Technology for improving the backlight efficiency of LCD TVs


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.
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: The use of LED backlights in LCD TVs has many advantages over CCFL backlights.
Table 1: The use of LED backlights in LCD TVs has many advantages over CCFL backlights.

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.

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.
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.

Technology for improving the backlight efficiency of LCD TVs
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