For the past 75 years, the vast majority of televisions
have been built around the same technology: the cathode ray
tube (CRT). In a CRT television, a gun fires a beam of
electrons (negatively-charged particles) inside a large
glass tube. The electrons excite phosphor atoms along
the wide end of the tube (the screen), which causes the
phosphor atoms to
light up. The television image is produced by lighting up
different areas of the phosphor coating with different colors
at different intensities (see How Televisions
Work for a detailed explanation).
Photo courtesy Sony
A plasma display from
Sony
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Cathode
ray tubes produce crisp, vibrant images, but they do have a
serious drawback: They are bulky. In order to increase
the screen width in a CRT set, you also have to
increase the length of the tube (to give the scanning
electron gun room to reach all parts of the screen).
Consequently, any big-screen CRT television is going to weigh
a ton and take up a sizable chunk of a room.
Recently, a new alternative has popped up on store shelves:
the plasma flat panel display. These televisions have
wide screens, comparable to the largest CRT sets, but they are
only about 6 inches (15 cm) thick. In this edition of HowStuffWorks,
we'll see how these sets do so much in such a small space.
What is Plasma?
If you've read How Televisions
Work, then you understand the basic idea of a standard
television or monitor. Based on the information in a video
signal, the television lights up thousands of tiny dots
(called pixels)
with a high-energy beam of electrons. In most systems, there
are three pixel colors -- red, green and blue -- which are
evenly distributed on the screen. By combining these colors in
different proportions, the television can produce the entire
color spectrum.
The basic idea of a plasma display is to illuminate tiny
colored fluorescent
lights to form an image. Each pixel is made up of three
fluorescent lights -- a red light, a green light and a blue
light. Just like a CRT television, the plasma display varies
the intensities of the different lights to produce a full
range of colors.
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Tuning
InMost plasma displays
aren't technically televisions, because they don't have
a television tuner. The television tuner is the device
that takes a television signal (the one coming from a cable
wire, for example) and interprets it to create a
video image.
Like LCD
monitors, plasma displays are just monitors that
display a standard video signal. To watch television on
a plasma display, you have to hook it up to a separate
unit that has its own television tuner, such as a VCR.
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The central
element in a fluorescent light is a plasma, a gas made
up of free-flowing ions (electrically charged atoms)
and electrons (negatively charged particles). Under
normal conditions, a gas is mainly made up of uncharged
particles. That is, the individual gas atoms include
equal numbers of protons (positively charged particles in the
atom's nucleus) and electrons. The negatively charged
electrons perfectly balance the positively charged protons, so
the atom has a net charge of zero.
If you introduce many free electrons into the gas by
establishing an electrical voltage across it, the situation
changes very quickly. The free electrons collide with the
atoms, knocking loose other electrons. With a missing
electron, an atom loses its balance. It has a net positive
charge, making it an ion.
In a plasma with an electrical current running through it,
negatively charged particles are rushing toward the positively
charged area of the plasma, and positively charged particles
are rushing toward the negatively charged area.
In this mad rush, particles are constantly bumping into
each other. These collisions excite the gas atoms in the
plasma, causing them to release photons of energy. (For
details on this process, see How
Fluorescent Lamps Work.)
Xenon and neon atoms, the atoms used in plasma screens,
release light photons when they are excited. Mostly,
these atoms release ultraviolet light photons, which
are invisible to the human eye. But
ultraviolet photons can be used to excite visible light
photons, as we'll see in the next section.
Inside the Display
The xenon and neon gas in
a plasma television is contained in hundreds of thousands of
tiny cells positioned between two plates of glass. Long
electrodes are also sandwiched between the glass plates, on
both sides of the cells. The address electrodes sit
behind the cells, along the rear glass plate. The transparent
display electrodes, which are surrounded by an
insulating dielectric material and covered by a
magnesium oxide protective layer, are mounted above the
cell, along the front glass plate.
Both sets of electrodes extend across the entire screen.
The display electrodes are arranged in horizontal rows along
the screen and the address electrodes are arranged in vertical
columns. As you can see in the diagram below, the vertical and
horizontal electrodes form a basic grid.
To ionize the gas in a particular cell, the plasma
display's computer charges the electrodes that intersect at
that cell. It does this thousands of times in a small fraction
of a second, charging each cell in turn.
When the intersecting electrodes are charged (with a
voltage difference between them), an electric current flows
through the gas in the cell. As we saw in the last section,
the current creates a rapid flow of charged particles, which
stimulates the gas atoms to release ultraviolet photons.
The released ultraviolet photons interact with phosphor
material coated on the inside wall of the cell. Phosphors are
substances that give off light when they are exposed to other
light. When an ultraviolet photon hits a phosphor atom in the
cell, one of the phosphor's electrons jumps to a higher energy
level and the atom heats up. When the electron falls back to
its normal level, it releases energy in the form of a
visible light photon.
The phosphors in a plasma display give off colored light
when they are excited. Every pixel is made up of three
separate subpixel cells, each with different colored
phosphors. One subpixel has a red light phosphor, one subpixel
has a green light phosphor and one subpixel has a blue light
phosphor. These colors blend together to create the overall
color of the pixel.
By varying the pulses of current flowing through the
different cells, the control system can increase or decrease
the intensity of each subpixel color to create hundreds of
different combinations of red, green and blue. In this way,
the control system can produce colors across the entire
spectrum.
The main advantage of plasma display technology is that you
can produce a very wide screen using extremely thin materials.
And because each pixel is lit individually, the image is very
bright and looks good from almost every angle. The image
quality isn't quite up to the standards of the best cathode
ray tube sets, but it certainly meets most people's
expectations.
To learn more about plasma displays, as well as other
television technologies, check out the links on the next page.
