Edge-lit Backlight Development

With the continuous development of technology, the side-side LED backlight will develop from a single LED on the upper and lower sides to the final single-sided single LED. Generally, a single LED backlight TV on both sides of a 32" that can be seen in the market uses about 120 to 150 LEDs. If the TV backlight is replaced by a single LED, the number of LEDs can be reduced to 80-100 LEDs (which can eventually be reduced The number of LEDs depends on the brand technology). If the technology is matched, in the foreseeable future, a single LED will be switched from the long side (up or down) to the short side (left or right). This kind of change will use less LED Number of particles.

Simple layout of LED backlight (click on the picture to enlarge)

Life extension

Reducing the use of LEDs not only has a positive effect on cost control, but we also see other positive effects on modules. For example, the temperature of the module will be reduced due to less use of LEDs. If we take the above 32” LCD TV as an example, reducing the number of LEDs can reduce the module temperature by about 10%-15%. Although we cannot scientifically calculate how much this number can extend the life of electronic parts or even TVs today, From a general technical point of view, the temperature reduction must have a positive effect on the life of electronic parts. This help is more obvious in larger area LED backlight TVs, because there are fewer LEDs that are relatively seldom used.

Wider viewing angle

In addition, the use of high-performance brightness enhancement film solutions also play a positive role in the TV viewing angle. Because the technical principle of the high-efficiency brightness enhancement film is to transmit the polarized light to the backlight module to circulate and reflect until it penetrates the glass. The backlight module that uses the brightness enhancement film improves the brightness by about 30% compared with the module that does not use the optical film. Since the high-efficiency brightness enhancement film is different from the general prism film, it does not need to sacrifice the viewing angle to increase the brightness, so such a high-efficiency brightness enhancement film is very popular with domestic and foreign TV manufacturers. With the increasing area of LCD TVs, consumers have begun to have certain requirements for viewing angles. A 47” inch LCD TV is placed in the middle of the living room. Of course, the head of the household hopes that guests sitting at any angle can enjoy the same quality of the TV screen.

Simple schematic diagram of high-efficiency brightening film (click on the picture to enlarge)

Energy saving

Of course, the public can most directly experience the benefits of edge-lit LED backlights, which is the reduction in the overall energy consumption of the TV. Ordinary 32" LED backlight TVs generally consume about 80W at the current level. This level is equivalent to Level 3 in the latest national energy efficiency standards.

If manufacturers want to improve TV energy consumption standards, there are many corresponding solutions, but using high-performance brightness enhancement film should be the simplest and direct and effective way to improve energy consumption performance. If combined with a high-performance brightness enhancement film, the energy consumption can be reduced by about 20%-30% while maintaining the same level of brightness (the final performance depends on the technology of each brand). From a numerical calculation, the energy consumption of the TV can basically be improved from 80W to about 60W through the high-performance brightness enhancement film. The improvement of energy consumption not only allows manufacturers to vigorously cooperate with national environmental protection policies, but also helps consumers with related electricity bills.

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, edge-lit single-sided single LEDs must be the ultimate destination of LED backlights.

Comprehensive analysis of edge-lit advantages

The side-light type is also called the side-in 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 must be uniformly output to the TV screen by the light guide plate. Therefore, it is difficult to adjust the brightness of a certain area of the screen. In the direct type, since a certain local LED switch can be realized through the LED driver chip, the local dimming has obvious advantages. Therefore, a new locally dimmable matrix backlight solution that combines the advantages of direct-down and side-in technologies has emerged, and has become a key research direction for related backlight manufacturers.

Regarding the matrix backlight solution that can be locally dimmed, it should be said that it is a backlight technology that combines side-in and direct-down. According to Lu Yihang, a senior engineer from 3M's optical system department, related backlight factories have been doing some basic research on this technology as early as 2009. At present, it seems that many of these backlight methods are classified as direct backlight technology, referred to as matrix backlight for short.

The advantages of this technology should be said to concentrate the advantages of the two technologies of direct down and side entry:

1. Partial dimming: When the early LED TVs were just emerging, there were two kinds of technologies: direct-type and side-type. One of the biggest weaknesses of the side-entry type is that it cannot be locally dimmed. Since the LED lights are concentrated on the edge of the TV, the light must be uniformly output to the TV screen by the light guide plate. Therefore, it is difficult to adjust the brightness of a certain area of the screen. In the direct type, since a certain local LED switch can be realized through the LED driver chip, the local dimming has obvious advantages. As a combination of the two technologies, the matrix backlight can also perform the function of local dimming, and the controllable partition is completely determined by the LED drive. In some of the current early matrix products, there will be more controllable partitions than the current direct type.

2. Thickness: Regarding the thickness, it is a fatal weakness of the direct backlight. Even if Sharp uses LED+lenslet to greatly reduce the thickness of the elimination of LED lights, the thickness of the TV is still much higher than that of the edge-type guide. The thickness of the light board is also the main reason for the complete failure of the direct type among the two LED backlight technologies. It can be said that the matrix backlight perfectly solves this contradiction.

3. Picture contrast: As an important point of image quality expressiveness, relatively speaking, contrast is a weakness of LCD. Since the twist of the liquid crystal can never achieve full switch 0,1 control, the contrast of the side-type liquid crystal is usually only a few thousand even if the dimming technology is applied. The direct-down dimming technology, if the LED is turned off when the display is black, its contrast can be greatly improved. According to LG, their contrast ratio can reach 7 million:1.

4. Energy saving: In the darker part of the image, reducing the power consumption by turning off or reducing the LED current is also an advantage of the direct backlight. Similarly, the matrix backlight also has the same advantages.

5. Heat dissipation: As a technical point of the side-lit backlight, since the matrix backlight disperses a large number of LEDs in the backlight instead of focusing on one side of the TV, the heat dissipation design is relatively simple.

Although the matrix backlight has some of the above advantages, it should be said that it is a new direction for LED design in the future, but its shortcomings are also obvious at present:

1. Since the matrix backlight requires precise control of the segmented backlight, generally speaking, the design of the circuit will be much more complicated; in addition, the selection of LED chips and some control chips will also increase the cost.

2. In terms of assembly, the matrix backlight is correspondingly more complicated than the current direct type and side 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. Because the direct type is under the diffuser, the light will be homogenized, while the matrix type is the lower diffuser. The film is uniform, so that the influence between adjacent zones needs to be smaller than the direct type, and the contrast will be more finely controlled. However, since the partitions have less mutual influence, if the LED in a certain area is damaged, the optical effect defect will be much more obvious than the direct type. In addition, its thickness advantage is extremely obvious. However, since each partition of the matrix is a relatively independent side-lit backlight, the complexity of the mechanism will be much higher. At present, many direct-type partial light control are controlled by the light of some LEDs in one area, while the matrix type must control each sub-block, so the circuit is relatively complicated.

Compared with the side-entry type, the matrix type solves the problems of local light control and intelligent energy saving, greatly improves the picture contrast, and has a relatively simple heat dissipation design; and because the distance of the light guide backlight is much lower than that of the side-entry type, its optical The efficiency will be higher than that of the side-entry type, and the thickness can be completely comparable to that of the side-entry backlight. The only disadvantage compared with the side-lit backlight is the cost and manufacturing complexity mentioned above. Usually a light guide plate needs to be realized through hundreds of partitions, which means that the number of parts and assembly complexity are correspondingly increased by hundreds of times. Since the backlight components are display-related, the defect rate in the assembly process I believe it is also a big problem.