The term "local dimming" gets used frequently in LED LCD marketing. It's one of the few such terms that can actually denote improved picture quality, but not all local-dimming methods are created equal. Here's the difference.
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. It's also vital to get the intense highlights possible with High Dynamic Range (HDR) content.
But not all local dimming works the same, and certain types of local dimming work better than others.
This article is a distilling/expansion of a sliver of the longer LED LCD backlights explained. Check out that article for even more info. If you're really only interested in local dimming... read on!
LED LCDs don't have the contrast ratio potential of OLED (or plasma, may it RIP). As such, their images don't have quite the depth and three-dimensionality that's possible with other technologies.
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 advanced, local dimming has as well. As the LCD industry moved towards the cheaper, thinner edge-lit 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. 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."
At its best, full-array backlit local dimming produces the best images you can get with LCD. It's also always more expensive than TVs from the same company with one of the other backlighting methods discussed below.
To get the most out of HDR content with an LCD, full array local dimming is best.
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.
The top-of-the-line models from most manufacturers are full-array... but not always. Since edge-lit TVs are thinner and cheaper to produce, they're far more common. Today, "local dimming" is used as a blanket term, so it's important to check the specs (or our reviews) for which models are full array.
Edge-lit local dimming
The most common variety of LED LCD is edge-lit. 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 bit looser of a term. Yes, the TV can still dim areas of the screen, but those areas are much larger than they are with full array, as you can see in the image above.
Worst-case, the "local dimming" could be nearly invisible, or dim huge swaths of the screen at a time, neither offering any benefit. In some cases, it could result in a worse picture.
Best-case, there's a noticeable improvement in the picture quality, though not as much as there would be with full array. The pin-point highlights of HDR aren't possible here, though some models might still offer Wide Color Gamut (which is related to HDR, but separate).
Depending 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.
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. As in, the entire backlight functions as one single light. This is common among the least expensive LCD TVs.
It's common on these models, for example, that 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 the TV seem like it has a really good black level.
This is fake, of course. If anything should appear, the LEDs kick back on, and the black level jumps up, revealing the TV's true (and far more muted) contrast ratio. There are some tiny energy-saving benefits to turning off the LEDs, but visually this is can be distracting.
Another variation of this theme senses the average brightness of the scene and, during darker scenes, ratchets down the whole backlight. Again black levels improve because the whole screen is darker, but this is at the the expense of bright highlights. Sometimes this causes 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-OLED levels of picture quality. Or, it can offer some improvement compared with basic TVs, creating 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 do really well.
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, TV resolutions explained, LED LCD vs. OLED, and more. Still have a question? Tweet at him @TechWriterGeoff then check out his travel photography on Instagram. He also thinks you should check out his bestselling sci-fi novel and its sequel.