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What is it?
A type of display technology well suited to making large, flat screen displays. Plasma screens are now very popular in large televisions, having made cathode ray tubes largely obsolete.
The technology was invented in the 1960's but it is only recently that full colour screens have become practical.
What is a Plasma?
'Plasma' is an electrically charged gas that gives off light (the 'light' might be ultraviolet and so is invisible to humans - but there are ways around that)
Factoid: Sometimes a plasma is said to be the fourth state of matter after Solid, Liquid and Gas.
A lightning bolt is a huge example of a plasma being created. An electric current flows from the clouds down to the ground and as it does so, the air molecules are ripped apart by the sheer amount of energy flowing, so forming a plasma. When they come back together again, light is given off, which is why you see a lightning flash.
A plasma display makes use of this effect (although much less dramatically!).
How does it work?
One picture element (sub-pixel) of the display is made up of a tiny hollow glass cube/cell filled with a noble gas such as Xenon and Neon. An electric current is made to flow through the gas which creates a plasma. The glass on the front of the cube is coated with a special material called a 'phosphor' that gives off a coloured light when struck by particles (photons) coming out of the plasma.
The sub-pixels are arranged in a group of Red, Green and Blue very close to one another to form a single 'pixel'. When they each shine with different intensity, your eye combines the Red, Green and Blue light so you sense a different colour. With Red Green and Blue you can produce the illusion of any colour including white. Black is simply all the element being off.
Do they wear out?
Yes eventually, just like a fluorescent tube. It is estimated they last about 60,000 hours or about 25 years in heavy use.
Why large displays rather than small ones?
The gas-filled glass cubes need to be certain size to work well and this lends itself well to large displays, as each pixel can be larger. Trying to make even tinier cubes for small displays is much more expensive and difficult.
How does Plasma compare to other technologies?
Compared to a Cathode Ray Tube (CRT)
- It is much thinner
- It is much lighter for large screens
- Easier to make into large screens.
Compared to a Liquid Crystal Display
- It is not as suitable for small displays
- It uses more power
- It has a faster response, good for fast moving images
- It is brighter
- It has higher contrast (difference between black and white)
- It has a better 'black', which is good for watching movies.
And the future?
It is all down to economics.
Makers of television and display screens have to decide what is the most economic technology to use for a given sized screen.
At the moment Plasma has the edge over Liquid Crystal for sizes above 36 inches or so. But the developers of LCD technology are fighting back, making their systems lighter, faster and cheaper so perhaps Plasma will be replaced by LCD technology in the future.
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