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What is a direct-lit LED LCD TV?

Consumer Reports News: May 08, 2012 04:38 PM

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During the Consumer Electronics Show back in January, we noticed that some TV manufacturers were debuting LCD TVs with a new type of LED backlight, a direct-lit LED backlight.

At the time we didn't know a lot about direct-lit LED backlights, but based on where these TVs were positioned in the manufacturer's lineup—typically in or close to the entry-level series—it appeared that direct-lit LED backlight technology was able to bridge the price gap between lower-cost CCFL (fluorescent) backlights and the newer edge LED backlights that have become increasingly common in many LCD TVs. This year, for example, both Samsung (EH series) and LG (LS3400 series) are offering lower-priced LCD TVs with direct-lit LED backlights.

So what exactly is a direct-lit LED backlight?

Most of the LCD TVs in our TV Ratings now come with edge LED backlights, where the LEDs are arrayed around the perimeter—typically the sides—of the TV. A waveguide (or lightguide, or diffuser) then spreads the light across the entire panel, hopefully in a uniform fashion.

The other type of LED-based backlighting we've seen—now less common—is a full-array LED backlight, where rows of LEDs are spread across the entire back panel of the TV. Using a feature called local dimming, the LEDs are divided into a number of zones that can be individually controlled, so some portions of the backlight can be dimmed while other remain illuminated. In some instances, we've seen this improve contrast and black levels. Some edge LEDs also have a form of local dimming, but this has had a negligible effect on contrast or black levels on most of the TVs we've tested.

Direct-lit LED backlights are an offshoot of full-array backlighting, in that they use LEDs spread across the entire back panel of the TV. (The TV's spec page may just refer to these TVs as having a full-array backlight.) However, there are a few key differences compared to the more expensive full-array LED sets we've tested previously. One is that they use significantly fewer LEDs across the back of the panel. Another is that these sets lack the local dimming feature.

In addition, these TVs are much deeper than previous LED-backlit models, especially the ultra-thin edge LED sets. In fact, they more closely resemble LCD TVs with CCFL backlights. The reason: Because fewer LEDs are used, they have to be moved farther away from the screen to provide adequate light coverage across the panel, much the way the beam of a flashlight gets wider as you move it away from an object.

But the primary reason we're seeing direct-lit LED backlights is price. Though they do cost a bit more than CCFL models, they're less expensive than edge LED models, since they don't require the lightguide plates. And in a tough economy, this lets manufacturers offer less-expensive models without having to forgo what has become perceived as a key LCD feature—an LED backlight. Although direct-lit LED backlights are no slimmer than CCFL-based LCD TVs, they do offer an advantage over models with fluorescent lights: better energy efficiency.

We're currently testing a few Samsung models that use direct-lit LED backlights, so make sure to check out our TV Ratings in the next week or so to see how these sets fared. We'll also be watching the market to see if more manufacturers embrace this type of backlight in their lower-priced LCD TV models.

James K. Willcox

   

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