Development trend of LED backlight components in the new era

As the application of LCD panels has expanded from notebook computers to mobile phones, car navigation, home televisions and other fields, the natural color reproduction of LCD has become the focus of attention from all walks of life. Some special fields even require that the color reproduction range of LCD exceed the color specifications of NTSC.

Due to the inherent characteristics of CCFL, it is unable to break through certain color barriers, making it impossible for consumers to enjoy rich and colorful images similar to those in nature in terms of color performance, especially the inability to perfectly display bright red colors.

However, in pursuit of the goal of high color rendering, technologies to replace CCFL light sources have been proposed one after another. Among them, the most promising is the use of multi-color LEDs as backlight sources. Its wide color gamut has attracted the attention of many industry players, who have also invested in related development.

1. Traditional CCFL red light performance is weak

Currently, most display manufacturers use cold cathode lamps as the light source of the display, and use the three primary colors of RGB as color filters in an array distribution. Generally speaking, the color temperature of CCFL is around 4800K. When reflected in the color gamut table, it can be found that it is only about 80% of the NTSC specification.

Figure 1 shows the spectrum of CCFL and the distribution characteristics of color filters. From the figure, we can see that when CCFL is combined with RGB color filters, the color performance near wavelengths of 490nm and 590nm is poor, and some problems such as narrow color gamut occur after color mixing by color filters.

Of course, for general display screens or applications, there will not be too much color problem or visual deterioration, but in the face of instrument measurement and specific color performance environment, the subpeak phenomenon of impure luminous lines can be clearly compared. In particular, CCFL is weaker in red light performance, which is the most difficult point for CCFL to meet strict requirements in color performance.

▲Figure 1: CCFL spectrum and color filter distribution characteristics

However, when CCFL is used as the backlight source, there are certain factors that affect the color gamut performance, but it is not necessarily completely unthinkable. It can be improved based on the traditional three-color color filter to make up for this defect.

▲Picture: CCFL backlight LCD display - Samsung 931C can also achieve a high color gamut of 97% NTSC

2. Use multi-color filters to make up for the shortcomings

Based on the module that also uses CCFL as the backlight source, Chi Mei Electronics has developed three panels that use more than 4 color filters for color expression. They are panels that add 5-color filters of Y (yellow) and C (cyan) to the original 3 colors of R (red), G (green) and B (blue), panels that add 4-color filters of Y (yellow) on the basis of RGB, and panels that add 4-color filters of W (white) on the basis of 3 colors of RGB.

In fact, such a design also expands the color range. For example, the color range of the panel with added Y (yellow) and C (cyan) and Y (yellow) is 115% and 109% respectively compared with the NTSC range. The purpose of adding a white color filter is only to increase the overall brightness of the panel. Of course, this is based on the original light source, using color filters to achieve the purpose of improving the color gamut, but after all, due to inherent limitations, it is impossible to significantly expand the color range. Perhaps changes are still needed in the backlight source to achieve the goal of expanding the color range.

Based on current technology and components, a very suitable approach is to use LED as a backlight source. Since LED has the characteristics of multiple wavelengths, unique wavelengths can be produced according to demand, and brightness control can be achieved using circuit design.

3. Three-color LED backlight module achieves high color gamut ideal

In contrast, using three-color or multi-color LEDs as backlights will not result in the aforementioned color gamut narrowing problem in the performance of mixed colors. Figure 2 shows the use of three-primary-color LEDs as backlights, which performs better than CCFL, especially in the red light portion, where a very wide color reproduction range can be obtained, and will not cause subpeaks of impure brightness lines similar to those seen in CCFLs, and greatly improves the color purity of each primary color.

▲Figure 2: Spectra of the three primary colors of LEDs and distribution characteristics of color filters.

In addition, the color gamut can be expressed in a wider range. The following table shows the three-color LED backlight module published by Japan LEIZ. From the table, we can see that the chromaticity it expresses, after testing, the LEIZ backlight module can reach 100% of the NTSC color gamut. Japan LEIZ uses 40 high-brightness three-color LEDs on this module, and sets Heatsink on both sides of the module to allow the three-color LEDs to mix light in the module, providing the light source required by the LCD.

4. SONY takes the lead in launching LED backlight LCD TV

At the end of 2004, SONY launched an LCD TV with R, G, and B three-color LED as the backlight, making the multi-color LED backlight module reach the stage of practical application. The color gamut of RGB three-color LED exceeds 150% of CCFL, and the color expression ability greatly exceeds the sRGB commonly used in traditional TVs. In the past, TVs using traditional CRT as display components could not display sky blue, dark green, dark red, and some bright colors in nature, but LCD TVs using R, G, and B three-color LED as the backlight are no longer a problem. If compared with Pointer's 768-color high-color color chart, LCD TVs using LED backlight modules can cover up to 82% of the color range, especially in green and red, which can show a very wide color saturation, and the yellow and orange parts even exceed the color gamut of sRGB. However, compared with CCFL or traditional CRT screens, they can only reach about half of the color range.

Special arrangement makes up for the weakness of color system

According to experiments, the human eye's highest sensitivity to light color is green light, red light is about 1/3 of green light, and relative to blue light, it is 10 times of blue light. Based on such characteristics, there are also some changes in the LED color matching. Because it is necessary to meet the visual sensitivity, most of them are designed with a ratio of red light × 1, blue light × 1, and green light × 2, but considering the lower color temperature of red light. Therefore, SONY uses "green-red-blue-red-green" as the arrangement structure of the backlight LED to achieve the best color output. Because the color gamut of the LCD screen is not just based on the backlight source, the front-end color filter is one of the important determining factors, so overall, the color gamut range of the LCD screen depends on the light source characteristics of the backlight module and the combination of the transmittance characteristics of the color filter. Of course, in the face of a three-color LED backlight module that can display such a wide color gamut, SONY has made considerable improvements in the color filter. Since the color distribution of LEDs has a certain range, it is necessary to use a color filter that can narrow the wavelength range of the penetrating light and maintain a certain color purity. It is hoped that the color effects of the coordinated colors can be fully utilized in conjunction with the characteristics of the LED light source.

6-color LED with wide color gamut is amazing

If RGB or multi-color LEDs with different wavelengths are used as backlight sources, it is not necessary to use RGB on the color filter. You can even use purple and chrysanthemum color filters to match a higher color gamut display capability and present more colors. Mitsubishi Electric and Samsung have both published technologies that use 6-color LEDs as backlight modules. Samsung uses a new technology of 6-color light source plus 6-color color filter panel, using a 6-color filter with RGBCYM spectroscopic characteristics on a light source with RGBCYM6-color (red, green, blue, cyan, yellow, magenta) wavelengths. The LED lighting method is to stagger the display time by lighting alternately in field sequence, turning on R (red), G (green), and B (blue) LEDs in turn, with a resolution of 1366×768, a brightness of 500cd/㎡, and a contrast ratio of 1000: 1. Since the LED light can be seen directly, the color performance reaches 110% of the NTSC specification. The power consumption is 82W, which is equivalent to 60% of the energy consumption of the old LCD panel with the same brightness. According to the information, the 6-color LED backlight module released by Mitsubishi Electric, in addition to being able to expand the color reproduction range, another feature is that it will not suddenly increase the production cost.

Technically, Mitsubishi Electric uses different wavelengths of LED to achieve 6 different colors, namely 410nm (blue), 540nm (green), 615nm (red) for the first group, and 430nm (blue), 510nm (green), 625nm (red) for the second group. The chromaticity coordinates can be achieved as follows: the first group: 615nm (red 1) (0.664, 0.321), 540nm (green 1) (0.291, 0.666), 410nm (blue 1) (0.154, 0.060).

The second group: 625nm (Red 2) (0.682, 0.308), 510nm (Green 2) (0.131, 0.580), 430nm (Blue 2) (0.112, 0.173). In a 23-inch, 1280×768, 80cd/m2 panel, 26 blue and red LEDs are used, while 56 green LEDs are used. The sRGB ratio is increased to 175%, which can cover about 96% of the colors of natural objects (based on the MunsELlColorCascade standard).

LED driving circuit composed of sequential interactive lighting

In the LED circuit drive design, multi-color LED backlights mostly use field sequence interactive lighting to form a "FieldSequence". In this way, the 6-color LEDs in the backlight module can operate synchronously with the Sub-Pixels of the LCD panel and the 3-color color filters.

In the past, most LCD panels used three sub-pixels to form one pixel. However, this method not only allows a wider range of color expression, but also allows for more detailed images. However, this will cause some overall development problems, because such structural changes make it impossible to continue to use some components, including some backlight films, overall module structures, color conversion circuits, etc. These must be redeveloped.

However, because such a reform can greatly improve color performance, and Mitsubishi Electric claims that it will not cause much cost increase, perhaps the use of such a light source design only requires a larger initial development cost, and overall, the material cost structure will not change much.

In addition, the 6-color LED backlight module released by Mitsubishi Electric has a brightness of only 80cd/㎡. This result may be due to the overall backlight module design problem. Although LED currently has good performance in brightness, it is limited by the module material. Therefore, in the future, more efforts must be made to overcome the overall brightness of the module in order to reach a level of commercial acceptance.

Low light transmission efficiency is the biggest fatal point of the backlight module

From the current LED brightness technology, it is not too difficult to improve the brightness, but the high power consumption and heat dissipation problems that come with it are troubling all engineers. Moreover, it is unrealistic to blindly develop in this direction and expect to solve the problem. Because the improvement of brightness will always reach a bottleneck, if the service life is shortened due to efforts in this aspect, it seems that the gains do not outweigh the losses. Therefore, on the whole, it is necessary to start from improving the transmittance of the backlight module, which is the fundamental solution.

Since the backlight source must use optical films such as reflectors and diffusers to achieve the purpose of even projection of the light source, light loss often occurs. According to research, if the light emitted from a traditional backlight source is 100%, after passing through optical films such as reflectors and diffusers, only about 60% of the light will pass through the backlight module and enter the polarizing film, and finally only 4% of the light will come out after passing through LC and Surface (Figure 3).

Figure 3: The light transmission capacity of the backlight module is quite limited. (Image by Lu Qingru)

That is to say, if the backlight source is 10,000 nits, then the final projected light will only be 400 nits. Assuming that the LCD panel specification requires 500 nits, the brightness of the backlight source must be able to provide 12,000 to 13,000 nits.

5. Multicolor LED is the mainstream backlight technology in the future

Since SONY released the LED backlight module in 2004, the backlight module for LCD monitors can be said to have officially entered the LED era. Although LED itself still has many technical problems to be overcome, in the future, part of the market for traditional cold cathode lamps will gradually be replaced by LED light sources. It is believed that the image quality and color of LCD monitors will be more vivid and delicate in the future.

The color reproduction characteristics of LED backlight modules, LCD displays using R/G/BLED backlight modules are suitable for applications in the fields of medical, printing, PC, etc., especially home LCD TVs with a color temperature range of 3000~9300K can obtain a wide range of color reproduction. Since there is still a lot of room for improvement in the power consumption and production costs of LED backlight modules, in addition to the optical system of LED backlight modules, the heating problem of LED itself needs to be suppressed in the future. (References: Optoelectronic Technology Magazine, Japan NE Magazine, Mitsubishi Electric, SONY, Japan LIEIZ related materials)