The biggest disadvantage of LEDs relative to CCFLs is their low luminous efficiency. CCFLs are typically between 80 and 100 lumens per watt, while LEDs have luminous efficiencies of only 60 to 80 lumens per watt. But because the CCFL is cylindrical, the light it emits is non-directional. When it is placed under the LCD screen, despite the use of a semi-circular mirror, it can be considered that a considerable portion of the light is still wasted. LEDs usually use side-light radiation, and their light can be used more efficiently. Therefore, overall, its efficiency is still higher than CCFL, and the luminous efficiency of LED is still increasing year by year.
In the past, white LEDs were used as backlights. In order to get better color gamut performance, RGB three-color LEDs were used to mix them into white, but this requires more LEDs. Moreover, since it is necessary to display the color originally, it is not necessary to first mix the RGB colors into white, and then filter the color with the RGB color filters. To this end, Samsung has introduced a field-sequential RGB LED backlight technology without color filters.
Use the field sequential method to remove the color filter
However, it seems impossible to replace the filter directly with RGB LEDs, which is impossible. Because the color filter divides each pixel into three different color sub-pixels, if you directly replace the color filter with three color LEDs, you must use as small as the sub-pixel size to illuminate each sub-pixel. Pixels don't make red light on sub-pixels of green light. In this way, the number of LEDs will be several million. In order to maintain the same number of pixels, the only way is to reuse these pixels, that is, using time division multiplexing.
Thus, as long as red, green, and blue light are sequentially emitted while controlling the thin film transistor (TFT) of each pixel, the liquid crystal light valve is turned on correspondingly according to the intensity that the pixel should have in such a color. However, in order to sequentially emit three colors of red, green, and blue, and to form a color video image, it is necessary to utilize the visual residual effect of the human eye.
As long as the repetition period of the three colors is less than the visual residual time of the human eye, a colored image can be formed in the human brain. The visual residual time is actually the field frequency period of the TV, which is one-fifth of a second in the PAL system. This also requires the display of three images of red, green and blue in half a second. Therefore, this method is called the "field order" system, and some people call it the "color order" system. Its working sequence is shown in Figure 1.
Figure 1: Working sequence of field order.
The field order is actually to let the liquid crystal screen sequentially emit images of three different colors, and then the human brain to synthesize the color image. The effect from the human eye is exactly the same. Although there are great differences in hardware implementation, it brings many advantages.
Table 1: Compared to CCFL backlights, there are many advantages to using LED backlights in LCD TVs.
First of all, this method eliminates the color filter, which is the main absorber of light energy, which will lose more than 70% of the light energy. Figure 2 shows the loss of light energy after it has been emitted from the light source.
Figure 2: Loss of light energy from a backlight.
After removing the color filter, not only can 70% of the optical energy loss of the color filter be eliminated, but also the cost of the color filter can be saved. The cost of the color filter is 19% of the total cost, which is a considerable part (see Figure 3). In fact, after the filter is omitted, the yield can be improved and the production time can be reduced. These intangible improvements are very important.
The 25% cost of the backlight is based on CCFL data. If LED is used as the backlight, the cost will be much higher than this ratio, which will account for about 50% of the cost. After adopting the field sequential system, removing the color filter is equivalent to a 70% improvement in luminous efficiency, which is very impressive. This means that the number of LEDs can be reduced by 70% and the cost of LEDs can be reduced by 70%. Moreover, after the number of LEDs is reduced by 70%, the heat generation can be reduced by 70% and the power consumption can be reduced by 70%.
Not only that, after adopting the field order, since it is not necessary to divide each pixel into three sub-pixels, the thin film transistor TFT can be reduced by three times, so that the pixel area is increased by three times; or, if the pixel area is not maintained Change, in the same screen size, you can increase the resolution by three times (see Figure 4).
Figure 4: The field order does not require dividing each pixel into three sub-pixels.
It can be seen that the benefits of adopting the field order are countless.
The main problem with the field order is that the response time of the liquid crystal is required to be fast. Especially if a field frequency of 100 Hz or 120 Hz is used, the response time is required to be less than 3 ms or even shorter. Recent research has shown that the incorporation of metal nano-does can reduce the response time of liquid crystals to 500 microseconds.
However, only after adopting the field order can the LED backlight be commercialized, and it can truly compete with CCLF.
In 2006, Samsung introduced several LED-backlit LCD TVs with field sequential. One of them is a 40-inch LE40M91 with a resolution of 1366×768, a color gamut of 110%, a contrast ratio of 10,000:1, a brightness of 450 cd/m2, and a field rate of 100 Hz. The backlight system consumes less power than the CCFL. Reduced by 70%. The life of the backlight board is more than doubled, and more importantly, it is mercury-free and RoHS compliant. It is priced at $3,000.
Area brightness control
When using LED backlights, there has been a problem of how to make the brightness of the screen uniform. By using the area brightness control, the problem of uniform brightness can be solved, and the backlight power consumption can be further reduced. This is because the actual image brightness is different in each specific part, and some areas may be brighter and some areas are darker, so there is no need to use a uniform brightness backlight on the entire screen, which can be based on image content. Determined area brightness control. The method is: dividing the entire screen into, for example, 20 equal parts, measuring the highest brightness according to the image content, determining the required backlight brightness of the area, and then using the PWM technology for real-time control of the backlight LED brightness of the area. Thereby achieving the purpose of energy saving. After the area brightness control using the backlight, the image signal of the area should also be compensated accordingly to avoid brightness distortion.
Obviously, it is impossible to control the brightness of this area in CCFL backlight. On the one hand, because the shape of CCFL is long, it can't be divided into 20 equal parts by screen; on the other hand, it is not because of its brightness. It may be adjusted quickly according to the content of the image. However, when using LED as the area brightness control, only the direct type backlight can be used, and the side shot type cannot be used.
At the 2006 SID conference, Taiwan Jiaotong University proposed a kind of direct-down type as a whole, but in a single view, it uses side-emitting LEDs, which improves its light utilization, reduces the number of LEDs required, and increases the number of LEDs. effectiveness. This technology mainly uses side-emitting LEDs with a hexagonal light guide unit (see Figure 5).
Figure 5: "Direct-down" LED light guide system with side-emitting LEDs and hexagonal light guides.
As can be seen from the figure, in the area of ​​270 × 480 mm, a total of 14 R, G, and B are required, for a total of 42 side-emitting LEDs (Fig. 6).
Figure 6: Directional diagram of a side-emitting LED.
For a 32-inch screen, the area is 2.22 times larger than the above area, so about 93 LEDs are needed, which is far less than the typical direct-lit backlight system. However, because of the hexagonal structure, only RGB can be used, and RGGB cannot be used. This requires a green LED to have a sufficiently large light output.
Samsung recently introduced the 70-inch LED-backlit LCD TV PAVV LED 70 (LA70F91BD), which achieves a 180-degree viewing angle from top to bottom and left and right, using a 120Hz drive to increase image response time to 8ms. And Samsung's LED Smart Light Technology (Local Lighting) is used. It divides the entire screen into 192 areas. The brightness of each area is individually adjusted according to the content of the input image, thereby significantly improving the contrast and color performance of the picture. The resolution is 1920×1080, the brightness is 600cd/m2, the contrast ratio is 2,000:1, and the dynamic contrast ratio is 500,000:1, which can display 1.07 billion colors. Moreover, the power consumption is also reduced by 50%, which extends the life of the LCD screen. Moreover, the high-speed image mode (LED scanning) enables image quality without image blurring and image loss. The thickness of the product is controlled at 149mm.
South Korea's LG said that this technology is AFLC (Area Focused Luminance Control) technology, while Japan's NEC calls it AGCPS (Auto Gama Control and Power Saving). With this technology, power consumption can be reduced by half.
Therefore, if the field order is combined with the area backlight, it is possible to reduce the power consumption of the entire backlight system to less than half of the CCFL. This is a very amazing power saving.
Summary of this article
The use of LEDs as backlights for LCD TVs is an irreversible trend. According to estimates made by Insight Media in 2006, LCD TVs with LED backlights will exceed CCFL-backlit LCD TVs in 2010. The estimated absolute number is much smaller, but the relative value is still credible.
The LED backlight looks like there is nothing too complicated, just turn the LED on. But if you want to apply to LCD TVs, there are many very complicated technologies involved. This kind of technology cannot be done by enterprises that produce backlights. It must be done by the close cooperation between the companies that produce LCD TVs, the companies that produce backlights, and the companies that produce LED driver chips. If we don't start early, we will fall far behind Japan and South Korea, and we will once again face the dilemma of paying huge patent fees. "China is a big country in home appliance production, but it is not a strong country in home appliance production." We must completely change this situation! The only way is to increase investment in research and development. Many companies in the United States use more than 20% of their operating income for research and development. I hope that in the future, the government will also encourage and support enterprises to increase investment in research and development. This is the way out for Chinese companies.
(Edit: Xiaotang)
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