LED TVs: 10 things you need to know

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 while they've become best-known this 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 types of LED backlighting.

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. But to create superthin 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.

Samsung is the main maker of such sets this year with three series of edge-lit sets, although Sony did release one model earlier this year, the KDL-40ZX1M, and has another flagship series, the KDL-XBR10 models, waiting in the wings. Samsung, Sony, Sharp Toshiba, LG, and Vizio all have non-edge-lit, or "full array" models, available today. See our comparison of edge-lit vs. local dimming for more info.

Edge-lit LED-based LCD by Samsung explodes--photos

3. Of the two, local dimming can produce deeper black levels, but also creates "blooming."

Local dimming LED backlights can dim or turn off individually as needed.

The type of backlighting can impact how deep a shade of black a TV can produce. All current LED-based LCDs with rear-placed, "full-array" LED backlighting--except the Sharp LC-LE700UN series--feature a technology called "local dimming." With local dimming, specific areas of the backlight can be dimmed or brightened when different areas of the picture get darker or brighter.

With fluorescent backlighting and edge-lit LED backlighting, by contrast, the entire backlight dims or brightens at once, if at all.

Being able to dim specific quadrants 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 great example of local dimming done right is Samsung's UNB8500 series, which is one of the best-performing TVs 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 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 8500. 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. Blooming isn't an issue on other kinds of displays, including edge-lit models.

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.

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 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. But 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, 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. Samsung's LED-based, 46-inch UN46B6000, for example, costs just $18.73 per year to run (after calibration to equalize light output).

On the flip side, standard fluorescent backlighting is getting more efficient itself. To make a comparison, Samsung's same-size CCFL-based LN46B650 costs $25.96 per year. That's about 28 percent more than its LED brother--but the overall numbers are so low to begin with, we're talking about less than $8 total annually. In other words, it's not really enough to affect anyone's purchasing decision between CCFL and LED flavors of LCD TVs.

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 46-inch TC-P46G10 costs $60.69 per year after calibration, making the LED model more than three times as efficient. Check out our TV power consumption guide for more info.

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

While 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. With consumers seemingly smitten with super-thin TVs, engineers are trying improve edge-lit backlighting to the point where 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=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. On average, an "LED TV" costs at least $400 more than its non-LED counterpart. And top of the line models will run up to $2,000 more, depending on screen size (example: Samsung's 46-inch 8500 costs $3,500, while the non-LED LN46B750 costs around $1,700).

While many of these high-end sets are certainly excellent performers and slick to look at, the premium you pay for them usually doesn't measure up to how much better they perform on a percentage basis. In other words, a TV may cost 25 percent more but only have a picture that's 10 percent better.

Eventually, of course, as LED backlighting becomes a more universal feature, you won't have to pay the same premium as you do now.

9. Top-of-the-line LED TVs deliver as good a picture as plasma TVs--with one caveat.

LCD TVs have long been knocked for not producing the deep blacks of plasma TVs. Well, with the introduction of LED backlighting, blacks on high-end LCD 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 on LCDs remains a sticking point. With plasma, by comparison, you can sit to the side of the TV and the picture won't degrade.

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

You can have the best LCD TV 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.

Additional reading:

• LED TVs compared: edge-lit vs. local dimming
• CNET's Best LCD TVs
• See all LED-backlit LCD TVs