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Local Dimming on Monitors

What it is: The lights behind the LCD layer adapt to the picture displayed, improving the contrast ratio.
When it matters: Dark scenes viewed in a dark room.
Score components: Subjectively assigned
Score distribution

Local dimming is a feature found on some monitors that dims the backlight behind darker areas of the screen. This makes blacks appear deeper and darker on those parts of the screen, which can significantly improve the viewing experience when watching videos or playing games, especially in HDR. Unfortunately, no local dimming feature is perfect, and issues with the picture are common. Very few monitors currently support local dimming, although this is expected to change in the coming years, as HDR on monitors grows in popularity and technology improves.

Test results

When it Matters

Acer Predator X27 with Local dimming offLocal dimming off
Acer Predator X27 with Local dimming onLocal dimming on

Local dimming is meant to increase contrast by making black look deeper. Local dimming functionality will, therefore, make the most difference when watching a dark scene in a dark room. Lights being on in the room would lead the TV to reflect that light, which means you won't really see the benefit of local dimming in a bright room.

Local dimming can be of noticeable benefit to picture quality, but it can also lead to blacks losing detail, or light blooming off of bright objects and into darker portions of the screen. If local dimming is a feature that sounds appealing, you should pay close attention to the results of this test, just to make sure you get a monitor that does this feature well.

Our tests

Original test footage (mp4)

Our main goal with this test is to determine how well the monitor's local dimming feature works. We use a video test pattern and compare the local dimming performance to other monitors. We use this comparison to subjectively assign a score for local dimming performance. We also use thermal imaging to determine the type of backlight used by the monitor. Although the backlight type can make a difference in some areas, we don't score this directly.

Local Dimming Video

Our local dimming video represents how well the backlight can adjust when dimming a moving image involving bright and dark elements. For our video, we always show the maximum local dimming setting in this video, but we test all the different options and will mention which mode we consider the best to use, and why.

This test is meant to show a couple of things:

  • Number and size of dimming zones: With local dimming, different groupings of LEDs – called zones – are dimmed or brightened at the same time. With many small zones, it's easier to illuminate just the parts of the image that are supposed to be brighter, with little light bleeding into darker portions. Fewer, larger zones, on the other hand, will dim and brighten big portions of the image, often leading things that should be dark to look lighter. By looking at how much space is brightened around the white dot at any given time, you should be able to get an idea of the size and number of zones.
  • Responsiveness: Local dimming is not perfectly synchronized with the movement of bright objects on a dark background. By looking at how much lag there is between the repositioning of the dot and the brightening/dimming of the backlight, you can get an idea of how responsive the monitor’s local dimming is.

In this test, we play the above video on a monitor while the local dimming feature is enabled, and then record how the monitor plays the video. The backlight setting is also adjusted to have a 100 cd/m² white on a checkboard pattern when local dimming is enabled.

Local Dimming

What it is: Whether it has a feature that controls the LEDs behind the LCD layer, to match the picture and darken the dark portion of it.
When it matters: On LED monitors only. Dark scenes viewed in a dark room.

Our local dimming score is based on the subjective impression we get from enabling dimming on a monitor. We consider performance with the video test from above, but also how well the monitor's dimming performs with regular video and still images.

We check the following:

  • Sizes of the zones: The smaller, the better.
  • Responsiveness: There is a trade-off here. Local dimming that responds too quickly to movement will make blooming more apparent, but a monitor that is too slow will darken bright objects that are in motion.
  • Aggressiveness: Another trade-off here. A monitor that turns off the backlight as soon as something is dark will lose detail in the surrounding shadows (and similarly, a monitor that turns the backlight on to the max for bright objects will create too much blooming). At the opposite end of the spectrum, local dimming that is too weak won't darken the blacks enough to make a noticeable difference.

Because of these trade-offs, how 'good' any individual local dimming feature comes down to personal preference. You shouldn't only look at our score to determine whether you will like the local dimming on a given monitor. Ideally, a monitor will offer a few settings options for you to choose whatever compromise you like.

A perfect score would go to a monitor that can display very bright highlights next to deep blacks, without any blooming or loss of fine details. This is nearly impossible for an LED monitor. Although there aren't many on the market, this is one of the advantages of OLED monitors.

Backlight

What it is: Configuration of the lights of the backlight.
When it matters: Effectiveness of the local dimming.
Good value: Full-array/direct lighting is better for local dimming. As for the uniformity of the screen, it depends on the implementation. Some edge-lit monitors have more uniform blacks than some full-array monitors.

Our backlight test verifies the configuration of the monitor's backlight. Direct backlighting (sometimes referred to as full-array) means there are LEDs placed all over behind a monitor’s LCD panel. Edge-lit monitors only have LEDs along the sides of the screen (usually along the bottom), and these LEDs are responsible for lighting the entire screen. They perform quite differently for local dimming, so this result is quite important.

For this test, we use thermal imaging to try and identify the location of the LEDs, which usually appear as bright spots, due to the heat generated by the LEDs. For example, this thermal image of the Samsung CHG70 clearly shows the LEDs on each side of the display.

Direct-lit monitors just need to analyze the video, figure out where the spots are that need to be dimmed, and then dim the LEDs that are directly behind that part of the screen. This makes them better at only dimming dark portions of an image and brightening only the parts that should be lighter.

Edge-lit monitors can’t just dim LEDs located behind the relevant parts of the screen (they aren’t there), and so edge-lit local dimming tends to be much less precise. Using local dimming on this kind of monitor will usually lead to either horizontal or vertical bands of the screen becoming dimmer, corresponding to the locations of the LEDs on the edges.

In short, if you want decent local dimming, get a direct-lit monitor and not one that is edge-lit, although these are extremely rare.

Additional information

Why local dimming exists

Despite other advances in technology, the contrast ratio on monitors hasn't improved much, and few monitors achieve contrast ratios above 2500:1. This is because the majority of displays are backlit, meaning they rely on a light behind the panel to make the picture visible, and the LCD layer is not able to prevent all light from escaping out of the screen. These imperfections result in some light bleeding, even when the screen is completely black, which reduces contrast.

In an attempt to mask this shortcoming, some monitors use local dimming to target dark portions of the screen and dim the backlight in those areas. In a perfect world, the local dimming feature would be able to dim only dim or dark areas, leaving everything else at its original brightness. In reality, though, no display, whether it's a TV or monitor, is able to do this for several reasons. One of the biggest limitations is the size of the LED backlights themselves. The effectiveness of the local dimming is partially limited by the number of LED backlights, more commonly known as zones. This isn't the only limitation, though; more zones doesn't necessarily mean better local dimming.

Limitations of Local dimming

Local dimming can introduce minor problems on some monitors.

  • Light blooming around bright objects. Due to the relatively large size of the backlight zones on most monitors, it's almost impossible to perfectly dim the backlight around a bright object on a black background, like in the test cross above.
  • A trail of light following moving bright objects. The backlight can't always keep up with fast-moving objects, and sometimes, the light changes trail the actual object a bit.
  • A loss of detail in dark areas. Similar to the issue with blooming, sometimes the opposite can occur. By dimming a dark scene too much, some small dim details can be lost.

How to get the best results

Some monitors offer different local dimming settings. Low settings will usually dim the backlight less, but the downsides of local dimming (see the section above) also usually aren't as noticeable. Higher settings will dim more, but will also make downsides more noticeable.

Local dimming preferences are subjective, so if you have multiple options, try out the different settings and choose whichever one you like best. Apart from that, there is no way to get better results from local dimming.

Related settings

  • Contrast: This setting affects the contrast of the monitor’s panel, not the effectiveness of local dimming. If you’re after deeper contrast, setting your contrast setting to the maximum is a good first step. Note that this can sometimes reduce the amount of detail in highlights.
  • HDR: a monitor with a good local dimming implementation will do well with HDR because it will be able to further brighten the highlights.

The Future of Local Dimming

Local dimming is still very rare on monitors, but as technology improves, it is expected to become more common. There are a few new technologies that stand to significantly improve local dimming performance, including:

  • Mini-LED: These monitors use many extremely small LED backlights instead of a few large ones. This allows the monitor to more precisely control the dimming zones. With hundreds or even thousands of dimming zones, these monitors should be able to dim very small portions of the screen, significantly reducing the amount of visible blooming and limiting the loss of fine details. One of the first mini-LED monitors on the market is the ASUS ProArt PA32UCG, which should be released in early 2020, and has 1152 dimming zones.
  • OLED: OLED displays don't have a backlight; instead, each pixel is self-emissive. This results in perfect blacks and perfect black uniformity, with no local dimming feature. Since each pixel can dim or turn off individually, there is no blooming around bright objects, and no loss of fine-details. Although very few OLED monitors exist, they can be found on some laptops.

Conclusion

Local dimming is a feature that dims the backlight of monitors to improve the depth of blacks. It’s useful for people who are watching dark scenes and want the black to look deeper and darker. To test local dimming performance, we play a pattern on each monitor with local dimming to see how well the feature is implemented. We then subjectively score the monitor based on whether a positive difference is made to the blacks and the picture in general, or if unintended downsides are introduced.

Apart from choosing a local dimming setting that you like (when available), there isn’t anything else that can be done to improve local dimming. For that reason, if you want good local dimming performance, the most important thing is to get a monitor that scores highly on our local dimming tests. Unfortunately, very few monitors have a local dimming feature, and even fewer perform well.

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