by Chris Woodford. Last updated: September 4, 2014.
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. It used to be the case that if you wanted a very large TV, you'd almost certainly have to buy a plasma screen, but LCD screens are now available in sizes up to 250cm (100 inches)—or even more.
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.
But what happens if you don't stop there? What if you keep on
heating a gas? The molecules and atoms inside
it break apart, releasing some of their electrons so they move freely in and around
it. When atoms disintegrate like this, 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. 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. 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 John Suits courtesy of US Navy.
How a plasma TV set makes its picture
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 squares) 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 invisible light into visible white light. 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.
Step by step
- Much like the picture in an LCD screen, the picture made by a plasma TV is made from an array (grid) of red, green and blue pixels (microscopic dots or squares).
- Each pixel can be switched on or off individually by a grid of horizontally and vertically mounted electrodes (shown as yellow lines).
- Suppose we want to activate one of the red pixels (shown hugely magnified in the light gray pullout circle on the right).
- The two electrodes leading to the pixel cell put a high voltage across it, causing it to ionize and emit ultraviolet light (shown here as a turquoise cross, though it would be invisible in the TV itself).
- The ultraviolet light shines through the red phosphor coating on the inside of the pixel cell.
- The phosphor coating converts the invisible ultraviolet into visible red light, making the pixel light up as a single red square.
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 more quickly than older ones, but it's generally true that plasma
screens are 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 for showing pictures to a roomful of people).
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
Generally speaking, plasma TVs are much cheaper
than they used to be, while LCD TVs are much faster than they used to be, and the two
technologies are now very broadly comparable for ordinary household
viewing—just pick whichever you like best!
On this website
- Choosing a big TV: Wired, 15 November 2011. Lists some of the things you need to consider when deciding between LCD, plasma, and other TV technologies.
- What's greener: an LCD or plasma TV? by Umbra Fisk, The Guardian, 26 February 2009. Which technology should eco-friendly TV lovers opt for?
- Call to tax 'wasteful' plasma TVs by Paul Rincon, BBC News, 15 May 2007. Energy-hungry plasma TVs should be taxed more, according to critics.
- Do flat-screen TVs eat more energy? by Sean Coughlan, BBC News, 7 December 2006. Plasma TVs can use up to three times more energy than old-style cathode-ray TVs or larger LCD TVs.