How A Plasma Display Works by forrests


									How A Plasma Display Works

A plasma display is an emissive flat panel display where light is created by phosphors
excited by a plasma discharge between two flat panels of glass. The gas discharge
contains no mercury (contrary to the backlights of an AMLCD); a mixture of noble gases
(neon and xenon) is used instead. This gas mixture is inert and entirely harmless.
The glass panels seem to be vacuum sealed, because when they are broken the plasma
breaks up,seemingly from the addition of air to the space.

The Plasma display panel was invented at the University of Illinois at Urbana-Champaign
by Donald L. Bitzer and H. Gene Slottow in 1964 for the PLATO Computer System. The
original monochrome (usually orange or green) panels enjoyed a surge of popularity in
the early 1970s because the displays were rugged and needed neither memory nor refresh
circuitry. There followed a long period of sales decline in the late 1970s as semiconductor
memory made CRT displays incredibly cheap. Nonetheless, plasma's relatively large
screen size and thin profile made the displays attractive for high-profile placement such
as lobbies and stock exchanges. In 1983, IBM introduced a 19" orange on black
monochrome display (model 3290 'information panel') which was able to show four
simultaneous 3270 virtual machine (VM) terminal sessions. In 1992, Fujitsu introduced
the world's first 21-inch full color display. It was a hybrid based on the plasma display
created at the University of Illinois at Urbana-Champaign and NHK STRL, achieving
superior brightness.
In 1997 Pioneer started selling the first Plasma TV to the public
Screen sizes have increased since the 21 inch display in 1992. The largest Plasma display
in the world was shown at the CES (Consumer Electronics Show) in Las Vegas in 2006.
It measured 103" and was made by Matsushita Electrical Industries (Panasonic).
Until quite recently (c. 2004) the superior brightness and viewing angle of color plasma
panels, when compared to LCD, made them one of the most popular forms of display for
HDTV. However since that time improvements in LCD technology have closed the gap
dramatically. The much lower weight, price and power consumption of LCDs have seen
them make large inroads into the former plasma market. Sony now only sells a very
limited range of Plasma screens, and appears set to quit this market altogether.

General characteristics
Plasma displays are bright (1000 lx or higher for the module), have a wide color gamut,
and can be produced in fairly large sizes, up to 260 cm (102 inches) diagonally. They
have a very high "dark-room" contrast, creating the "perfect black" desirable for watching
movies. The display panel is only 6 cm (2 1/2 inches) thick, while the total thickness,
including electronics, is less than 10 cm (4 inches). Plasma displays use as much power
per square meter as a CRT or an AMLCD television; in 2004 the cost has come down to
US$1900 or less for the popular 42 inch (107 cm) diagonal size, making it very attractive
for home-theatre use. Real life measurements of plasma power consumption find it to be
much less than that normally quoted by manufacturers. Nominal measuments indicate
150 Watts for a 50" screen. The lifetime of the latest generation of PDPs is estimated at
60,000 hours to half life when displaying video. Half life is the point where the picture
has degraded to half of its original brightness, which is considered the end of the
functional life of the display. So if you use it at an average of 2-1/2 hours a day, the PDP
will last approximately 65 years.
Competing displays include the Cathode ray tube, OLED, AMLCD, DLP, SED-tv and
field emission flat panel displays. The main advantage of plasma display technology is
that a very wide screen can be produced using extremely thin materials. Since each pixel
is lit individually, the image is very bright and looks good from almost every angle.
Because many plasma displays still have a lower resolution the image quality is often not
quite up to the standards of good LCD displays or cathode ray tube sets, but it certainly
meets most people's expectations. Also, most cheaper consumer displays appear to have
an insufficient color depth - a moving dithering pattern may be easily noticible for a
discerning viewer over flat areas or smooth gradients; expensive high-res panels are much
better at managing the problem.
With prices starting around US$2,000 and going all the way up past US$20,000 (as of
2004), these sets did not sell as quickly as older technologies like CRT. But as prices fall
and technology advances, they have started to seriously compete against the CRT sets.
Some 42" sets fell below $1,500 at major retailers like Best Buy and Costco during the
2005 Christmas season, and many of the retailers reported that plasma TVs were among
the hottest selling items for that season.

Functional details
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.
In a monochrome plasma panel, control circuitry charges the electrodes that cross paths at
a cell, causing the plasma to ionize and emit photons between the electrodes. The ionizing
state can be maintained by applying a low-level voltage between all the horizontal and
vertical electrodes - even after the ionizing voltage is removed. To erase a cell all voltage
is removed from a pair of electrodes. This type of panel has inherent memory and does
not use phosphors. A small amount of nitrogen is added to the neon to increase hysteresis.
To ionize the gas in a color panel, the plasma display's computer charges the electrodes
that intersect at that cell 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. The current creates a
rapid flow of charged particles, which stimulates the gas atoms to release ultraviolet
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 visible spectrum. Plasma displays use the same
phosphors as CRTs, accounting for the extremely accurate color reproduction.

Contrast ratio claims
Contrast ratio indicates the difference between the brightest part of a picture and the
darkest part of a picture, measured in discrete steps, at any given moment. The
implication is that a higher contrast ratio means more picture detail. Contrast ratios for
plasma displays are often advertised as high as 5000:1. On the surface, this is a great
thing. In reality, there are no standardized tests for contrast ratio, meaning each
manufacturer can publish virtually any number that they like. To illustrate, some
manufacturers will measure contrast with the front glass removed, which accounts for
some of the wild claims regarding their advertised ratios. For reference, the page you're
reading now (on a computer monitor) is actually about 50:1. A printed page is about 80:1.
A really good print at a movie theater will be about 500:1.

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