Plasma display

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A plasma display panel (PDP) is an emissive flat panel display where visible 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 active matrix LCD); an inert mixture of noble gases (neon and xenon) is used instead.

Contents

[edit] History

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, 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 1970s 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" 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 that one of Dr. Bitzer's students, Dr. Larry F. Weber founded with Stephen Globus, and James Kehoe, who was the IBM plant manager. 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 1996, Matsushita purchased Plasmaco and its color AC technology and American facility. In 1997 Pioneer started selling the first plasma television 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, the superior brightness, wider color range, and wider viewing angle of color plasma displays, when compared to LCD televisions, 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 lower weight, price, and power consumption of LCDs have seen them make large inroads into the former plasma market.

[edit] 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 262 cm (103 inches) diagonally. They have a very high "dark-room" black level, creating the "perfect black" 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 power per square meter as a CRT or an AMLCD television. Real life measurements of plasma power consumption find it to be much less than that normally quoted by manufacturers. Nominal measurements indicate 150 watts for a 50" screen.

The lifetime of the latest generation of plasma displays is estimated at 60,000 hours[citation needed] 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, which is considered the end of the functional life of the display.

Competing displays include the CRT, 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 has a wide viewing angle. Most cheaper consumer displays appear to have an insufficient color depth - a moving dithering pattern may be easily noticeable for a discerning viewer over flat areas or smooth gradients; expensive high-resolution panels are much better at managing the problem.

[edit] Pros and cons

Pros

  • Slim, wall-mountable design
  • Larger maximum practical size than LCD screens
  • Wider viewing angle than LCD screens and better color consistency throughout this range
  • Better contrast ratio than LCD, though LCDs are improving rapidly
  • Faster response time than LCD, though LCDs are improving rapidly
  • Able to achieve darker black than LCD

Cons

  • PDPs are fragile, making them difficult to ship and install.
  • Expensive, although currently cheaper than LCDs per unit of size at larger sizes.
  • Older panels were notoriously subject to burn-in, although due to improvements in phosphors, in modern PDPs the effect is largely caused by polarization of the gas particles and can often be reversed by leaving the screen on a "snow" or static channel for an hour. Some home theater afficionados claim that, while burn-in is less likely now than in the past, it is still possible in some circumstances, and many plasma televisions have functions (such as "orbiting", in which the image is periodically moved imperceptibly) to minimize the problem. In any case, LCDs are not susceptible to permanent burn-in.
  • The display is brightest during its first 2000 hours. Thereafter, the display gradually dims. LCD backlights exhibit dimming as well, but they are replaceable. A plasma display cannot be recharged since the panel is a fixed pixel device with each pixel etched into the glass substrate. However, as the phosphors in a modern panel have a 60,000 hour half-life, most users will never see a plasma reach the end of its life.
  • At higher elevations, usually 6000 ft (1,800 m) or higher, PDPs exhibit noticeable humming or buzzing.
  • Sufferers of the DLP Rainbow Effect may encounter a similar problem with PDPs in high contrast situations. This typically manifests itself as a green flash during sudden changes from white to black and is most obvious in films such as Underworld.

[edit] 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; 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. These phosphors are excited to give off colored light by the ultraviolet photons emitted by the plasma. 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 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.

[edit] 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 10,000:1. On the surface, this is a significant advantage of plasma over other display technologies. Yet there are no standardized tests for contrast ratio, meaning that each manufacturer can publish virtually any number. Some manufacturers measure contrast with the front glass removed, which accounts for some of the wild claims made in advertising. For reference, text viewed on a CRT computer monitor usually has a contrast ratio of about 50:1. A printed page has a ratio of about 80:1. A very good print at a movie theater may reach 500:1[1].

Plasma displays achieve their high contrast ratios mostly thanks to their brightness, which is typically much higher than other display technologies, which permits the use of especially dark glass. However, running a plasma display at its maximum brightness can significantly reduce its life, so many owners run them at well below maximum brightness (which is usually still brighter than CRTs).

[edit] See also

[edit] References

  1. ^ Da-Lite, Angles of View vol. III, "Contrast - From Dark to Light"[1]

[edit] External links


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