One of the top buzzterms in LED LCD marketing is "local dimming." Ostensibly, local dimming can dim the area of the screen that needs it, while keeping the bright parts of the screen bright. This technology can really increase the contrast ratio to make a better image.
But not all local dimming works the same, and certain types of local dimming work better than others.
Here's what to look for.
This article is a distilling/expansion of a sliver of the longer LED LCD backlights explained. Check out that article for even more info, or if you're really only interested in local dimming... read on!
Local dimming was developed to improve this aspect of LED LCD performance. By dimming the parts of the screen that should be dark (a character in shadow, perhaps), and keeping bright the parts of the screen that should be bright (a nearby well-lit window, say), you can improve the apparent contrast ratio. As LCD technology has advanced, local dimming has as well. As the LCD industry moved towards the cheaper, thinner edge-lighting methods, local dimming was adapted to work with these TVs too.
Full-array local dimming
This is the full monty. The name refers to an array of individual LEDs behind the LCD panel, all pointing out through the screen toward your eyeballs. A mock-up example is shown at right, if the front LCD layer was removed and the LED backlight exposed.
While individual control of all these LEDs would be ideal (though rarely implemented), the most common method is a set number of "zones." Depending on the TV, these could number in the dozens or more. Vizio's high-end Reference series has 384 zones, the most we've ever heard of. Unfortunately, most LED TV makers don't disclose the number.
Each zone is responsible for a certain area of the screen. Objects smaller than the zone (stars in the night sky, for example), don't benefit from the local dimming, and can look muted as a result. Also, if a zone is lit, and an adjacent zone isn't lit, you could see a halo/bloom as that part of the screen becomes brighter than its neighboring zone. This artifact is commonly known as "blooming."
The downside is size and cost. The LEDs have to be set back from the screen a little (the farther away, the fewer you need to cover the same area), so there's additional depth compared to the edge-lit models.
That's also a lot of LEDs back there, and those cost money. Last year there weren't any TVs with full-array local dimming beyond the $40,000 Samsung S9. In 2014 more models are coming out for prices in the four, rather than five, figures. The cheapest we know of so far will be Vizio's P series, which features a 50-inch size for $1000.
Direct local dimming
An offshoot of full-array, direct local dimming simply means fewer LEDs. They're still arranged behind the screen, but there are fewer of them, and not as many dimmable zones.
Vizio is the first, and still the only, TV maker to market direct local dimming as distinct from full-array. Select sizes of its 2013 and 2014 E series TVs include the feature, and in CNET's reviews of the 42-inch and 50-inch models, it made a real improvement in picture quality. As you'd expect, the dimming is less precise and evinces more blooming than full array, but it's still better than no local dimming in our book. We're looking forward to 2014's iterations to see whether the company can improve its dimming -- or if other makers of direct-backlit LED TVs adopt it.
Edge-lit local dimming
The most common variety is local dimming on edge-lit LED LCDs. With edge-lit LCDs, all the LEDs are along the edge of the TV, facing the center of the screen.
Local dimming, in this case, becomes a little looser of a term. Worst-case, "local" could be nearly invisible dimming, or dimming huge swaths of the screen at a time, offering little benefit, or, in some cases, a worse picture. Best-case, for example, the Samsung F8000/F9000 and the Sony KDL-55W900A, there's a noticeable improvement, though not as much as with full array.
Depending on where the LEDs are (along all four sides of the screen, just the right and left, just the top and bottom, or just the bottom or the top), edge-lit local dimming can have widely different performance.
For the full story on all the different methods, plus illustrations of what each can look like in practice, check out LED LCD backlights explained.
Global dimming One last trick isn't really "local" dimming as much as it's just "dimming," or maybe "global dimming." The entire image will get darker with dark scenes, and stay bright with bright scenes. On many models when given a full black image (like the fade-out at the end of a movie, but before the credits start) the LEDs will shut off completely, making it seem like the TV has a really good black level.
This is fake, of course, as you can't see anything else on the screen. If anything should appear, the LEDs kick back on, and the black level jumps up, revealing the TV's true contrast ratio. There are some tiny energy-saving benefits to turning off the LEDs, but visually this is distracting. It's also really common among projectors (where an auto-iris tracks the incoming signal).
Another variation of the theme senses the average brightness of the scene and, during darker scenes, ratchets down the whole backlight. Again black levels improve at the expense of highlights and sometimes-visible fluctuations in overall brightness.
It all comes down to "don't believe the marketing hype," at least not at face value. Local dimming can be a way to get near-plasma levels of picture quality. Or, it could offer some improvement compared with other TVs, and create a pleasing, if not class-leading, image. Or, it could be a marketing label for something that's not really much benefit at all. As usual, the best way to know is to check the reviews, where TVs with good local dimming usually dominate.
Got a question for Geoff? First, check out all the other articles he's written on topics like why all HDMI cables are the same, LED LCD vs. plasma, active versus passive 3D, and more. Still have a question? Send him an e-mail! He won't tell you what TV to buy, but he might use your letter in a future article. You can also send him a message on Twitter @TechWriterGeoff or Google+.