The edge-lit type places the LEDs on the upper and lower sides of the backlight, and the layout adjustment greatly reduces the use of LEDs, thereby reducing production costs and energy consumption, which has great benefits for manufacturers and consumers. However, its weakness of non-local dimming has led to the birth of a local dimmable matrix backlight solution, which is a backlight technology that combines side entry and direct illumination.
Recalling the history of TV development, the traditional CRT TV at home must be the product that everyone thinks about for the first time. In 2000, plasma TVs began to appear; around 2003, CCFL (Cold Fluorescent Tube Light Source) LCD TVs began to appear and become popular; until 2010, LED backlight technology gradually replaced CCFL. Most people inside and outside the industry should agree that LED will completely replace CCFL as the main TV backlight source in two to three years.
Most people know that LED backlight has several advantages, including thinner TV, lower energy consumption, richer colors, etc. These advantages are reflected in the fact that the general public can easily compare with CCFL TV. In addition, the price of LED TV has been declining, which makes LED TV more advantageous in cost performance. These are the main reasons why LED-backlit TVs can be popularized in the market instantly.
During the development of LED backlight, in fact, technology has been improving. From the initial dot matrix development to today's edge-lit type is one of the important technical improvements. Under various obvious advantages, manufacturers have actively developed LED-backlit TVs in recent years. Domestic brands such as Hisense, TCL, Konka, and Skyworth are all working hard to develop their own LED backlights. Among them, technological improvement and cost control are important parts. In order to allow modern LED backlights to quickly become popular in the market, everyone has begun to consider using fewer LEDs to reduce product costs. But the most important thing is to significantly improve energy consumption, in order to match the country's latest environmental protection and energy conservation policies. Less LEDs are used to adjust the backlight layout. The original dot matrix type placed the LEDs evenly on the TV backlight, while the edge-lit type placed the LEDs on the top and bottom of the backlight. This layout adjustment greatly reduces the use of LEDs, thereby reducing production costs and energy consumption.
But most people think that LEDs are less used, will the brightness of the screen decrease? Hasn't the TV picture quality also decreased? This problem has been predicted by the manufacturers early in the morning, so most manufacturers will find the most ideal way to solve it.
Figure title: Dot-matrix and edge-lit LED layout
The most common solution in the market today is to use high-efficiency brightening films to improve brightness. The use of brightening films is different from other solutions. It is simpler and more direct than increasing the glass transmittance. It does not rely on the cooperation of the upstream glass supplier ’s research and development progress. Problem (generally high-efficiency LEDs are two to three times more expensive than ordinary LEDs). Brightening film is simply an ultra-thin plastic optical sheet. Using it in an edge-lit LED backlight not only does not affect the overall thickness of the TV, but also complements the brightness required by the TV screen under cost control. Now the DBEF produced by the American 3M company can generally increase the brightness of about 30%. If the TV backlight itself has only 300 nits of brightness, using DBEF without any changes, the brightness can be increased to about 390 nits.
Use high-efficiency brightening film to improve picture brightness
Development of edge-lit backlight
With the continuous development of technology, the side-type LED backlight will develop from a single LED on the upper and lower sides to the final single-sided single LED. Generally, the 32 "single LED backlight TVs on both sides of the market can be used, ranging from about 120 to 150 LEDs. If the TV backlight is changed to a single side single LED, the number of LEDs can be reduced to 80-100 (finally can be reduced) The number of LEDs depends on the brand technology. If the technology is matched, in the future, a single LED will turn from the long side (up or down) to the short side (left or right), such changes are less used LED Number.
Simple layout of LED backlight
Life extension
Reducing the use of LEDs not only has a positive effect on cost control, we also see other positive effects on the module. For example, the module temperature will be reduced due to less use of LEDs. Taking the above 32 â€LCD TV as an example, reducing the number of LEDs can reduce the module temperature by about 10% -15%. Although we ca n’t scientifically calculate how long this number can extend the life of electronic parts or even TVs, but From a general technical point of view, the reduction in temperature will definitely have a positive effect on the life of electronic parts. This help is more obvious in larger area LED-backlit TVs because there are fewer LEDs that are relatively rarely used.
Wider viewing angle
In addition, the use of high-efficiency brightening film solutions also plays a positive role in helping TV viewing angles. The technical principle of the high-efficiency brightness enhancing film is to make the polarized light circulate and reflect in the backlight module until it penetrates the glass. The backlight module using the brightness enhancement film has improved the brightness by about 30% compared with the module without the optical film. Since the high-efficiency brightening film is different from the general prism sheet, it does not need to sacrifice the viewing angle to increase the brightness, so such high-efficiency brightening film is very popular with domestic and foreign TV manufacturers. The LCD TV area is getting larger and larger, and consumers have certain requirements for viewing angles. A 10,000-inch 47 â€LCD TV is placed in the middle of the living room. The owner of course hopes that guests sitting at any angle can enjoy the same picture quality.
Simple schematic diagram of high-efficiency brightening film
Energy saving
Of course, the public can most directly experience the benefits of the edge-lit LED backlight, that is, the overall energy consumption of the TV is reduced. Ordinary 32 "LED-backlit TVs, the current level of general energy consumption is about 80W. This level is equivalent to the third level in the latest energy efficiency standards launched by the country.
If the manufacturer wants to improve the TV energy consumption standard, there are many corresponding solutions, but the use of high-efficiency brightening film should be the simplest and directly and effectively improve energy consumption performance. If combined with a high-efficiency brightening film, the energy consumption can be reduced by about 20% -30% while maintaining the same brightness level (the final performance depends on the technology of each brand). Calculated numerically, TV energy consumption can basically be improved from 80W to about 60W through high-efficiency brightening films. The improvement in energy consumption not only allows manufacturers to vigorously cooperate with national environmental protection policies, but for consumers, the related electricity cost expenditures are also slightly helpful.
From the above technical analysis, we see that the edge-lit backlight design is of great benefit to manufacturers and consumers. In the near future, the edge-lit single-sided single LED must be the ultimate destination of LED backlight.
Comprehensive analysis of edge-lit advantages
The side-light type also has a name called side-entry type, and its biggest weakness is that it cannot be locally dimmed. Since the LED lights are concentrated on the edge of the TV, the light guide plate must uniformly output the light onto the TV screen. Therefore, it is difficult to adjust the brightness of a certain area of ​​the screen. The direct type, because the LED driver chip can be used to achieve a certain local LED switch, so local dimming has obvious advantages. Therefore, a new locally dimmable matrix backlight solution that combines the advantages of direct and side-entry technology has emerged, and has become a key research direction for related backlight manufacturers.
Regarding the matrix-type backlight scheme that can be locally dimmed, it should be said that it is a backlight technology that combines side-in and direct-down. Lu Yihang, a senior engineer from the optical system department of 3M Company, introduced that the technology had already done some basic research in the backlight factory as early as 2009. At present, many people have summarized this backlighting method as direct-type backlighting technology, referred to as matrix backlighting for short.
The advantages of this technology should be said to focus on the advantages of the two technologies of direct and side entry:
1. Local dimming: At the beginning of the early days of LED TV, there were two types of technology, direct-type and side-entry. One of the biggest weaknesses of the side-entry type is that it cannot be dimmed locally. Because the LED lights are concentrated on the edge of the TV, the light guide plate must evenly output the light onto the TV screen. Therefore, it is difficult to adjust the brightness of a certain area of ​​the screen. The direct type, because the LED driver chip can be used to achieve a certain local LED switch, so local dimming has obvious advantages. As a combination of the two technologies, the matrix backlight can also achieve the function of local dimming, and the controllable partition is completely determined by the LED driver. In some of the early matrix products, there are more controllable partitions than the current direct type.
2. Thickness: Regarding the thickness, it is a fatal weakness of the direct-lit backlight. Even if Sharp uses LED + lenslet to significantly reduce the thickness of eliminating LED lights, the thickness of its TV set is still much higher than that of the side-entry guide. The thickness of the light board is also the main reason why the direct type has completely failed in the two LED backlight technologies. It can be said that the matrix backlight perfectly solves this contradiction.
3. Picture contrast: As an important aspect of image quality, contrast is relatively a weak point of LCD. Since the twist of the liquid crystal cannot always achieve the 0,1 control of the full switch, the contrast of the side-entry liquid crystal is usually only a few thousand even if the dimming technology is applied. The direct dimming technology, such as turning off the LED when displaying a black screen, can greatly improve the contrast. According to LG, their contrast ratio can reach 7 million: 1.
4. Energy saving: In the darker part of the image, reducing power consumption by turning off or reducing the LED current is also an advantage of direct backlight. Similarly, matrix backlight has the same advantages.
5. Heat dissipation: As a technical point of the side-entry backlight, the matrix-type backlight disperses a large number of LEDs in the backlight instead of focusing on one side of the TV, which is relatively simple for heat dissipation design.
Although the matrix backlight has some of the above advantages, it should be said that it is a new direction of LED design in the future, but its shortcomings are also obvious at present:
1. Since matrix backlighting requires precise control of the block backlighting, in general, the design of the circuit will be correspondingly more complicated; in addition, the selection of LED chips and some control chips will also increase the cost.
2. In terms of assembly, matrix backlighting has correspondingly higher process complexity than current direct type and side entry type. Therefore, the yield and cost of assembly are also a major factor restricting it.
Compared with the traditional direct backlight, the accuracy of the local dimming of the matrix type backlight is higher than that of the direct type. Since the direct type has uniform light in the lower diffuser, the matrix type is in the lower diffuser film. Even light distribution, so that the influence between adjacent partitions needs to be less than the direct type, and the contrast control will be finer. However, due to the small mutual influence of the partitions, if the LED in a certain area is damaged, the defect of its optical effect will be much more obvious than that of the direct type. In addition, its thickness advantage is extremely obvious. However, since the partition of each block of the matrix is ​​a relatively independent side-entry backlight, the complexity of its mechanism will be much higher. At present, many direct-type localized light control of the partition is controlled by the light of some LEDs in an area, while the matrix type must control each sub-block, so the circuit is relatively complicated.
Compared with the side entry, the matrix type solves the problems of local light control and intelligent energy saving, greatly improves the contrast of the picture, and is relatively simple in heat dissipation design; and because its light guide backlight distance is much lower than the side entry type, its optical The efficiency will be higher than that of the side-entry type, and the thickness can also be completely comparable to that of the side-entry backlight. The only shortcoming compared to the side-entry backlight is the aforementioned cost and manufacturing complexity issues. Usually a light guide plate needs to be realized by hundreds of partitions, which means that the number of parts and assembly complexity are also increased by hundreds of times. Because the backlight components are display-related, the defect rate during the assembly process I believe it is also a big problem.
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