Gray uniformity describes how well a TV is able to maintain a single, uniform color on the screen. Uniformity issues can look like darker patches around the screen, with corners and edges especially susceptible to looking darker than intended. It’s particularly an issue for sports, where darker patches on the screen can affect the look of playing surfaces.
To evaluate gray uniformity of TVs, we take a photograph of the screen while it is displaying a single, solid color. Using that picture, we calculate both the standard deviation of the color values of the pixels across the entire screen, and the number and severity of darker patches in the center of the screen (where they matter most).
You’re most likely to notice gray uniformity issues while watching sports, since playing surfaces are usually large stretches of a single color. Dirty screen effect will also be most obvious when there is a panning shot during sports. Most non-sports media tends to be a bit more varied, so you shouldn’t have gray uniformity issues with anything else unless your TV has particularly poor uniformity.
Because it's only usually really bad gray uniformity that is obvious, it’s not hugely important most of the time. If a particular TV has really bad gray uniformity, or if a person is very sensitive to this kind of problem, then the importance could increase some. If you get a TV with gray uniformity that you can’t live with, try making an exchange for a different unit.
Our picture test captures the gray uniformity imperfections on a TV screen, and to show you the quality of the uniformity as you would actually see it on a TV. Of our tests, we consider this the most useful for most people who are trying to get a sense of what kind of uniformity they can expect from a TV.
To evaluate the gray uniformity on TVs, we take a photograph (F4, 1/4 second exposure and ISO 200) of this 50% white (medium gray) image while it is being displayed on the screen. The image shows any imperfections in the reproduction.
Similar to the 50% gray uniformity picture test, the 5% gray is to show you the quality of the uniformity as you would see it on a TV, but with a darker image. This test is more useful to visualize the uniformity of TVs using emissive technologies like OLED TVs.
To evaluate the 5% gray uniformity on TVs, we take a photograph (F4, 1/4 second exposure and ISO 200) of this 5% white (dark gray) image while it is being displayed on the screen. The image shows any imperfections that would appear in very dark shots.
Our 50% standard deviation test measures the evenness of color reproduction across the screen, and tells us the average squared difference of the color values of pixels when a full-screen, 50% white (medium gray) or 5% white (dark gray) image is displayed. A lower standard deviation means less difference is present, and that the picture is better.
This allows us to score the overall uniformity of the entire screen objectively. It’s useful for comparing TVs in absolute terms but isn’t as useful as the picture test for comparing TVs for what you’ll actually experience.
For this test, we take the photo of the screen from the picture test and process it with a low pass filter (Gaussian blur) to remove noise and artifacts (like moiré) created by the camera. We then calculate the standard deviation of the color values of the pixels using the following formula:
Our 50% dirty screen effect test also evaluates the amount of difference the color values of pixels have from the target color but focuses only on the area around the middle of the screen, where variance tends to look like dirty patches.
This is a bit more important for sports than the standard deviation test is, because while the standard deviation test gives us an idea of how far the uniformity is from ideal in absolute terms, it doesn’t fully represent the issues people will notice with poor gray uniformity. Compare these two images. On the left is a TV with a higher standard deviation, but less DSE. On the right is a TV with lower standard deviation, but higher DSE. You can see the second has more dark patches around the middle, where they are likely to be problematic.
To take this into account, we once again use the photo from our 'Picture' test, only this time we pass it through a high pass filter so that low frequencies are removed from the image. The result is something like the image below and to the left (result multiplied by 50 for purposes of illustration). This isolates all the variations that cause the dirty screen effect but does so for the entire screen.
In order to test just the middle of the screen, we multiply the values of that picture with those of the image in the below-middle, resulting in the below image to the right.
Finally, we calculate the root of the average of the squared values of the final image. The equation is similar to that of the standard deviation calculation above but uses ‘0’ instead of the mean.
Our 5% standard deviation test use the same procedure as the 50% standard deviation test, but run it on our 5% gray picture instead. For more information on the procedure, refer to the 50% standard deviation test.
Once again here, a lower standard deviation means less difference is present, and that the picture is better.
Our 5% dirty screen effect test uses the same procedure as the 50% dirty screen effect test, but run it on our 5% gray picture instead. For more information on the procedure, refer to the 50% dirty screen effect test.
The 5% dirty screen effect is based on a darker image, so this test is more important for any content that has dark scenes.
Gray uniformity is unique to each individual panel. This means that no two TVs, even of the same model, will have matching uniformity. Generally, though, higher-end TVs will have better gray uniformity, as the manufacturers will have stricter standards for the panels being used.
With LED TVs, gray uniformity issues are caused by a couple of factors. LCD panels are pretty sensitive to pressure, so extra pressure caused by misalignment of the TV's components, or by mishandling of the panel during manufacturing, could lead to defects appearing. Likewise, improper pressure can interfere with the light diffuser doing its job correctly, which can cause certain portions of the screen to be lit darker or lighter than intended.
With LED TVs, full-array and direct backlighting can also contribute to worse gray uniformity. It's often possible to see a faint grid on the screen, corresponding to the placement of the LEDs.
With OLED TVs, the gray uniformity issues are more related to imperfections in the panel itself, independent of pressure-related problems. Also, LG's 2015 OLED TVs suffered from vignetting that was particularly visible when displaying a dark gray image, but since then most OLED TVs do much better when tested under our new 5% gray image.
Unfortunately, gray uniformity is entirely down to the panel you get. There are no steps available that will help you improve the gray uniformity.
Gray uniformity refers to how well a TV can display a single, solid color across the screen. It matters for any images containing a wide expanse of a single color, and in particular sports, where bad gray uniformity affects the appearance of playing surfaces. For each TV, we take 2 photos of the grays uniformity (dark and medium gray), calculate the standard deviation of the color values of the pixels, and then calculate the amount of dirty screen effect that is present for each picture.
Unfortunately, there are no steps that can be taken to improve gray uniformity – it’s entirely down to the panel you get. If you find yourself with uniformity that you cannot live with, you should exchange your TV for a different unit, or even a different model.