamut and reproduce richer natural colors. So, can the LED display achieve 3 plus 3 multi-primary color display?
We know that in the visible light range, yellow and cyan are monochromatic light, and we already have high-saturation yellow and cyan LEDs. While purple is polychromatic light, single-chip purple LEDs do not exist. Although we can not achieve red, green, blue plus yellow, cyan, purple 3 plus 3 multi-primary color LED display. However, it is feasible to study the red, green, blue, yellow, and cyan 3 plus 2 multi-primary color LED display screens. Because there are a lot of high-saturation yellow and cyan colors in nature; therefore, this research is of certain value.
In the current various television standards, the video source has only the three primary colors of red, green and blue, but not the two colors of yellow and cyan. So, how to drive the yellow and cyan colors of the display terminal? In fact, when determining the driving strength of the two primary colors of yellow and cyan, we follow the following three principles:
(1) The purpose of increasing the two primary colors of yellow and cyan is to expand the color gamut, thereby increasing the color saturation. And the overall brightness value cannot be changed;
(2) While increasing the color saturation, the hue must not be changed;
(3) Take D65 as the center; take the RYGCB color gamut boundary as the end point, and linearly expand each point in 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 plus 2 multi-primary chromaticity processing method. However, if we want to truly realize a 3 plus 2 multi-primary full-color screen, we have to overcome the insufficient brightness of the yellow and cyan LEDs; the increase in cost and other difficulties are currently limited to theoretical discussions.
In summary, we mainly discussed three aspects:
(1) How to improve the chromaticity uniformity of the LED display screen;
(2) How to improve the color reproduction of the LED display;
(3) How to expand the color gamut and restore more natural colors.
The above-mentioned chromaticity processing technologies are all related to each other when they are implemented in practice, and some aspects are not even compatible with fish and bear's paws. The integrated LED display screen also needs to perform brightness uniformity correction, gray-scale nonlinear transformation, noise reduction processing, image enhancement processing, dynamic pixel processing, etc., and the entire signal processing process is very complicated. Therefore, we must comprehensively weigh various performances from the perspective of the system, grasp the sequence of various processing, and increase the depth of signal processing, so that the LED full-color display can show a colorful, colorful and wonderful world.
