Interpretation of various chromaticity processing technologies for LED display screens

LED display technology has evolved from monochrome displays in the early 1980s, to dual-primary-color displays in the late 1980s, to three-primary-color (full-color) displays in the mid-1990s, to the multi-primary-color (greater than three-primary-color) processing technology that we are widely discussing in the field of flat-panel displays today. The chromaticity processing technology of LED displays ranges from the most basic selection of primary color wavelengths, to the adjustment of white field color temperature, to the color space transformation processing for improving color reproduction and the chromaticity uniformity processing for improving image quality, to the multi-primary-color (greater than three-primary-color) processing that we adopt today to expand the color gamut and reproduce more natural colors. Various chromaticity processing technologies run through the development history of LED displays and have become one of the most core technologies in the comprehensive discipline of LED displays.

Various chromaticity processing technologies

1. Selection of primary color wavelength

LED display screens are widely used in all walks of life, and different application sites have different requirements for the wavelength of the primary color of LEDs. Some of the selections of the wavelength of the primary color of LEDs are to achieve good visual effects, some are to conform to people's habits, and some are stipulated by industry standards, national standards, and even international standards. For example, in the selection of the wavelength of the primary color of the green tube in the full-color LED display screen; 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 screen was small, the color reproduction was poor, and the brightness was low. After selecting a pure green tube with a wavelength of 525nm, the color gamut of the display screen was nearly doubled, and the color reproduction was greatly improved, which greatly improved the visual effect of the display screen. For another example, in the stock market display screen, people are usually accustomed to using red to indicate rising stock prices, green to indicate falling stock prices, and $ to indicate flat prices. 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 the primary color wavelength is one of the important links of the LED display screen.

2. 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.

3. Chromaticity uniformity processing.

The color uniformity of 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.

As people's requirements for LED displays become higher and higher, simply segmenting and screening LED color coordinates can no longer satisfy people's discerning eyes. It is feasible to perform comprehensive correction processing 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 it can greatly improve the color uniformity. However, 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 should be distributed as concentratedly as possible so that the mixing distance of red, green and blue can be as close as possible to achieve better results. At present, the industry usually adopts the LED uniform distribution method, which will bring confusion to the color uniformity correction. In addition, how to carry out the measurement of 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.

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:

First, the color coordinate space of the red, green and blue primary color LEDs is transformed 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.

Second, only the skin color gamut, to which the human eye is most sensitive, is properly corrected, while the original color saturation of other gamuts, to which the human eye is less sensitive, is reduced as little as possible. This process can achieve a balance between color reproduction and color saturation.

5. 3+2 multi-primary color processing method

In spring, everything comes back to life. Under the blue sky, the grass is green. In autumn, the wheat waves are rolling. Under the sunshine, the sky is golden. The colorful nature is so beautiful. Unfortunately, the existing LED display screen cannot fully reproduce this beautiful scenery. Although LED is a monochromatic light, each color LED still has a half-wave width of about 30~50nm, so its color saturation is limited. As can be seen from Figure 3: in the extremely colorful $ and cyan areas of nature, the color saturation of the LED full-color screen is seriously insufficient.

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 red and cyan LEDs. Purple is a complex color 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, it is feasible to study a 3+2 multi-primary color LED display screen of red, green, blue plus yellow, cyan. Since there are a large number of highly saturated red and cyan in nature, this research is of certain value.

In the current TV standards, the video source only has three primary colors: red, green and blue, but no yellow or cyan. So how do the display terminals drive the two primary colors of yellow and cyan? In fact, when determining the driving strength of the two primary colors of yellow and cyan, we should 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) While increasing the color saturation, the color tone shall not be changed;

(3) With D65 as the center and the RYGCB color gamut boundary as the endpoint, linear expansion is performed 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 color full-color screen, we still have to overcome the difficulties such as insufficient brightness of yellow and cyan LEDs and large cost increase, so it is currently limited to theoretical discussion.

In summary, we have mainly discussed three aspects:

(1) How to improve the color uniformity of LED display screens;

(2) How to improve the color reproduction of LED display screens;

(3) How to expand the color gamut and restore more natural colors.

The above-mentioned chromaticity processing technologies are interrelated in their specific implementation, and some aspects are even mutually exclusive. The comprehensive 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.