Edge-lit white LED backlight will shift from NB applications to LCD TVs

Japanese manufacturers conducted a survey on consumers, and most of them believed that 50mm thick LCD TVs were not attractive, while 30mm products were favored by consumers. In order to produce attractive products, edge-lit white LEDs will shift from NBs to LCD TVs.

It is generally believed that edge-lit LEDs are mostly used in portable device displays or notebook computer (NB) backlights. Now, as TFT LCD-related companies are moving towards ultra-thin LCD TV/panel designs, edge-lit white LED backlights have more advantages than edge-lit cold cathode fluorescent lamps (CCFLs) and RGB LEDs, as well as direct-lit CCFLs and LEDs, and therefore related applications are gradually attracting attention.

The thinnest edge-lit white LED backlight module design

Hitachi Ltd. will be the world's first manufacturer to market an ultra-thin LCD TV. It will start selling a 32-inch LCD TV with a thickness of only 35mm in December 2007. The TV uses CCFL backlight and will be priced at 230,000 yen (about US$2,116).

Hitachi Ltd. recently conducted a questionnaire survey among 60 respondents on "Preference for LCD TV thickness of 50mm and 30mm". Among them, only 1.6% of the respondents thought that 50mm thick LCD TV is attractive. In contrast, as many as 55.6% of the respondents said that 30mm LCD TV is attractive, indicating that ultra-thin LCD TV is one of the future TV design trends.

CCFL is the preferred backlight source for ultra-thin LCD TVs due to its low cost and high reliability. However, in terms of thickness alone, LED backlight has the advantage of being "ultra-thin" over CCFL.

Among the ultra-thin TV LCD panels recently released by various manufacturers, the thinnest one is the 40-inch LCD panel displayed by Samsung Electronics in late October. It uses an edge-lit white LED backlight and is expected to start mass production in mid-2008. Although Samsung Electronics' launch schedule is more than half a year later than Hitachi, its thickness is comparable to that of OLED TV (Sony's 27-inch OLED TV is 9mm thick).

Although the size of Sony's recently launched OLED TV is only 11 inches, the size of OLED will be developed to 40 inches in the future. Taking Samsung SDI's blueprint for future OLED size planning as an example, it will mass-produce 42-inch OLED panels in 2010. Therefore, if the LCD camp wants to be closer to OLED TV in terms of ultra-thin TV thickness, it must give priority to introducing edge-lit LED backlights into LCD panels for TVs.

Edge-lit white LED saves energy and space

There are two main factors that determine the thinness of LCD TVs: the thickness of the backlight module and the power substrate. However, since the entire area of ​​the backlight module is similar to that of the LCD panel, and the power substrate only occupies a part of the area of ​​the LCD TV, in other words, the thickness of the thinnest part of the LCD TV is closely related to the backlight module.

Edge-lit LEDs are used in TV LCD backlight modules, and their thickness is thinner than CCFL direct-lit, edge-lit, and LED direct-lit modules. For example, Samsung Electronics' 40-inch, 10mm-thick TV LCD panel has a backlight module thickness of only about 5mm.

Why is it difficult to reduce the thickness of a backlight module with a direct-type design? The reason is that if the light source (whether CCFL or LED) is too close to the optical film above, Mura (uneven light) will easily occur, so a certain distance must be maintained between the backlight source and the optical film; and if RGB LED is used as a direct-type backlight source, a longer light mixing space is required.

For edge-lit CCFL backlight modules, since the diameter of CCFL lamp tubes is higher than the thickness of LED packages, the thickness of LCD panels with edge-lit LED backlights can be made thinner than that of CCFL edge-lit ones.

Although the thickness of the backlight modules of edge-lit RGB LED and white LED can be made very thin, ultra-thin LCD TVs are also moving towards low power consumption design. Among them, white LED light sources are more able to meet the requirements of lower power consumption because white LED has better luminous efficiency than RGB LED. Therefore, in LCD panels of the same size, such as 40-inch TVs, the number of white LEDs used and the power consumption of the latter are about 1 times higher than those of the former.

Disadvantages of edge-lit white LED backlights in LCD TVs

Although the advantages of using edge-lit white LEDs in LCD TV backlights are mentioned above, some problems with this design cannot be ignored, such as the inability to achieve regional control, the number of LEDs used and the power consumption being higher than that of direct-lit white LEDs, and the development of light guide plates.

One of the biggest advantages of direct-type LED backlight (whether RGB or white LED) is that because LED is a point light source, it can be controlled by area. This feature allows the brightness of the LCD TV screen to be adjusted in different areas, the dynamic contrast ratio is greatly improved to more than 100,000 to 1, the power consumption can be lower than that of CCFL backlight, and the dynamic image quality is better. The above are functions that CCFL backlight cannot achieve.

However, since edge-lit LEDs are composed of point light sources to form a linear light source, rather than being distributed over the entire area of ​​a TV LCD panel, they cannot perform a regional control function, and are therefore not helpful in improving dynamic contrast ratios and dynamic image quality. Furthermore, in order to evenly distribute light energy over the entire TV LCD panel, edge-lit LEDs require high light density and luminous flux. Therefore, under the same size, the number of edge-lit white light LEDs used is slightly more than that of direct-lit white light LEDs. In addition, since the edge-lit LED light source is in full brightness, its power consumption is higher than that of a regionally controlled direct-lit LED backlight.

The cost of ultra-thin light guide plates currently used in LED backlight NBs is still high. It will be a challenge to achieve a large area of ​​more than 40 inches. Not only must the ultra-thin light guide plates be prevented from breaking over a large area, but how to evenly distribute the side light source to the other end also requires a test of the microstructure design on the light guide plate.

Because each backlight source has different advantages in TV LCD backlight, and they have a trade-off relationship in different requirements or designs, if better color rendering is required, RGB LED backlight source is used, but the cost and power consumption are the highest; if only cost factors are considered, CCFL is the best choice in recent years; similarly, if only thinness is considered, edge-lit white LED is the best design for ultra-thin LCD TV, but it must sacrifice the characteristics of direct-lit LED backlight area control. In order to provide consumers with a variety of choices for LCD TV, although no backlight source can meet all designs, at least edge-lit white LED has gradually emerged in LCD TV backlight from being only used in portable product displays in the past.