Black Uniformity of TVs
Clouding and Flashlighting
- 75% Native Std. Dev.
- 25% Std. Dev. w/ L.D.
Black uniformity refers to a TV’s ability to display solid black all across its screen. Perfect black uniformity would mean shadows, night skies, and other black objects would look equally dark at any given point on the screen, and that the TV’s lighting is perfectly diffused across the screen. Unfortunately, all LED TVs have some imperfections, including flashlighting, which looks like a flashlight is pointed across the corners of the television; and clouding, which looks like brighter patches on parts of the screen. Both are caused by the TV improperly diffusing the backlight across the entire screen, and both are especially obvious when the TV is being viewed in a dark room.
For this test, we take a photo of the TV while it displays a mostly black frame, and we also calculate the standard deviation of the color values of the pixels when they try to display black.
Test results
When it matters
Black uniformity issues like clouding or flashlighting can only be seen during dark scenes, and only when the TV is being watched in a dark room. If you watch TV in a room that gets a bit of light, this isn’t so important.
No LED TV has a perfect uniformity, but in most cases, you won't notice it unless you really look for it. However, some TVs (like on the above, left) have exceptionally bad black uniformity. If you get something that bad, it will make a big difference in how much you enjoy what you are watching.
Our tests
Native Black Uniformity Picture
Our picture test captures the black uniformity imperfections on a TV’s screen and is meant to show you what the quality of the blacks is in terms of what you see. Of our two black uniformity tests, we consider this the most useful for most people.
To evaluate the black uniformity on TVs, we take a photograph of this pattern, in a pitch dark room, with the following camera settings: F4.0, ISO-200, 2 sec shutter time. The backlight of the TV is set so that white emits 100 cd/m2 of light. To show the TV’s true uniformity, we don't turn on local dimming for this test (the local dimming feature is scored separately in our reviews).
The camera settings we use were chosen so that what you see in our picture reflects what you see in a dark room.
Our black uniformity test patternNative Black Uniformity Std. Dev.
We also calculate the standard deviation of the color values of the pixels from the picture taken in the test above. This is done so that we can objectively score the TVs for uniformity, rather than needing to rely on our subjective opinions of each TV. This is a bit less useful for people who want an idea of what black uniformity looks like, but is necessary for us to be able to assign a fair score.
To score the TV clouding and flashlighting issues objectively, we process the photograph from above and calculate the standard deviation of the pixels in the picture. The result is a number that corresponds to how uniform the blacks are. A value of 0 means it is perfect. The greater the number, the more the blacks vary.
Black Uniformity with Local Dimming Picture
Similarly to the native black uniformity test, we take a picture with the same room and camera settings, but this time with the local dimming feature turned on and set to maximum.
This is useful to show how the local dimming is effective to get rid of black uniformity issues when displaying a black image when viewed in a dark room.
Black Uniformity with Local Dimming Std. Dev.
We calculate the black uniformity with local dimming standard deviation the same way we do the native black uniformity standard deviation, but this time base on the black uniformity with local dimming picture. This is not perfect as almost all TVs with local dimming experience some blooming around the white cross.
Additional information
LED vs OLED black uniformity
LED backlit LCD TVs are prone to this issue, but it isn't a problem on OLED TVs, because the pixels are self-emitting, and therefore don't need to worry about improper implementation of a backlight.
The cause of flashlighting and clouding
Flashlighting and clouding can occur for a few reasons:
- Poorly designed backlight placement. The type of the backlight and the position of it directly affect the uniformity of the screen. The worst uniformity can be found in edge-lit LED TVs, because the LEDs are only on the edge of the screen. Full-array LED TVs have good uniformity because their LEDs are spread out across the whole screen. This is less true today, though. Some edge-lit TVs have great uniformity. You can learn more about different backlights here.
- Non-uniform frame pressure. Both the LCD screen and the diffuser layers are somewhat flexible. A slight difference in pressure on the frame of the television can change the uniformity of the picture. On some televisions, this can be fixed (see below).
- Uneven intensity of the lighting. Sometimes, the intensity between two lighting components can vary in a set (for example, one LED could be slightly brighter than an LED next to it). This is less likely to happen with a brand new TV but can start appearing over time if some components age and degrade faster than others.
How to get the best results
Flashlighting can sometimes be fixed on some televisions. If your flashlighting issue is caused by an issue with the frame’s pressure, you can adjust it by slightly loosening or tightening the screws behind the edge of the screen. Some televisions do not expose the screws, however, so you won’t have that option with those. You should also keep in mind that a small change in the tightness of a screw can have a big impact on the uniformity of the screen, so be careful if ever you do try this step.
For clouding, there's a pretty simple fix that - while not guaranteed - can have good results. Power on the TV and display a black image. This will allow you to see the lighter spots on the screen. Next, take a soft cloth and massage those brighter spots to improve the uniformity. You should only apply very gentle pressure, just barely touching the screen. You can also try a few different stroking patterns. If you are patient enough, this works surprisingly often, especially on edge-lit TVs.
Alternatively, you might consider returning or exchanging your television. Some models are more prone than others to having issues. Even within the same model, some units can have more of this issue, because of slight variance in the manufacturing or the shipping process. It can be worth trying to exchange it for the same model before going with an entirely different one. Extreme cases of clouding are also covered under a TV's warranty.
Related settings
- Reducing 'Backlight' can help mask the problem by making the whole screen darker.
- The local dimming feature, if the TV is equipped with it, can also help. We talk about local dimming in more detail here.
Other notes
- All LED TVs have black uniformity problems.
- The bigger the LED TV, the more likely it is to have uniformity issues.
- Backlight uniformity issues don't increase over time unless a large amount of stress is put upon the panel (like in the case of a big temperature swing, or if the TV is moved).
- Sometimes, just letting the TV settle for a few days after unpacking it can solve the issue. This is especially true if the TV is laid on the side during shipping/moving.
Conclusion
A TV’s black uniformity refers to how evenly a TV can display black all across the screen. It’s important when watching dark scenes in a dark room, and while most people shouldn’t notice black uniformity issues like flashlighting and clouding, they can become more apparent when the issues are especially bad. You won’t need to worry about these problems in rooms with lights on, though. For each TV, we take a photo of a black frame in order to capture the black uniformity, and then we calculate the color values of the pixels in order to find the standard deviation of the uniformity.
You may be able to remove flashlighting (to some degree) by tightening the screws of the TV’s frame. This can have pretty dramatic effects on the picture, though, so you should be very careful when attempting this. For clouding, gently massaging problem areas with a soft cloth often does a good job of fixing bad uniformity.
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