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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.

How coloured pixels in LCD TVs work

There's a bright light at the back of your TV; there are lots of colored squares flickering on and off at the front. What goes on in between? Here's how each colored pixel is switched on or off:

How pixels are switched off

Artwork showing how an LCD TV's pixels are switched off
  1. Light travels from the back of the TV toward the front from a large bright light.
  2. A horizontal polarizing filter in front of the light blocks out all light waves except those vibrating horizontally.
  3. Only light waves vibrating horizontally can get through.
  4. The liquid crystal for this pixel is switched off, so light travels straight through it unchanged.
  5. Light waves emerge from the liquid crystal still vibrating horizontally.
  6. A vertical polarizing filter in front of the liquid crystal blocks out all light waves except those vibrating vertically. The horizontally vibrating light that travelled through the liquid crystal cannot get through the vertical filter.
  7. No light reaches the screen at this point. In other words, this pixel is dark.

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How pixels are switched on

Artwork showing how an LCD TV's pixels are switched on
  1. The bright light at the back of the screen shines as before.
  2. The horizontal polarizing filter in front of the light blocks out all light waves except those vibrating horizontally.
  3. Only light waves vibrating horizontally can get through.
  4. A transistor switches on the liquid crystal for this pixel. The crystal rotates light waves by 90° as they travel through it.
  5. Light waves that entered the liquid crystal vibrating horizontally emerge fromit vibrating vertically.
  6. The vertical polarizing filter in front of the liquid crystal blocks out all light waves except those vibrating vertically. The vertically vibrating light that emerged from the liquid crystal can now get through the vertical filter.
  7. The pixel is lit up. A red, blue, or green filter gives the pixel its color.

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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Copyright © Chris Woodford 2007.

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