Flatscreen televisions
Last updated: May 22, 2007.
Televisions used to be hot, heavy,
power-hungry beasts
that sat in the corner of your living room. Not any more! Now they're
slim enough to hang on the wall and they use a fraction as much energy as they used to. New televisions have
flatscreens and use LCD (liquid-crystal display) or plasma technology.
For a general introduction to TV, and details of how traditional
(cathode-ray tube or CRT) TVs work, take a look at our main article on television.
If you just want to learn about LCD and plasma, read on!
What's the difference?
You probably know that an old-style cathode-ray tube (CRT) TV makes
a picture using three electron guns. Think of them as three very fast,
very precise paintbrushes that dance back and forth, painting a moving
image on the back of the screen that you can watch when you sit in
front of it. LCD and plasma screens work in a completely different way.
If you sit up
close to a flatscreen TV, you'll notice that the picture is made from
millions of tiny blocks called pixels
(picture elements). Each one of these is effectively a separate red,
blue, or green light that can be switched on or off very rapidly to
make the moving color picture. In an LCD television, the pixels are
switched on or off using liquid crystals that rotate polarized
light (don't worry, we'll explain that in a minute). In a plasma
screen, each pixel is a tiny fluorescent lamp
switched on or off electronically. To understand LCD screens, we need
to look more closely at light and how it travels.
Photo: This iPod screen is another example of
LCD technology.
Its pixels are colored black and they're either on or off, so the
display is
black-and-white.
In an LCD TV screen, much smaller pixels colored red, blue, or green
make
a brightly colored moving picture.
Tricks of the light
Light's a mysterious thing. Sometimes it behaves like a stream of
particles—like a constant barrage of microscopic cannonballs carrying
energy we can see, through the air, at extremely high speed. Other
times,
light behaves more like waves on the sea. Instead of water moving up
and down, light is a wave pattern of electrical and magnetic energy
vibrating through space.
When sunlight streams down from the sky, the light waves are all
mixed up and vibrating in every possible direction. But if we put a
filter in the way, with a grid of lines arranged vertically like the
openings in prison bars (only much closer together), we can block out
all the light waves except the ones vibrating vertically (the only
light waves that can get through vertical bars). Since we block off
much of the original sunlight, our filter effectively dims the light.
This is how polarizing sunglasses work: they cut out all but the
sunlight vibrating in one direction or plane. Light filtered in this
way is called polarized or plane-polarized light (because it can travel in
only one plane).
If you have two pairs of polarizing sunglasses (and it won't work
with ordinary sunglasses), you can do a clever
trick. If you put one pair directly in front of the other, you should
still be able to see through. But if you slowly rotate one pair, and
keep the other pair in the same place, you will see the light coming
through gradually getting darker. When the two pairs of sunglasses are
at 90 degrees to each other, you won't be able to see through them at
all. The first pair of sunglasses blocks off all the light waves except
ones vibrating vertically. The second pair of sunglasses does exactly
the same. If both pairs of glasses are pointing in the same direction,
that's fine—light waves vibrating vertically can still get through
both. But if we turn the second pair of glasses through 90 degrees, the
light waves that made it through the first pair of glasses can no
longer make it through the second pair. No light at all can get through
two polarizing filters that are at 90 degrees to one another. (If you
want to read more about polarized light, there's a very good page on
The
Physics Classroom.)
LCD televisions
An LCD TV screen uses the sunglasses trick to switch its colored
pixels on or off. At the back of the screen, there's a large bright
light that shines out toward the viewer. In front of this, there are
the millions of pixels, each one made up of smaller areas called
sub-pixels that are coloured red, blue, or green. Each pixel has a
polarizing glass filter behind it and another one in front of
it at 90 degrees. That means the pixel normally looks dark. In between
the two polarizing filters there's a microscopic liquid crystal that
can be switched on or off electronically. When it's switched on, it
rotates the light passing through it through 90 degrees, effectively
allowing light to flow through the two polarizing filters and making
the pixel look bright. Each pixel is controlled by a separate
transistor (a tiny electronic
component) that can
switch it on or off many times each second.
Plasma televisions
A plasma screen is similar to an LCD, but
each pixel is effectively a microscopic fluorescent
lamp glowing with plasma. A plasma is a very hot form of gas in
which the atoms have blown apart to make negatively charged electrons
and positively charged ions (atoms minus their electrons). These move
about freely, producing a fuzy glow of light whenever they collide.
Plasma screens can be
made much bigger than ordinary cathode-ray tube televisions, but they
are also much more expensive.
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