Science Fair Project Encyclopedia
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 non-harmful.
The Plasma display panel was invented at the University of Illinois 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 1970's because the displays were rugged and needed neither memory nor refresh circuitry. There followed a long period of sales decline in the late 1970's as semiconductor memory made CRT displays incredibly cheap. Starting with his PhD disseration in 1975, Larry Weber of the University of Illinois sought to create a color plasma display, finally achieving that goal in 1995. Today the superior brightness and viewing angle of color plasma panels have caused these displays to have a resurgance of popularity.
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 200 cm (80 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 a AMLCD television; in 2004 the cost has come down to US$1900 or less for the popular 42-inch diagonal size, making it very attractive for home-theatre use. However, since the power comsumption is proportional to the square of the diagonal size, the larger screen sizes can use considerable power—"as much as 700 watts of power, enough to make some critics worry about the environmental consequences if the displays are widely adopted.". 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 and intensity, which is considered the end of the functional life of the display.
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. The image quality is not quite up to the standards of the best cathode ray tube sets (according to some), but it certainly meets most people's expectations. The biggest drawback of this technology has to be the high cost. With prices starting around US$2,000 and going all the way up past US$20,000 (as of 2004), these sets do not sell as quickly as older technologies like CRT. But as prices fall and technology advances, they may start to seriously compete against the CRT sets.
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 monochome 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 photons.
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. 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 standardised 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 (Da-Lite, Angles of View vol. III, "Contrast - From Dark to Light")
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