Local Dimming on TVs

Direct-Lit, Full-Array, and Edge-Lit

Local dimming's most obvious impact is on the TV's contrast. Measuring contrast, one of the most important aspects of picture quality is also one of our simplest tests. We begin by placing the TV in its post-calibration settings. Next, we display our custom-designed real-scene test image and adjust the backlight so the white area in the center of the cave measures about 200 cd/m², which serves as the white luminance measurement for our contrast calculation.

We chose 200 cd/m² for two reasons: any TV on the market can achieve it, ensuring comparable results, and it reflects a typical brightness level in a moderately lit room. Afterward, we select the local dimming setting that provides the best possible image quality.

Next, we measure the light output at the four corners of the scene and take one additional measurement to the right of the cave entrance. We then average these five readings to find the black luminance level for our contrast calculations. Finally, we divide the white luminance by the black luminance to determine the contrast ratio. We run this test twice: once to measure the panel's native contrast and once with local dimming enabled to see how much it improves performance.

Read more about contrast ratio.

We test the TV's blooming performance by taking a photo with the camera angled between +40° and +50° horizontally and +4° to +6° vertically. We then assign a subjective score based on how blooming appears in real-world usage. When choosing the representative photo, we focus on how harsh the blooming looks, not just how widely it spreads. Extremely bright highlights that abruptly cut off in dark scenes can be more distracting than a gradual brightness gradient.

Below, you can see how blooming affects the Hisense U6N, with the effect being most visible around the subtitles. Note the angle at which the photo was taken—this angle best represents the real-world impact of blooming when watching darker content in a completely dark room.

Finally, we test lighting zone transitions using our local dimming video to see how well the backlight adjusts when a moving bright image appears in a dark scene. We typically showcase the maximum local dimming setting in this video, but we also test every available setting and settle on the one that performs best.

When analyzing this video, we pay special attention to Zone Transition Visibility and Speed. As a bright object moves between dimming zones, we look for obvious blooming or multiple zones lighting up at once. Such artifacts can create a distracting glow around bright objects, like street lamps or stars. We also evaluate how quickly each zone responds: does the new zone illuminate too early, and how long does the old zone stay lit on the screen?

Number and Size of Dimming Zones: With local dimming, clusters of LEDs—called zones—dim or brighten in unison. A TV with many small zones can better control each zone's effect on the image, while fewer zones that cover larger areas might lead to more blooming. By observing how much space around the bright dot remains lit at any moment, you can estimate the zone size. We also use this video to count the number of zones the TV has.

In the example video below, recorded on the LG QNED85T, you can clearly see its lighting zone transitions because this TV only uses six edge-lit dimming zones. As the bright object shifts from one zone to another, you'll notice a bright band of light whenever a new zone lights up. You'll also see the size of each zone, as it remains lit behind the moving object until the next zone activates.

In contrast, the footage below shows the Hisense U8N, which uses a more effective local dimming feature. Although some haloing appears, you won't see the distracting bands of light found on the edge-lit LG QNED85T.

Local dimming also affects a TV's black uniformity. A well-implemented local dimming feature often reduces—or even eliminates—clouding issues that might otherwise be visible. However, this usually comes at the cost of increased blooming around bright elements, so we monitor backlight bleed and blooming closely.

In some cases, such as with the LG QNED85T shown below, local dimming can actually worsen uniformity. Instead of a uniformly cloudy image, there's a bright band of light surrounded by darker blacks.

We also check each LED TV's backlight type, though it doesn't affect our scoring. Direct or full-array backlighting places LEDs across the entire area behind the LCD panel. Edge-lit TVs, like the LG QNED85T, only position LEDs along the sides of the screen—usually two opposing edges—and rely on those LEDs to light the entire display. Because these different approaches affect local dimming performance, we consider the results important.

We label a TV as "Full-Array" if it uses direct/full-array backlighting and supports local dimming (like the Hisense U8N). We call it "Direct LED" if it's direct-lit but lacks local dimming (like the TCL Q5/Q550G). Because direct-lit TVs place LEDs behind the entire screen, their local dimming feature can more effectively control each zone and easily turn off specific areas.

In contrast, edge-lit TVs like the LG QNED85T can't dim LEDs located directly behind the relevant parts of the screen, so edge-lit local dimming tends to be less precise. These TVs usually provide minimal contrast improvements and can cause entire vertical or horizontal zones to illuminate when a bright object appears, as seen earlier. Although most TVs now use full-array local dimming, some still rely on edge-lit panels. Many monitors also use edge-lit local dimming; this example shows how it lights multiple zones at once.

As part of our Test Bench 1.9 update, we introduced the "Contrast and Dark Details in Game Mode" test, which replaces our previous "Local Dimming in Game Mode" test. Although this new test is conducted on all TVs—including those without local dimming—it serves to emphasize how local dimming impacts performance or image quality in Game Mode. With this test, we replicate our standard local dimming tests, but we switch the TV to Game Mode and enable its variable refresh rate feature if supported. We then see if this impacts performance or image quality in any way. While most TVs deliver similar local dimming results in Game Mode, certain models show more noticeable blooming or zone transition issues in this mode.