LED display technology from the monochrome display in the early 1980s, to the dual primary color display in the late 1980s, to the three primary color (full color) display in the mid-1990s, until today we are in the field of flat panel display Multi-primary (greater than three primary) processing techniques are discussed. The color processing technology of the LED display screen is selected from the most basic primary color wavelength selection, to the white field color temperature, to the color space conversion processing for improving the color reproduction degree, and the chromaticity uniformity processing for improving the image quality until Today we are dealing with multiple primary colors (greater than three primary colors) in order to expand the color gamut to reproduce more natural colors. Various color processing technologies have penetrated the development history of LED displays and become one of the core technologies in the comprehensive discipline of LED displays.
Various color processing techniques
1, the choice of the primary color wavelength
LED display has a wide range of applications in all walks of life, and different requirements for the primary color wavelength of LED in different applications, the choice of LED primary color wavelength is to achieve good visual effects, some to meet people's Habits, and some are more industry standards, national standards and even international standards. For example, the choice of the wavelength of the green tube in the full-color LED display; in the early days, the yellow-green LED with a wavelength of 570 nm was generally used. Although the cost is low, the color gamut of the display is small, the color reproduction is poor, and the brightness is low. After selecting a pure green tube with a wavelength of 525nm, the color gamut of the display has nearly doubled, and the color reproduction degree has been greatly improved, which greatly improves the visual effect of the display. For example, in the securities market display screen, people are usually accustomed to using red to indicate that the stock price has risen, green to indicate that the stock price has fallen, and to use yellow to indicate the flat. In the transportation industry, the blue-green band is strictly defined by the national standard, and the red band is prohibited. Therefore, the choice of the primary color wavelength is one of the important links of the LED display.
2, the allocation of white field color coordinates
White field color coordinate modulation is one of the most basic technologies for full color LED display. 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 the human eye and by feeling, resulting in random color cast and white-field color temperature. With the promulgation of industry standards and the complete testing methods, many manufacturers have begun to regulate the full color screen color matching process. However, some manufacturers still use the theoretical guidance of lack of color matching, often at the expense of the gray level of some primary colors to match the hundred field color coordinates, the overall performance is not improved.
3, chroma uniformity processing
The problem of uniformity of chroma of LED display has always been a big problem for people in the industry. It is generally believed that the brightness of LEDs is not uniform and can be corrected by single point to improve 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 display screens are getting higher and higher, only subdividing and screening LED color coordinates can not satisfy people's critical eye, and comprehensive correction processing on the display screen can improve chromaticity uniformity. of.
We found that even the same brand of the international first brand LED has large wavelength deviation and color saturation deviation, and the deviation range greatly exceeds the human eye's threshold for green color difference identification. Therefore, it is important to perform chromaticity uniformity correction. meaningful.
In the CIE1931 chromaticity diagram, according to the law of gravity center, we found that any point in the G range (â–¡abcd) can be mixed with a certain proportion of red and blue, and the color coordinates of the mixed color can be adjusted to the straight line cR and The intersection point O of the straight line dB.
Although the chromaticity uniformity can be greatly improved. However, the corrected color saturation is significantly reduced. At the same time, another premise of 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 concentrated as much as possible so that the red, green and blue color mixing distances are as close as possible to achieve better. Effect. At present, the LED uniform distribution method is usually used in the industry to bring confusion to the chromaticity uniformity correction. In addition, how to measure the tens of thousands of red, green and blue LED color coordinates is also a very difficult problem. We gave a hint for this.
4, color reproduction processing
The birth of pure blue and pure green LEDs has made the full-color LED display popular in the industry for its wide color gamut and high brightness. However, due to the large deviation between the chromaticity coordinates of the red, green and blue LEDs and the chromaticity coordinates of the PAL TV red, green and blue (see Table 1), the color reproduction of the LED full color screen is poor. Especially when expressing the skin color of a person, there is a visually significant deviation. As a result, color reproduction processing technology came into being. Here I recommend two methods of color reproduction:
Firstly, the color coordinate transformation of the red, green and blue primary color LEDs is performed, so that the three primary color coordinates between the LED and the PAL television are as close as possible, thereby greatly improving the color reproduction degree of the LED display. However, this method greatly reduces the color gamut of the LED display screen, and the color saturation of the screen is greatly reduced.
Second, only the skin color gamut that is most sensitive to the human eye is properly corrected; and the color gamut that is not sensitive to other people's eyes is less likely to reduce the original color saturation. This is done to achieve a balance between color reproduction and color saturation.
5, 3+2 multi-primary color processing method
In the spring, everything recovers. Under the blue sky, the green grass is green; the autumn wheat waves are rolling; under the sunshine, it is golden. The colorful nature is so beautiful, unfortunately the existing LED display can not fully reproduce this beautiful scenery. Although the LEDs are monochromatic, the LEDs of each color still have a half-wave width of about 30 to 50 nm, so the color saturation is limited. It can be seen from Fig. 3 that the color saturation of the full-color LED of the yellow and cyan area in the color of nature is seriously insufficient.
In recent years, in the field of flat panel display, I am keen to discuss 3+3 multi-primary color display (red, green, blue, yellow, blue, purple) to expand the color gamut and reproduce more abundant natural colors. So, can the LED display achieve 3+3 multi-primary display?
We know that in the visible range, yellow and blue are monochromatic lights. We have high-yellow yellow and cyan LEDs. Purple is multi-color light, and single-chip purple LEDs do not exist. Although we can not achieve red, green, blue plus yellow, blue, purple 3 + 3 multi-primary LED display. However, it is feasible to study red, green, blue, yellow, and blue 3+2 multi-color LED displays. Because of the large number of high-saturation yellows and cyan in nature, the study has some value.
In the current various TV standards, the video source has only three primary colors of red, green and blue, and no yellow or blue. Then, how to display the terminal yellow and blue primary colors? In fact, when determining the driving strength of the primary colors of yellow and cyan; we follow the following three principles:
(1) The purpose of increasing the primary colors of yellow and cyan is to enlarge the color gamut and thereby increase the color saturation. The overall brightness value cannot be changed;
(2) While changing the color saturation, the color tone shall not be changed;
(3) Centered on D65; with the RYGCB color gamut boundary as the endpoint, linear expansion at each point in the gamut.
Under the guidance of the above three principles; according to the gravity center law, we can find 3+2 multi-primary chromaticity processing methods. However, in order to truly realize the 3+2 multi-primary full-color screen, we have to overcome the difficulties of insufficient brightness of yellow and cyan LEDs;
In summary, we mainly discuss three aspects:
(1) How to improve the chroma uniformity of the LED display;
(2) How to improve the color reproduction degree of the LED display;
(3) How to expand the color gamut and restore more natural colors.
The above-mentioned various color processing techniques are interrelated in the specific implementation, and some aspects are not even compatible with fish and bear's paw. The integrated LED display also needs to perform brightness uniformity correction, grayscale nonlinear transformation, noise reduction processing, image enhancement processing, dynamic pixel processing, etc., and the whole signal processing flow is very complicated. Therefore, we must comprehensively weigh the performance from the perspective of the system, grasp the order of each treatment, and increase the depth of signal processing, in order to make the LED full-color display show a colorful, colorful and wonderful world.
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