Editors' note: Updated June 3, 2010

I've written articles in the past explaining various TV technologies, including the differences between 720p and 1080p and 120Hz and 240Hz LCD TVs. But with Samsung, LG, Sony, and other manufacturers pushing so-called LED TVs these days, it's high time that I--with an assist from our resident video guru, David Katzmaier--sort through all the marketing mumbo jumbo and provide some insight into just what an LED TV is. Here goes.

1. An LED TV is not a new kind of TV.

I appreciate a good marketing ploy as much as the next guy, but an LED TV is just an LCD TV that's backlit with light-emitting diodes (LEDs) instead of standard cold-cathode fluorescent lights (or CCFLs). And though they became well-known last year with Samsung's ultrathin models, LED-backlit LCDs have been on mainstream store shelves since 2007, when Samsung's LN-T4681F debuted.

Unlike plasma and OLED, which are emissive technologies where each pixel is its own discrete light source, LCD is a transmissive technology where each pixel has to be illuminated from behind, or backlit.

2.There are two LED backlight configurations

Initially, LED-based displays like the Samung LN-T4681F were backlit by what's referred to as a "full array" of LEDs behind the LCD, across the back of the panel--just like a standard CCFL backlight. But to create even thinner TVs, engineers needed to eliminate that extra layer of LEDs and move it to the sides of the display. With this form of backlighting, the LEDs are affixed to all four sides of the TV and light is projected inward to the middle of the TV via "lightguides." These types of TVs are commonly referred to as "edge-lit" LED-based LCDs, and are by far the most common available today.

3. Each configuration may also offer "local dimming."

All current LED-based LCDs with rear-placed, full-array LED backlighting--except the Sharp LC-LE700UN series from 2009--feature a technology called "local dimming." With local dimming, portions of the backlight can be dimmed or brightened independently when different areas of the picture get darker or brighter. For example, the LEDs behind the words in a credit sequence can illuminate while the ones behind the black background remain dim.

Being able to dim portions of the screen helps reduce the amount of light that leaks through to darkened pixels, and the end result is blacks that appear darker and more realistic. Since black levels are crucial to contrast ratio, the deeper the blacks, the more the picture--and colors--appear to pop. Also, the image as a whole will seem crisper. A couple of examples of local dimming done right are Samsung's UNB8500 series and LG's LH8500 series--respectively the best and second-best LCDs we've ever tested.

One downside to local dimming is an effect called "blooming," where brighter areas bleed into darker ones and lighten adjacent black levels. This "blooming" effect varies widely from model to model; it's pretty noticeable on the Toshiba 46SV670U, for example, and much more difficult to notice on the Samsung and LG 8500 sets. Incidence of blooming is directly related to how many local-dimming LED elements ("dimmable zones") are behind the screen, but some manufacturers won't divulge that information.

With standard CCFL backlighting and most edge-lit LED backlighting, the entire backlight dims or brightens at once (aka "global dimming"), if at all. There are a few 2010 LED TVs from Samsung and LG, however, that can perform a semblance of local dimming from an edge-lit configuration. Samsung calls its technology "precision dimming" to help differentiate it from true local dimming, and it's found only on the high-end UNC8000 series. (Samsung has not announced a 2010 TV with full-array local dimming.) LG calls its version "LED Plus," and it's found on the LH5500 and LX6500 series. In our tests we liked Samsung's version better, but neither performed as well as true local dimming.

4. Edge-lit TVs are really thin, but uniformity suffers.

As I said, the key benefit to an edge-lit LED-backlighting scheme is that manufacturers can make thinner TVs. However, the downside is that the backlighting isn't quite as uniform. With edge-lit displays, if you put a white image up, you may notice that the outer edges of the screen appear brighter, or "hotter." Also, when you put up an all-black image, the edges of the screen will appear lighter (grayer).

5. LED backlighting of either variety doesn't improve LCD's poor off-angle viewing.

Unlike with plasma, one of the big downsides to LCD TVs is that the picture degrades if you're sitting off to the side or the TV is placed too high or low, based on your eye level. LED backlighting doesn't change any of this and, in some cases, may actually make things worse.

The Samsung 8500, for example, currently offers among the best black levels of any LCD we've seen--so long as you're sitting in the sweet spot, with the middle of the screen between your eyes. Move a few feet to the left or right and you'll notice that the picture doesn't look as good. Why is this so apparent? Well, the problem is that you're starting with such a good picture, so you're more apt to notice the difference when you move to the side or stand up and look down at the TV. With a TV picture that doesn't look as good to begin with, the difference doesn't look as stark when you move off axis. Make sense?

6. LED backlighting is even more efficient than standard fluorescent backlighting.

It's definitely true that LED backlighting can cut down on power use, and some LED-backlit LCDs are, inch for inch, the most efficient flat panels available. On the flip-side, standard fluorescent backlighting is getting more efficient itself.

Our survey of TVs we've reviewed since 2008 finds that LED TVs use, on average, about 101 watts, compared with 111 watts for standard LCD TVs. Of course, screen size and picture brightness can affect power use significantly, but even in the largest screen sizes LED saves only about $20 per year.

On the other hand, it's also worth noting that LCD TVs of both varieties continue to be significantly more energy efficient than plasma TVs. For example, Panasonic's 50-inch TC-P50G20 plasma costs $46.55 per year after calibration, whereas the 52-inch Sony KDL-52NX800 LED costs $20.96.

7. LED backlighting will get better--but how much better is debatable.

Though it's true that manufacturers' backlighting schemes will continue to improve with time, we've heard from industry insiders that the real advancements will be in edge-lit technology. Engineers are trying to improve edge-lit backlighting to the point that it equals or even surpasses full-array LED backlighting.

One of the issues with full-array backlighting (that features local dimming) is that to truly perfect it would require 2.1 million LEDs to individually light 2.1 million pixels (in a 1080p TV). Adding that many LEDs simply isn't cost efficient, and sources tell us that for the foreseeable future, engineers have maxed out the number of LEDs they can add to a panel without making the end product prohibitively expensive.

8. LED equals higher price tag.

While we're on the topic of expensive...as you're probably aware, LED-powered LCD TVs cost more than non-LED models. The profusion of LED models in 2010 has improved selection, especially among smaller screen sizes, but the cost premium for LED is typically at least $400.

LG's least-expensive 32-inch LED, for example, is the 32LE5400 ($1100 at Best Buy currently), whereas the company's most-expensive non-LED at that size is the 32LD550 ($700). Sony's KDL-46EX600 (LED: $1,350) and KDL-46EX500 (non-LED: $990) provide another example at 46 inches, as do Samsung's UN46C5000 (LED: $1,350) and LN46C550 ($950). In 2010 LED is priced as the major step-up feature of LCD TVs.

On the other hand, with the entry of brands like Vizio and even Westinghouse into the LED TV market, we expect competition to intensify and prices to fall quickly over the next year. The excellent Vizio VF551XVT ($1,400) is a 55-incher that costs hundreds less than similar models from traditional brands.

9. Top LED TVs can come close to the picture quality of the best plasmas, but they still have drawbacks.

LCD TVs have long been knocked for not producing the deep blacks of plasma TVs. Well, with the introduction of LED backlighting with local dimming, blacks on the best LED TVs can go toe-to-toe with the blacks on some of the best plasmas, and the picture is outstanding. Also, as noted, LED-backlit LCD TVs are more energy efficient than plasmas and weigh less. But off-angle viewing and picture uniformity remain a sticking point. With plasma, by comparison, you can sit to the side of the TV and the picture won't degrade, and blooming and other uniformity problems are nonexistent.

On the other hand, we haven't seen any major picture quality benefits to edge-lit LED technology--with or without dimming. In some cases, LED TVs have worse picture quality than their standard CCFL-backlit cousins. As usual, performance varies widely from model to model, but don't expect LED to automatically equate to a better picture.

10. If you don't have your picture settings correct, LED or non-LED won't make a difference.

You can have the best HDTV in the world with the latest and greatest technology, but if it's not set up correctly, it can look pretty run of the mill. Luckily, with every TV David Katzmaier reviews here at CNET, he posts his optimal settings in the HDTV picture settings forum.

As always, feel free to add your own comments.

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