LED LCD TVs: Back or Side-Lit?

Until recently, just about every LCD TV used cold cathode fluorescent lamps (CCFL) to provide the very bright backlights required to create an image using an LCD panel. (Even when showing an all-white screen, about 95% of the light is absorbed by the panel, so the backlight needs to produce a lot of light.) CCFL technology has some limitations, however, including the fact that it relies on some materials that are environmental hazards. As a result, TV makers are moving to using LEDs for to light up their LCD panels. These solid state light sources are highly efficient and produce a more natural range of color than CCFL, which makes for a more life-like image.

The question is where to put these LEDs. At first, manufacturers simply used them as a replacement for the CCFL modules, and created a huge matrix of LEDs behind the LCD panel. One bonus advantage of this approach is that you can create “localized dimming“. When a portion of the image is dark, such as a shadowy doorway, the LEDs behind that portion of the screen can be turned down. The net result is a darked black in those areas, which can benefit the “dynamic contrast” which refers to the amount of difference between the darkest part of the screen and the lightest part. Localized dimming has helped LCD TVs approach the black levels normally found only in plasma displays, so that they can achieve similar contrast ratings.

The problem with this matrix approach is that it requires hundreds of LEDs, which can be expensive and adds to the manufacturing assembly costs. You also need to provide sufficient space for the mixing of the light so that there are no hot spots on the screen over the individual LEDs. And perhaps most important of all, it creates problems with heat dissipation; the LEDS can put out a lot of heat, and they can fail if they get too hot.

Another solution is to put the LEDs along the edge of the LCD panel. This approach requires fewer LEDs but they must put out more light. This approach also requires complex diffusion plates to evenly distribute the light from the edge across the whole screen. On the other hand, it places all the LEDs along the edge of the screen where it is much easier to manage the heat problems. And as an added benefit, the diffusion layer can be very thin, resulting in a large panel that can be less than a half inch thick.

Both of these approaches cost more than a standard CCFL design, but they both can produce better quality images. The backlit approach offers the advantage of local dimming, but the edge-lit design gives you a thinner and lighter display that may be more reliable due to the more manageable heat.