Recently, the topic of LED backlight LCD TVs has been extremely enthusiastic. Although Korean manufacturers have adopted a LCD TV with LED backlights called "LED TV" to enhance the brand image strategy, the author believes that the name "LED TV" is likely to be an overstatement of advertising language that misleads consumers ( It is easily confused with a display device using an LED unit, so it will be referred to as "LED LCD TV" hereinafter. Regarding the LED LCD TV market, Samsung Electronics, a leading South Korean company, will be the leader, and the market is expected to grow significantly this year. Sharp also intends to equip LED backlights on a new generation of TVs with ultra-low power consumption as a "selling point".
Recently, various LCD TV manufacturers have started the LED LCD TV business mainly because of the following points.
First, the luminous efficiency of white LEDs has increased year by year, and the mass production effect has continuously lowered the unit price. At present, high color rendering LEDs also achieve a luminous efficiency of about 100 lm/W, which is recognized by the industry to be around 150 lm/W around 2015. At the beginning of the LED backlight LCD panel, the RGB tri-color LED was used, and the NTSC ratio of the color performance range was over 100%. But the power consumption is about two times, the price is also very high, and can only be used on a very small number of high-end models. If a white LED is used, although the color performance range is not significantly larger than the cold cathode tube (CCFL) type, it is very competitive in terms of power consumption and cost.
Second, with the advancement of the light guide plate technology, the Edge Light method can obtain uniform diffused light by using the thin shape of the LED regardless of the end type or the direct type. Thus, a module having a thickness lower than that of the original and having a power consumption equal to or lower than that of the CCFL type and having a substantially equivalent cost can be realized. LED backlights make it easy to adjust the brightness of the display, so you can also use the dynamic contrast adjustment technology widely used in portable terminals. Therefore, the power consumption of the standard image can be reduced as compared with the power consumption at the time of all lighting. That is to say, if it is an average image, the power consumption can be completely lower than the CCFL type. If local dimming is used, it is expected to further reduce power consumption and improve contrast.
Possibility of using local dimming technology
A two-dimensional local dimming technique that achieves maximum contrast while minimizing the power consumption of a liquid crystal television. The technology divides the screen into hundreds of blocks, adjusts the brightness of the direct-type LED unit corresponding to each block in real time, and corrects the signal so that each block forms the driving mode of the optimal contrast curve.
This technology has a fatal problem because of the increase in circuit scale and the increase in the number of electronic components without cost advantages. That is, the absolute brightness must be used to compensate the output brightness of all LED units from low to high brightness. To solve this problem, pre-delivery inspections and adjustments take a lot of time and are very costly. Moreover, each LED unit produces different degrees of brightness degradation, and aging or uneven brightness may occur. Therefore, it is generally believed that the possibility of two-dimensional local dimming technology popularizing in the mass space of liquid crystal televisions is very low.
If two-dimensional local dimming is not required, the use of an end-illuminated LED backlight is easier than in the direct type. From this point of view, Samsung's strategy of "full use of end-illuminated white LED backlights" is very clear.
Samsung's end-illuminated white LED backlight LCD module performance indicators. The author took photos at IMID2009/KES2009
As shown in the above figure (Figure 1), Samsung's TV-based end-illuminated white LED backlight LCD module exhibited at the Korea Display Related Society and Exhibition "IMID2009/KES2009" has a power consumption value of "32" Max 55W Typ 38W. And "40-inch Max 76W Typ 53W". The reason why the power consumption value at Typ (standard image display) is smaller than Max (all white display) is considered to be the result of dynamic contrast adjustment in which the brightness of the backlight can be changed. If the power consumption of the digital tuner, image processing circuit, and audio circuit is about 20W, the power consumption of the 32-inch TV can be reduced to less than 60W when the standard image is displayed, and the 40-inch model can be reduced to less than 75W. In the future, white LED LCD TVs may become the flagship models of various manufacturers that are promoting low-power consumption strategies.
At the display related exhibition "IDW2009" held in December 2009, there was a technical report on the local dimming of the end-illuminated white LED backlight (Paper No. DES2-3). Since the dimming is performed by the six end-illuminated LED lamps, the dimming block cannot be clearly divided, but the dimming function is realized by compensating for the light diffusion characteristics on the light guide plate (Fig. 2, Fig. 3). Even a darker image can only be dimmed by a quarter or so, so the dimming effect is not obvious. However, if the optical characteristics of the light guide plate and the arrangement of the LEDs can be optimized, the low power consumption effect by the dimming can be further improved. In the future, it is also possible to use this dimming technology for end-illuminated LED backlights.
The dimming operation state of the end-illuminated LED backlight. The power consumption of the 32-inch TV is 9.9W (paper number DES2-3). There are 6 LED positions, two on the upper and lower sides, one on each side and one on the middle. The upper part of the screen is brighter and the lower part is darker. With this type of backlight, only the central portion (about 10% of the area) has a brighter image. The author took a photo of the developer meeting at IMID2009
When the end-illuminated LED backlights are all lit. The 32-inch TV consumes 36.1W (paper number DES2-3). The author took a photo of the developer meeting at IMID2009
Perhaps few people can compare and distinguish images of white LED backlight LCD TVs and CCFL backlight LCD TVs. However, if LED backlights can be used to enhance their image as “green goodsâ€, then it is of great significance for companies to use LED backlights. Moreover, the luminous efficiency of white LEDs is expected to increase year by year, so it is also a major advantage to develop a future low power consumption plan. With this in mind, LCD TVs with white LED backlights should be faster than expected.
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