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Plasma display

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. 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. Plasma displays have a very high "dark-room" contrast, creating the "perfect black" , essential for watching movies. The display panel is only 6 mm 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, and in 2004 the cost has come down to $1900 or less for the popular 42-inch diagonal size, making it very attractive for home-theatre use. The life-time of the latest generation of PDP's is guaranteed at 60,000 hours when displaying video. Competing displays include the Cathode ray tube, OLED, AMLCD, DLP, and field emission flat panel displays.

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. 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 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 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, 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 will not sell as quickly as other competing technologies. But as prices fall and technology advances, they may start to edge out the CRT sets.

Contrast ratios

Contrast ratio indicates the difference between the brightest part of a picture and the darkest part of a picture at any given moment. Contrast ratios for plasma displays are often advertised as high as 4000:1. 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. For reference, the page you're reading now 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.

Last updated: 02-09-2005 23:45:07
Last updated: 02-26-2005 20:49:46