LED display technology from the monochrome display in the early 1980s, to the double primary color display in the late 1980s, and then to the three primary color (full-color) display in the mid-1990s. Until today, we have widely discussed the multi primary color (greater than three primary colors) processing technology in the field of flat panel display. The chromaticity processing technology of LED display screen ranges from the most basic primary color wavelength selection, to the allocation of white field color temperature, to the color space transformation processing to improve the color reduction degree and the chromaticity uniformity processing to improve the image quality. Until today, we have adopted the multi primary color (greater than three primary colors) processing in order to expand the color gamut and reproduce more natural colors. Various color processing technologies run through the development history of LED display screen and become one of the core technologies in the comprehensive discipline of LED display screen.
Various color processing technologies1. Selection of primary color wavelength
LED display screen is widely used in all walks of life, and there are different requirements for the primary color wavelength of LED in different application places. Some of the selection of the primary color wavelength of LED is to achieve good visual effect, some is to meet people's habits, and some are the provisions of industry standards, national standards and even international standards. For example, the selection of the primary color wavelength of the green tube in the full-color LED display screen; In the early days, yellow green LEDs with a wavelength of 570nm were widely used. Although the cost was low, the color gamut of the display screen was small, the color reduction degree was poor and the brightness was low. After selecting the pure green tube with the wavelength of 525nm, the color gamut of the display screen is nearly doubled, and the color reduction is greatly improved, which greatly improves the visual effect of the display screen. Another example is the stock market display. People are usually used to using red to indicate the rise of stock prices, green to indicate the decline of stock prices, and yellow to indicate the flat market. 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 primary color wavelength is one of the important links of LED display screen.
2. Allocation of white field color coordinates
White field color coordinate allocation 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 field color coordinates were usually determined only by human eyes and feelings, resulting in serious color deviation 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 screen. However, due to the lack of theoretical guidance of color matching, some manufacturers often allocate 100 field color coordinates at the expense of the gray level of some primary colors, and the comprehensive performance can not be improved.
3. Chroma uniformity treatmentThe chromaticity uniformity of LED display screen has always been a major problem perplexing people in the industry. It is generally believed that the uneven brightness of LED can be corrected at a single point to improve the brightness uniformity. The chromaticity unevenness cannot be corrected, and can only be improved by subdividing and screening the LED color coordinates.As people have higher and higher requirements for LED display screen, only subdividing and screening led color coordinates can not meet people's critical eyes. It is feasible to comprehensively correct the display screen and improve the chromaticity uniformity.
We find that even the same LED of the first international brand has large wavelength deviation and color saturation deviation, and the deviation range greatly exceeds the threshold of human eyes for green color difference identification. Therefore, it is of great significance to correct the chromaticity uniformity.In CIE1931 chromaticity diagram, according to the law of the center of gravity, we find that the color coordinates of the mixed color can be adjusted to the intersection o of the straight line Cr and the straight line dB when green is mixed with a certain proportion of red and blue at any point within the range of G (& square; ABCD)
Although the chromaticity uniformity can be greatly improved. However, the corrected color saturation decreases significantly. At the same time, another premise of using red and blue to correct the uniformity of green chromaticity is that the three red, green and blue LEDs in the same pixel should be centrally distributed as far as possible, so that the color mixing distance of red, green and blue can be 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 measure the color coordinates of tens of thousands of red, green and blue LED is also an extremely difficult problem. We gave a hint.
4. Color restoration processingThe birth of pure blue and green LED makes the full-color LED display popular in the industry with its wide color gamut and high brightness. However, due to the large deviation between the chromaticity coordinates of red, green and blue LED and that of PAL TV (see Table 1), the color restoration degree of LED full-color screen is poor. Especially in the performance of human skin color, there are obvious visual deviations. Therefore, color restoration technology came into being. Here, the author recommends two methods of color restoration:First, the color coordinate space of red, green and blue LED is transformed to make the three primary color coordinates between LED and PAL TV as close as possible, so as to greatly improve the color restoration degree of LED display screen. However, this method greatly reduces the color gamut range of LED display screen and greatly reduces the color saturation of the picture.
Second, only the most sensitive skin color gamut of human eyes is properly corrected; The color gamut that is not sensitive to other human eyes should reduce the original color saturation as little as possible. In this way, a balance can be obtained between color reduction and color saturation.5. 3 2 multi primary color chroma processing methodEverything recovers in spring. Under the blue sky, the green grass is green; In autumn, wheat waves roll; Under the sun, it was golden. The colorful nature is so beautiful. Unfortunately, the existing LED display screen 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 LED full-color screen is seriously insufficient in the yellow and cyan areas with extremely rich colors in nature.
In recent years, in the field of flat panel display, they are keen to discuss 3 3 multi primary color display (red, green, blue plus yellow, green and purple) to expand the color gamut and reproduce more abundant natural colors. Then, can the LED display realize 3 3 multi primary color display?We know that in the visible light range, yellow and green are monochromatic light. We already have high saturation yellow and cyan LEDs. Purple is polychromatic light, and single chip purple LED does not exist. Although we can not achieve red, green, blue plus yellow, green and purple 3 3 multi primary color LED display. However, it is feasible to study red, green, blue plus yellow and green 3 2 multi primary color LED displays. Due to the existence of a large number of high saturation yellow and cyan in nature; Therefore, this study is of certain value.In the current TV standards, the video source has only three primary colors of red, green and blue, but no two colors of yellow and green. Then, how to drive the yellow and green primary colors of the display terminal? In fact, when determining the driving intensity of yellow and green primary colors; We follow the following three principles:
(1) The purpose of adding yellow and green primary colors is to expand the color gamut and improve the color saturation. The overall brightness value cannot be changed;(2) The hue shall not be changed while increasing the color saturation;(3) Centered on D65; Take the rygcb gamut boundary as the endpoint, and make linear expansion at each point within the gamut range.
Under the guidance of the above three principles; According to the law of gravity center, we can find a 3 2 multi primary color chroma processing method. However, in order to truly realize the 3 2 multi primary color full-color screen, we have to overcome the lack of brightness of yellow and cyan LEDs; At present, it is limited to theoretical discussion.To sum up, we mainly discussed three aspects:(1) How to improve the chromaticity uniformity of LED display screen;
(2) How to improve the color restoration of LED display screen;(3) How to expand the color gamut and restore more natural colors.