How Plasma Displays Work

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.

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.

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 article, we'll see how these
sets do so much in such a small space.

If you've read How Television Works, 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.

What is plasma?

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.

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.

Inside a Plasma 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.

The biggest drawback of this technology has been the price.
However, falling prices and advances in technology mean that
the plasma display may soon edge out the old CRT sets.