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.
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 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.
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:
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.
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:
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.
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.
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.
Local dimming can introduce minor problems on some monitors.
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.
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:
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.