LED display chromaticity processing skills

Although the LED high-definition display belongs to monochromatic light, the LED of each color still has a half-wave width of about 30~50nm, so its color saturation is limited. The colorful nature is so beautiful, unfortunately, the existing LED display cannot fully reproduce this beautiful scenery.

1. Allocation of white field color coordinates:

White field color coordinate allocation is one of the most basic technologies for full-color LED displays. Due to the lack of industry standards and basic testing methods, the white color coordinates are usually determined only by the human eye and feeling, resulting in severe color cast and randomness of the white 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, due to the lack of theoretical guidance on color matching, some manufacturers often sacrifice the gray level of some primary colors to allocate 100-field color coordinates, and the overall performance cannot be improved.

To sum up, we mainly discussed three aspects:

1. How to improve the chromaticity uniformity of LED high-definition display;

2. How to expand the color gamut and restore more natural colors;

3. How to improve the color reproduction of the LED display;

Various industries have a very wide range of applications, 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 good visual effects, some are to meet people’s habits, and some are to meet people’s habits. It is also the provisions of industry standards, national standards and even international standards.

The above-mentioned chromaticity processing technologies are all interrelated in specific implementation, and in some aspects, it is even impossible to have both. The integrated LED display 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 complex. Therefore, we must comprehensively weigh various performances from the perspective of the system, grasp the order of each processing, and increase the depth of signal processing, in order to make the LED full-color display display a colorful and splendid world.

For example, the choice of the wavelength of the green tube primary color in the full-color LED display; in the early days, people generally chose yellow-green LEDs with a wavelength of 570 nm. 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 color reproduction is greatly improved, which greatly improves the visual effect of the display.

2. 3+2 multi-primary chromaticity processing method:

In recent years, 3+3 multi-primary color display (red, green, blue plus yellow, cyan, purple) has been discussed keenly in the field of flat panel display to expand the color gamut and reproduce richer natural colors. So, can the LED display screen achieve 3+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+3 multi-primary color led display. However, it is feasible to study red, green, blue plus yellow, cyan 3+2 multi-primary color LED display. Because of the abundance of highly saturated yellows and cyans in nature, this research is of some value.

In the current various TV standards, the video source only has three primary colors of red, green and blue, but no two colors of yellow and cyan. Then, how to drive the yellow and blue primary colors of the display terminal?

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

1. The purpose of increasing the 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 hue shall not be changed;

3. Take D65 as the center; take the RYGCB color gamut boundary as the endpoint, and linearly expand 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 a 3+2 multi-primary full-color screen, we have to overcome difficulties such as insufficient brightness of yellow and cyan LEDs, and a large increase in cost, which is currently limited to theoretical discussions.

3. Treatment of chromaticity uniformity:

The problem of chromaticity uniformity of full-color LED displays has always been a major problem for people in 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 uneven chromaticity cannot be corrected, and can only be improved by subdividing and screening the LED color coordinates.

4. color restoration processing:

The birth of pure blue and pure green LEDs makes full-color LED displays popular in the industry for their wide color gamut and high brightness. However, because the chromaticity coordinates of the red, green and blue LEDs have a large deviation from the red, green and blue chromaticity coordinates of the PAL TV (see Table 1), the color reproduction of the LED full-color screen is poor. Especially in the expression of human skin color, there are obvious visual deviations. As a result, color reproduction processing technology came into being. Here I recommend two methods of color restoration:

1. Only properly correct the skin color gamut that is most sensitive to human eyes; and reduce the original color saturation as little as possible for the color gamut that is not sensitive enough to other human eyes. By doing so, a balance can be achieved between color reproduction and color saturation.

2. Transform the color coordinate space of the red, green and blue LEDs, 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.

5. the choice of primary color wavelength:

As people’s requirements for LED display are getting higher and higher, only subdivision and screening of LED color coordinates can no longer satisfy people’s critical eyes. Comprehensive correction processing is performed on the display to make the color uniformity Improvement is achievable.

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

In the CIE1931 chromaticity diagram, according to the law of the center of gravity, we find that: at any point in the G range (abcd), green is 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 straight line. The intersection of dB O.

Although the chromaticity uniformity can be greatly improved. However, the corrected color saturation drops significantly. At the same time, another premise of using 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 much as possible to make the mixing distance of red, green and blue as close as possible, in order to achieve better results. Effect.

However, the method of uniform distribution of LEDs that is usually used in the industry at present 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 a hint for this.