What is Local Dimming?
Vizio M Series
Vizio M Series
Local dimming refers to the technology that changes the uniformity of the backlight of an LED screen. The television changes the luminosity of some zones of the screen to better reflect the scene and to create deeper blacks. It improves the weak black level of an LCD/LED TV.
Unfortunately, local dimming on LED TVs is rarely effective. Most LED TVs are edge lit, so they cannot control precisely the backlight. It works better on a full array LED TV (like shown in the pictures above), but it creates a blooming effect.
In the pictures, you can see that the blacks for an LED with local dimming are darker than a normal LED because the lights behind that section of the screen are dimmed. However, you can see a slight halo effect, because the television still needs to have some bright LEDs behind the center cross. A plasma TV has no problem displaying deep blacks directly next to the cross.
Is local dimming worth it?
In theory, local dimming is great. It increases the perceived contrast of the picture by illuminating more the correct part of the screen and by turning off completely a dark section. LCD screens leak a little bit of light through the pixels, even when displaying a black color. By reducing the lights behind a region of the screen where the picture is mostly black, it reduces the leakage and produces a darker picture. The following illustrations show a backlight for a normal LCD, an edge-lit LED and a full array LED.
You can see that the backlight tries to match the picture to display, but there is some diffusion caused by the dispersion of the lights of the LEDs.
In practice, very few TVs can control precisely the backlight, due to the lack of a really granular backlight. The precision is not perfect and the size of the controllable zones varies between models. The worst case would be if you are watching a scene in space with stars. Overall, the scene is dark but the small stars are bright. The LEDs behind the screen affect a rather large area so it cannot only light up the backlight for the pixels of the stars. Depending on the implementation, it could produce a worse picture than before and could create a halo effect around the stars.
As you can see in the videos, the full array local dimming of the P Series is a lot better than the M Series. Not only the added zones make difference, but it is also faster to match the picture.
The backlight cannot catch up on the Vizio M552i-B2. At first, it fully illuminates the dot. Once it starts moving too fast though, the backlight doesn't have time to turn on completely, reducing the luminosity of the dot. This is not an issue on the P Series. The backlight is more responsive. However, the backlight takes some time to dim down, leaving a trail of blooming.
What is Micro Dimming and Micro Dimming Ultimate?
Micro Dimming is Samsung's marketing response to local dimming (which is why the name is similar). However, Samsung does not actually dim the LEDs of the backlight, so it is not really local dimming. Instead, it artificially tries to create a similar effect by changing the contrast of different zones of the picture. Micro Dimming Ultimate claims to have twice the number of zones (600) and changes the color and sharpness in addition to the contrast.
Of course, this is mainly just a marketing feature and is nowhere near real local dimming. Video purist especially do not like it, because it messes with the video settings of their televisions, changing the contrast from frame to frame.
You will not really see a difference between the different versions of Micro Dimming. As usual in the television business, the difference between them are more in term of marketing rather than real technology improvements. Therefore, we do not recommend the Micro Dimming technology. However, Samsung do put better panels in their higher end models, so the picture could be better (but not due to this technology).
Samsung TVs equipped with Micro Dimming are also equipped with CE Dimming. CE Dimming does dim the LEDs of the panel, but not locally. It dims the whole panel on darker scene. Some people do not like it, because you can actually see the whole screen dims sometimes, like in the end credits of a movie. There is no option to turn CE Dimming off, even in the service menu.
What is Smart Dimming?
Smart Dimming is Vizio's name for its local dimming technology. It is used on some of their edge-lit LED televisions to dim the darker portion of the picture (see above for more information on local dimming for edge-lit televisions). The number of controllable zones varies depending on the model, but because it is still an edge-lit LED TV, we do not recommend that feature.
What is Frame Dimming?
Frame Dimming is the equivalent of an edge-lit local dimming, but with even less zones. Instead of changing the light for a specific region of the screen, it is the whole frame of the screen that changes intensity. You can see it as a very basic version of local dimming. It is better than no dimming at all for dark scene, but it is nowhere near real local dimming.
Does local dimming also work for edge-lit LED?
The majority of televisions with local dimming are edge-lit LED TVs. The resolution of the backlight of an edge-lit is a lot worse than a full array (see pictures above). The lights are on the edge of the screen, making it harder to control exactly a section of the screen. Instead of dimming a specific part of the screen, it is roughly the whole screen that will dim. It cannot be precise and it does not want to dim the brighter part of the image, so it will only slightly dim the darker parts.
Here is a real example on an edge lit TV:
The difference is really subtile. Because it is edge lit, it cannot improve the blacks inside the actual picture. Instead, only the top and bottom black bars are dimmed. It does overlap on the picture though. The top and bottom edges of the picture are darken.
Therefore, local dimming on edge-lit LED TVs (the majority of them) is not worth it. Only full array LED TVs can be effective, and they are very rare.
Can you turn local dimming off?
Yes, the local dimming feature of most televisions can be turned off in the settings menu. It is sometimes hidden inside the service maintenance screen.
For Samsung TVs however, Micro Dimming cannot be turned off even in the service menu. If you want to watch television without it, you will need to set your TV in the Movie Mode, where this feature is disabled by default (among other features also).
Does a plasma TV also have local dimming?
Plasma TVs do not need local dimming, because they have no backlight. Each pixel emits their own light and are independent of each other, so it can produce a white color right next to a black color with no leakage. Therefore, plasma do not have the problem that the local dimming technology is trying to solve for LCD/LED displays.
Learn about how plasma works here.
Unless it is a full array LED TV with a good implementation of local dimming, skip that feature. It does help with the blacks, but it can create a blooming/halo effect. Local dimming is even less useful for edge-lit LED TVs (most of them), because it cannot dim only a specific part of the screen.
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Update: We tested the HU8550, and the picture quality is indeed almost identical (except for the resolution of course). Go for the one that cost less.
Update: The review of the M552i-b2 is up now.
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