Plasma TV
Last updated: October 31, 2008.
A box that makes pictures from a soup of hot gas? Whatever next?
Cars that can fly? Men on Mars? It may sound like something straight
out of Flash Gordon, but plasma television
is far from science fiction. It's a brilliant example of how cutting-edge science
can be applied to everyday problems to make our lives better and more
fun. Let's take a closer look at how it works!
Photo: A typical flatscreen television. Can you tell just by looking whether it's
LCD or plasma? Not necessarily!
What is plasma anyway?
In schools they teach us that all substances come in three basic
flavors or states of matter:
solid, liquid, and gas. But they're wrong! There's a fourth
flavor called plasma
(and, arguably, there are even more states of matter too that we won't get
into here). What exactly is a plasma and how does it relate to solids, liquids, and gases?
Suppose you have a lump of freezing cold ice (a solid). Heat it up
a bit and you'll get a liquid (water). Heat it
up a bit more and, pretty soon, you'll have a gas (steam). The more heat you supply, the
more energy you inject. The more energetically the molecules (or atoms) have,
the further apart they can push and the more they move about.
In a solid like water, the molecules are bound tightly together; in liquid water, the molecules
are free to move past one another (that's why water can pour and
flow); in steam (gaseous water), the molecules are completely free of
one another and have so much energy that
they spread out to fill all the space available.
Artwork: Plasmas are made when some of the electrons in a gas break free,
leaving behind a positively charged nucleus called an ion. The negatively charged electrons
and positively charged ions make it possible for the gas to conduct electricity.
But what happens if you don't stop there? What if you keep on
heating a gas? The molecules and atoms inside
it bust apart, releasing some of their electrons so they move freely in and around
it. When atoms break apart this way, they form positively charged
particles called ions. The mixture of positively charged ions
and negatively charged electrons in a plasma turns it into a kind of hot
soup that will conduct electricity very easily. That's what we mean
by a plasma. It's a special type of gas in which some of the atoms
have become ions (an ionized gas, in other words).
Photo: Playing with plasma. This glass sphere contains plasma: a hot, ionized gas produced with an electric current. When you put your hands on the glass, they attract free electrons so the plasma seems to move toward you!
Photo by Brookhaven National Laboratory courtesy of US Department of Energy.
How a plasma TV set works
If you've read our articles on energy-saving fluorescent lamps
(also known as CFLs) and neon lamps
(the lamps that make brightly
colored displays in our streets), you'll know how they make light by
buzzing electricity through a gas.
Imagine if you built a TV screen
out of millions of microscopically tiny CFLs or neon lamps, each of
which could be switched on or off very quickly, as necessary, by an
electronic circuit, to control all the
separate pixels (lit-up,
colored square) on the screen. That's pretty much how a plasma TV
works and it's very different to other kinds of television
technology: in a conventional (cathode-ray) television,
the picture is built up by scanning an electron beam back and forth over a screen
treated with chemicals called phosphors; in an LCD TV (liquid-crystal
display television), polarizing crystals make light rays bend to
switch the pixels on and off.
The pixel cells in a plasma TV have things in common with both neon
lamps and CFLs. Like a neon lamp, each cell is filled with tiny
amounts of neon or xenon gas. Like a CFL, each cell is coated inside
with phosphor. chemicals. In a CFL, the phosphor is the chalky white
coating on the inside of the glass tube and it works like a filter.
When electricity flows into the tube, gas atoms crash about inside it
and generate invisible ultraviolet light. The white phosphor
coating turns this into visible white light that we can see. In a plasma TV,
the cells are a bit like tiny CFLs only coated with phosphors that
are red, blue, or green. Their job is to take the invisible
ultraviolet light produced by the neon or xenon gas in the cell and
turn it into red, blue, or green light we can actually see.
What's the difference between plasma and LCD TV?
Plasma and LCD TVs look very similar but, as we've just seen, work
in totally different ways. Plasma TVs tend to cost an awful lot
more, so why not just buy an LCD? The main difference is that the
cells that make up the pixels in a plasma TV can switch on and off
thousands of times faster than the pixels in an LCD screen, so you
get clearer pictures with less blur, especially for moving images
during action movies or sports games. (The latest LCD screens switch
on and off much faster than older ones, but it's generally true that plasma
screens are still faster.) Plasma TVs are also typically brighter and
have higher contrast, which can be important if you watch a lot of TV
in the daylight. You can view plasma screens from a wider angle
without seeing distortion of colors (like you get on an LCD computer
screen), so they're often better for larger audiences
(projection TV is another option).
But there are drawbacks with plasma too. They're more power hungry
than LCDs and the screens are heavier and more fragile, so you have
to be very careful when you transport them. Plasma TVs also have
problems with "burn in" (where images that are
displayed for too long can permanently damage the screen) and they tend to "burn
out" (stop working through too much use) more quickly than LCDs, though
most people are likely to replace a TV for something newer before
this happens.
Generally speaking, plasma TVs are much cheaper
than they used to be, while LCD TVs are much faster, and the two
technologies are now very broadly comparable for ordinary household
viewing—just pick whichever you like best!
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