Plasma vs LED: How They Work
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.
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.
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.
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.
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 are three main configurations of LED as backlights for television screens: full array, edge lit, and direct lit.
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.
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.
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.
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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.
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