Plasma Display

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A plasma display panel (PDP) is a type of flat panel display now commonly used for large television displays (typically above 37-inch or 940 mm). Many tiny cells located between two panels of glass hold an inert mixture of noble gases (neon and xenon). The gas in the cells is electrically turned into a plasma (physics) which then excites phosphors to emission (electromagnetic radiation) light.

History The plasma display was invented at the University of Illinois at Urbana-Champaign by Donald Bitzer, H. Gene Slottow, and graduate student Robert Willson in 1964 for the PLATO. The original monochrome (usually orange or green, sometimes yellow) panels enjoyed a surge of popularity in the early 1970s because the displays were rugged and needed neither memory nor circuitry to refresh the images. A long period of sales decline followed in the late 1980s as semiconductor memory made CRT displays cheaper than plasma displays. 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-inch (483 mm) orange-on-black monochrome display (model 3290 'information panel') which was able to show four simultaneous IBM 3270 virtual machine (VM) terminal sessions. That factory was transferred in 1987 to startup company Plasmaco, which Dr. Larry F. Weber, one of Dr. Bitzer's students, founded with Stephen Globus, and James Kehoe, who was the IBM plant manager. In 1992, Fujitsu introduced the world's first 21-inch (533 mm) full-color display. It was a hybrid, based upon the plasma display created at the University of Illinois at Urbana-Champaign and NHK STRL, achieving superior brightness. In 1996, Matsushita Electrical Industries (Panasonic) purchased Plasmaco, its color AC technology, and its American factory. In 1997, Pioneer Corporation started selling the first plasma television to the public. Current plasma televisions are often seen around the home and are thinner and in greater sizes than their predecessors. Their thin size allows them to compete with other display technology such as projection screen.

Screen sizes have increased since the 21-inch (533 mm) display in 1992. The largest plasma video display in the world was shown at the 2006 Consumer Electronics Show in Las Vegas, Nevada, U.S.A, a 103-inch (261.6 cm) unit manufactured by Matsushita Electrical Industries (Panasonic).

Until quite recently, the superior brightness, faster response time, greater color spectrum, and wider viewing angle of color plasma video displays, when compared with Liquid crystal display television, made them one of the most popular forms of display for HDTV flat panel displays. For a long time it was widely believed that LCD technology was suited only to smaller sized televisions, and could not compete with plasma technology at larger sizes, particularly 40 inches and above.

However, since then, improvements in LCD technology have narrowed the technological gap. The lower weight, falling prices, higher available resolution, which is important for HDTV, and often lower electrical power consumption of LCDs make them competitive against plasma displays in the television set market. As of late 2006, analysts note that LCDs are overtaking plasmas, particularly in the important 40-inch (1.0 m) and above segment where plasma had previously enjoyed strong dominance a couple of years before. Another industry trend is the consolidation of manufacturers of plasma displays, with around fifty brands available but only five manufacturers.

General characteristics Plasma displays are bright (1000 lux or higher for the module), have a wide color gamut, and can be produced in fairly large sizes, up to 262 cm (103 inches) diagonally. They have a very low-luminance "dark-room" black level, creating a black some find more desirable for watching movies. The display panel is only about 6 cm (2½ inches) thick, while the total thickness, including electronics, is less than 10 cm (4 inches). Plasma displays use as much Electric power per square meter as a Cathode ray tube or an AMLCD television. Power consumption will vary greatly depending on what is watched on it. Bright scenes (say a football game) will draw significantly more power than darker scenes (say a movie scene at night). Nominal measurements indicate 400 watts for a 50-inch screen. Recent models, post 2006, consume between 220 and 310 watts for a 50-inch display when set to cinema mode. Most screens are set to 'shop' mode by default and this draws at least twice the power compared to a more comfortable 'home' setting.

The lifetime of the latest generation of plasma displays is estimated at 60,000 hours (or 27 years at 6 hours of use per day) of actual display time. More precisely, this is the estimated half life of the display, the point where the picture has degraded to half of its original brightness. It is watchable after this point, but is generally 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 display 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 has a wide viewing angle.

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, in front of and behind 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 in front of the cell, along the front glass plate. Control circuitry charges the electrodes that cross paths at a cell, creating a voltage difference between front and back and causing the gas to ionize and form a plasma (physics); as the gas ions rush to the electrodes and collide, photons are emitted.

In a monochrome plasma panel, 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.

In color panels, the back of each cell is coated with a phosphor. The ultraviolet photons emitted by the plasma excite these phosphors to give off colored light. The operation of each cell is thus comparable to that of a fluorescent lamp.

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, analogous to the Triad (computers) of a shadow-mask CRT. By varying the pulses of current flowing through the different cells thousands of times per second, the control system can increase or decrease the intensity of each subpixel color to create billions of different combinations of red, green and blue. In this way, the control system can produce most of the visible colors. Plasma displays use the same phosphors as CRTs, which accounts for the extremely accurate color reproduction.

Contrast ratio claims Contrast ratio is the difference between the brightest and darkest parts of an image, measured in discrete steps, at any given moment. Generally, the higher the contrast ratio, the more realistic the image is. Contrast ratios for plasma displays are often advertised as high as 20,000:1. On the surface, this is a significant advantage of plasma over other display technologies. Although there are no industry-wide guidelines for reporting contrast ratio, most manufacturers follow either the ANSI standard or perform a full-on-full-off test. The ANSI standard uses a checkered test pattern whereby the darkest blacks and the lightest whites are simultaneously measured, yielding the most accurate "real-world" ratings. In contrast, a full-on-full-off test measures the ratio using a pure black screen and a pure white screen, which gives higher values but does not represent a typical viewing scenario. Manufacturers can further artificially improve the reported contrast ratio by increasing the contrast and brightness settings to achieve the highest test values. However, a contrast ratio generated by this method is misleading, as content would be essentially unwatchable at such settings.

Plasma is often cited as having better black levels (and contrast ratios), although both plasma and LCD have their own technological challenges. Each cell on a plasma display has to be precharged before it is due to be illuminated (otherwise the cell would not respond quickly enough) and this precharging means the cells cannot achieve a true black. Some manufacturers have worked hard to reduce the precharge and the associated background glow, to the point where black levels on modern plasmas are starting to rival CRT. With LCD technology, black pixels are generated by a light polarization method and are unable to completely block the underlying backlight.

One shortcoming with plasma technology is that running a display at maximum brightness will significantly reduce the panel's lifespan. For this reason, many owners leave the brightness settings well below maximum, which typically still results in a brighter screen than CRT displays.

Screen burn-in

With phosphor-based electronic displays (including cathode-ray and plasma displays), the prolonged display of a menu bar or other graphical elements over time can create a permanent ghost-like image of these objects. This is due to the fact that the phosphor compounds which emit the light lose their luminosity with use. As a result, when certain areas of the display are used more frequently than others, over time the lower luminosity areas become visible to the naked eye and the result is called burn-in. While a ghost image is the most noticeable effect, a more common result is that the image quality will continuously and gradually decline as luminosity variations develop over time, resulting in a "muddy" looking picture image.

Plasma displays also exhibit another image retention issue which is sometimes confused with burn-in damage. In this mode, when a group of pixels are run at high brightness (when displaying white, for example) for an extended period of time, a charge build-up in the pixel structure occurs and a ghost image can be seen. However, unlike burn-in, this charge build-up is transient and self corrects after the display has been powered off for a long enough period of time, or after running random broadcast TV type content.

Plasma manufacturers have over time managed to devise ways of reducing the past problems of image retention with solutions with grey pillarboxes, pixel orbiters and image washing routines.

See also

References

External links

Laser TVs Set to Take Down Plasma (from DailyTech)
Plasma.com classroom main page Articles on Plasma TV technology and installation
Schematic drawing and explanation of a typical color plasma display
HowStuffWorks "How plasma displays work
LCD or plasma? (PCWorld.ca, written February 14, 2007)
Plasma Television: The Plasma Technology Explained
Plasma display panels: The colorful history of an Illinois technology by Jamie Hutchinson, Electrical and Computer Engineering Alumni News, Winter 2002-2003

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