Plasma screens work by exciting tiny pockets of gas (Xenon and Neon), changing them to a plasma state. In that state, the electrons of that gas emit ultraviolet light, which is not visible to the human eye. The ultraviolet light is then absorbed and re-emitted into the visible spectrum of light by the phosphor inside each cell. Each pixel consists of three sub pixels: one red, one blue, and one green. The more excited the gas, the brighter the color produced.
The pixel is not excited continuously, but in short pulses. The naked eye perceives this as flickering. Some people are very sensitive to this. The effect is reduced on higher-end models because the flickering is a lot faster.
Because each pixel emits its own light, the blacks are really deep. When the television wants to display black, it simply emits no light at all for the selected pixels. Also, there is less of the motion blur sometimes seen on an LCD screen, because the gas stops emitting light as soon as it is not excited.
Plasmas suffer from occasional image retention if they display the same image for a very long period of time. This has been improved a lot in the last few years, and permanent burn-ins are no longer an issue. The image retention, if it occurs, will typically disappear in a few minutes.
Unless you plan on displaying a static image for days at a time, you do not need to worry about this anymore. It is more of an issue with public displays where the same static ads are shown continuously.
A backlight is behind an LCD screen
An LCD screen is composed of two parts: the actual liquid crystal display and a light source at the back of the screen (called backlight). A light diffuser is placed between the backlight and the LCD screen to make the source of light uniform across the screen.
The LCD screen does not emit light by itself; it only acts as a filter to block the light on a per pixel basis. The opacity of a pixel can be controlled by applying an electric field to it. If the screen wants to display black, the LCD pixel will try to block the light completely. If it wants to display white, it will let it through. Because the display is only a filter, the blacks will not be as deep as with a plasma screen. An LCD pixel, even if it displays black, will always let through a small portion of the light.
Contrary with plasma, an LCD panel does not flicker. It works by a sample and hold method where the picture stays on the whole time. On some TVs, though, the backlight will flicker to adjust its brightness.
Applying an electric field to the screen and illuminating the back costs less energy than exciting the electrons in the plasma display, thus the overall power consumption of the television is lower (check out the chart here). Also, the luminosity of the screen can be higher because the backlight can be very bright. This makes an LCD TV more suitable than a plasma TV for a well-lit room.
The viewing angle of the television (the angle of the screen where you can see the picture) is limited, because the LCD filter screen has depth. This is less of a problem nowadays because the depth of the LCD layer has been considerably reduced, which has greatly improved the viewing angle range.
There are two main types of backlights used in LCD screens: CCFL and LEDs.
Conventional LCD backlight
When someone refers to an LCD TV, they usually mean a CCFL (cold-cathode fluorescent lamp) backlit LCD screen. This is how a normal LCD screen works. The backlight is a series of light tubes placed behind the screen. These tubes are very similar to the fluorescent lamps used almost everywhere, but smaller.
LCDs with CCFL backlight are on their way out of the market. Manufacturers stopped producing them, replacing them with LED TVs. They have the same screen but use LED lights instead of a CCFL tube for the backlight. This reduces the manufacturing cost of a TV.
An LED screen is actually an LCD screen, but instead of having a normal CCFL backlight, it uses light-emitting diodes (LEDs) as a source of light behind the screen. An LED is more energy efficient and a lot smaller than a CCFL, enabling a thinner television screen. Marketing made a lot of fuss about LED TVs, but it is only the backlight that changed, so there is actually no picture quality improvement over a normal LCD screen.
There have been prototypes of real LED TVs. They didn't have an LCD panel but instead had 3 small colored LEDs per pixel. These would have been a lot better, but unfortunately the manufacturing cost was way too high to be profitable. OLED TVs are very similar to this and will hopefully become affordable within a few years.
There are three main configurations of LED as backlights for television screens: full array, edge lit, and direct lit.
Full array LED backlight
This method is considered the best LED backlight type, but can only be found on a limited number of models.
In a full array LED screen, the LEDs are distributed evenly behind the entire screen. This produces a more uniform backlight and provides a more effective use of local dimming, where it can change the luminosity of only a specific part of the screen.
In some TVs, like Sony's XBR line, they use colored LEDs instead of white ones. Technically, this can create an even greater color range gamut by matching the backlight color with the picture. In practice though, you will not really see the difference.
Edge-lit LED backlight
This is the most common method for LED TVs.
With an edge lit LED screen, the LEDs are placed at the peripheral of the screen. Depending on the television, it can be all around the screen or only on the sides or the bottom. This allows the screen to be very thin.
However, it can cause some spots on the screen to be brighter than others, like the edges. This problem is called flashlighting or clouding. It can be seen when watching a dark scene in a dark environment.
Direct lit LED backlight
This is how the lower-end LED TVs are constructed.
Similarly to the full array method, the LEDs are directly behind the screen. However, there are very few of them and they cannot be controlled separately to match the luminosity of the picture.
These TVs are not very thin because of the space required behind the screen to add the LEDs and to diffuse the light over a big area.
Questions & Answers
11 ANSWERED QUESTIONS
I watch a lot of sports and movies. I plan to buy a 60" for my basement home theater.
The room is very dark, I'm torn between led or plasma. I'm told LEDs last longer vs
Plasma. How long does a plasma usually last? What do you recommend? I'm looking at the Samsung 8000.
It is hard to compare the longevity of such products. While technically LEDs will last longer, in real life scenarios it doesn't matter. Common components (like the electronics board) will fail at around the same time. Longevity is more impacted by the quality of the model and brand. If you buy a quality set, it should last long enough for that technology to become obsolete unless you are unlucky. Most electronics will break due to a capacitor failure. For example, there was a class action lawsuit against Samsung for putting cheap capacitors in all their TVs a few years ago.
How can a normal buyer differentiate between edge, full array,and direct lit as manufacturing do not usually disclose such information. Which is better?
It is quite easy to tell by looking at the thickness of the TV. Edge lit TVs are really thin. Direct lit are thick. Full array are practically non existent in 2013 (only the Sharp Elite line are full array).
Is it true that all plasma TVs produce a buzzing sound?
Yes, all plasma TVs generate a buzz sound. However, it is normally not noticeable at all, especially when watching the television. The volume of the sound will vary by model and even by individual unit. If you can hear it while watching TV, your unit is defective and you should return it.
Which has better picture quality, an edge lit or a direct lit TV?
They are very similar. The only difference is direct lit has slightly better backlight uniformity.
You said that plasma produces ultraviolet. Ultraviolet is harmful for eyes. Is it possible that some portion of ultraviolet won't be transformed into visible light and will instead illuminate from the screen? Does it happen that LED TVs that use cheap LEDs in a backlight produce blue light instead of white?
By definition, ultraviolet is not in the visible spectrum, so it will always be invisible. You do not have to worry about it, though, as you are exposed to a lot more UV from other sources than your TV. The hue of a white LED will never be perfect. However, this can be easily compensated for by calibrating the television correctly, so any imperfection in the hue is filtered out by the LCD layer. Some LED TVs do use three different colors of LEDs for the backlight, but it doesn't really help to produce a better color gamut.
I just bought a new Sony 50" LED smart TV. When I got it home and set up, I was appalled at the glare off the screen during daylight hours. My living room has lots of windows which have vertical blinds which can only reduce the amount of light coming through, not eliminate it. I am going to return this TV. Do you have any suggestions as to what kind / brand of TV I could buy to deal with this problem. My six-year-old Samsung 40" flatscreen worked fine in this environment.
You can check out the pictures that we took of TV models that we reviewed here
. We measure the amount of reflections on all TVs that we test.
Can I stop the dimming in my 50 inch plasma Panasonic TV?
No. All plasma TVs have this and this is called the Automatic Brightness Limiter (ABL).
I heard plasma TV is better for sports because of the fast refresh time. Is there truth to this?
Yes, plasma TVs usually have better motion handling and less blur than LEDs.
Is the Samsung 55" UN55HU9000
a full array LED screen?
Why don't I see Sharp under your brand category for ratings?
We didn't have time to review them this year.
I have a 2013 Vizio M series
in an almost pitch black environment, and it has flashlighting/clouding with a completely black screen input. That's even with the Backlight/Brightness settings at the lowest values, and with local dimming off it still seems the LEDs are lit. I would think with an MFG claim of "10M:1 dynamic contrast ratio," why wouldn't they just turn off at zero?
Is this common with all brands? Does the lighting type (Full array, Edge lit, Direct lit) have anything to do with it?
No LED TV has perfect uniformity, so some flashlighting and/or clouding is normal. Edge-lit TVs, like your 2013 Vizio M, only have their LEDs on the sides of the TV, which makes it difficult for them to evenly distribute light. On the other hand, full array TVs have the LEDs all over the back of the TV, which should mean better uniformity. Direct lit TVs are similar to full array, but have fewer LEDs, and so the uniformity is a bit worse (but likely still better than that of an edge-lit TV).
As for why the backlight doesn't just turn off at zero, it's because the TV requires the LEDs to be on for it to display an image. In this case, zero doesn't represent a complete absence of light, but rather the dimmest level that the TV is programmed to allow the LEDs to get.