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Plasma vs LED: How They Work

Updated Feb 17, 2014 By Cedric Demers

Plasma

Plasma screens works by exciting tiny pockets of gas (Xenon and Neon) turning them into a plasma state. In that state, the electrons of that gas emits 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 3 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 motion blur as seen sometimes on an LCD screen because the gas stop emitting lights immediately when it is not excited.

Plasmas suffer from occasional image retention if they display the same image for a very long period of time. They improved a lot on this subject in the last few years and permanent burn-ins are no longer an issue. The image retention, if one 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 in a public display where the same static ads are shown continuously.

LCD

How LCD works
A backlight is behind an LCD screen

An LCD screen is composed of 2 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 uniform the source of light equally 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 in a plasma screen. An LCD pixel, even if it displays black, will always let through a small portion of the light.

Contrary to a plasma, an LCD panel does not flicker. It works by a sample and hold method where the picture stays on during the whole time of the frame. On some TVs though, the backlight will flickers to adjust its brightness.

Applying an electric field to the screen and illuminating the back cost 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-lighted room.

The viewing angle of the television (the angle off the screen where you can see the picture) is limited because the LCD filter screen has a depth. This is less of a problem nowadays because the depth of the LCD layer has been considerably reduced, which has improved a lot the viewing angle range.

There are 2 main types of backlights used in LCD screens: CCFL and LEDs.

CCFL

How LCD CCFL works
Conventional LCD backlight

When someone refers to an LCD TV, he usually refers to a CCFL (cold-cathode fluorescent lamps) backlighted 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 lamp used almost everywhere, but smaller.

LCDs with CCFL backlight are on their way out of the market. Manufacturers stopped producing them. They are getting replaced by LED TVs. They have the same screen but have LED lights instead of a CCFL tube for the backlight. This reduces the manufacturing cost of a TV.

LED

An LED screen as normally referred 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 in a few years.

There are three main configurations of LED as backlights for television screens: full array, edge lit and direct lit.

Full array
How LED full array works
Full array LED backlight

This method is considered the best LED backlight type, but can only be found on a very 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 it provides a more effective use of local dimming, where it can change the luminosity of only a specific part of the screen.

On some TVs, like the XBR line of Sony, 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
How LED edge lit works
Edge-lit LED backlight

This is the most common method for LED TVs.

In 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
How LED direct lit works
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

Mar 19 2013
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.
9
Nov 28 2012
Is it true that all Plasma TVs are producing 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 varies by model and actual unit. If you can hear it while watching TV, your unit is defective and you should return it.
5
Nov 12 2013
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 by looking at the thickness of the TV. Edge lit are really thin. Direct lit are thick. Full array are practically non existent in 2013 (only the Sharp Elite line).
5
Mar 12 2013
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 lights and illuminates from the screen? As it occurs then in LED TV used cheap LED in a backlight, which 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, you are exposed a lot more by UV from other sources than your TV. The hue of a white LED will never be perfect. However, this can be easily compensated by calibrating the television correctly, so any imperfection in the hue is filtered out by the LCD layer. Some LED TVs do use 3 different colors of LEDs for the backlight, but it don't really help to produce a better color gamut.
3
Jan 19 2014
Which has a better picture quality, an edge lit TV or direct lit?
They are very similar. The only difference is direct lit has a slightly better backlight uniformity.
3
Nov 18 2013
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 blurs than LEDs.
2
Apr 30 2014
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).
2
Jan 28 2014
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.
1
Aug 23 2014
Is the Samsung 55" UN55HU9000 a full array LED screen?
No.
0
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