Plasma display

An example of a plasma display

A plasma display panel (PDP) is a type of flat panel display common in large TV displays (typically above 37-inch or 940 mm). Many tiny cells between two panels of glass hold an inert mixture of noble gases. The gas in the cells is electrically turned into a plasma which then excites phosphors to emit light. Plasma displays should not be confused with LCDs, another lightweight flatscreen display using different technology.

History

Plasma displays were first used in PLATO computer terminals. This PLATO V model illustrates the display's monochromatic orange glow as seen in 1981.

The monochrome plasma video display was co-invented in 1964 at the University of Illinois at Urbana-Champaign by Donald Bitzer, H. Gene Slottow, and graduate student Robert Willson for the PLATO Computer System. The original neon orange monochrome Digivue display panels built by glass producer Owens-Illinois were very popular in the early 1970s because they were rugged and needed neither memory nor circuitry to refresh the images. A long period of sales decline occurred in the late 1970s as semiconductor memory made CRT displays cheaper than the $2500 512x512 PLATO plasma displays. Nonetheless, the plasma displays' relatively large screen size and 1" thickness made them suitable for high-profile placement in lobbies and stock exchanges.

In the late-1960's graduate student Larry F. Weber became interested in plasma displays and completed his PhD studies in this area at the University of Illinois. After his graduation in 1975, he joined the university as a research professor and began a period of 15 years of intensive research into plasma displays, filing 15 patents and eventually developing the sustainer power-saving circuit and color plasma panels. It is no exaggeration to say that one man working in the wilderness for 20 years kept plasma technology alive when semiconductor-backed CRTs and LCD displays threatened to make them extinct.

In 1983, IBM introduced a 19-inch (48 cm) orange-on-black monochrome display (model 3290 'information panel') which was able to show up to four simultaneous IBM 3270 terminal sessions. Due to heavy competition from monochrome LCD displays, in 1987 IBM planned to shutter its factory in upstate New York. When Larry Weber heard about this he felt that his dream of practical plasma displays had evaporated. Taking a big chance, he co-founded a startup company Plasmaco with Stephen Globus, as well as James Kehoe, who was the IBM plant manager, and bought the plant from IBM. Weber stayed in Urbana as CTO until 1990, then moved to upstate New York to provide hands-on direction to Plasmaco.

In 1992, Fujitsu introduced the world's first 21-inch (53 cm) 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 1994, facing certain death from an onslaught of color LCD displays, Weber demonstrated color plasma technology on the last day of an industry convention in San Jose. Panasonic was interested, and began a joint development project with Plasmaco, which led in 1996 to the purchase of Plasmaco, its color AC technology, and its American factory.

In 1997, Fujitsu introduced the first 42-inch (107 cm) plasma display; it had 852x480 resolution and was progressively scanned. Also in 1997, Pioneer started selling the first plasma television to the public.

Screen sizes have increased since the introduction of plasma displays. The largest plasma video display in the world at the 2008 Consumer Electronics Show in Las Vegas, Nevada, U.S., North America was a 150-inch (381 cm) unit manufactured by Matsushita Electrical Industries (Panasonic) standing 6 ft (180 cm) tall by 11 ft (330 cm) wide and expected to initially retail at US$150,000.

Until the early 21 century, superior brightness, faster response time, greater color spectrum, and wider viewing angle of color plasma video displays, compared to LCD televisions, made them a popular 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 (100 cm) and above. Improvements in VLSI fabrication technology have narrowed the technological gap. The lower weight, falling prices, and often lower electrical power consumption of LCDs make them competitive with plasma television sets. 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. Another industry trend is the consolidation of manufacturers of plasma displays, with around fifty brands available but only five manufacturers. In the first quarter of 2008 a comparison of worldwide TV sales breaks down to 22.1 million for CRT, 21.1 million for LCD, 2.8 million for Plasma, and 124 thousand for rear-projection.

In 2004 Larry Weber retired, but as of 2008 he is back at work, looking for new ways to reduce the power consumption of plasma displays. If he is successful, plasma displays may once again start to take market share away from LCD displays.

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 381 cm (150 inches) diagonally. They have a very low-luminance "dark-room" black level compared to the lighter grey of the unilluminated parts of an LCD screen. The display panel is only about 6 cm (2.5 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. Power consumption varies greatly with picture content, with bright scenes drawing significantly more power than darker ones. Nominal power rating is typically 400 watts for a 50-inch (127 cm) screen. Post-2006 models consume 220 to 310 watts for a 50-inch (127 cm) display when set to cinema mode. Most screens are set to 'shop' mode by default, which draws at least twice the power (around 500-700 watts) of a 'home' setting of less extreme brightness.

The lifetime of the latest generation of plasma displays is estimated at 60,000 hours of actual display time, or 27 years at 6 hours per day. This is the estimated time over which maximum picture brightness degrades to half the original value, not catastrophic failure.

Competing displays include the CRT, OLED, AMLCD, DLP, SED-tv, and field emission flat panel displays. Advantages of plasma display technology are that a large, very thin screen can be produced, and that the image is very bright and has a wide viewing angle.

Functional details

Composition of plasma display panel

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 put together 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. 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" 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 1,000,000:1. On the surface, this is a significant advantage of plasma over display technologies other than OLED. 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. However more recent LCD panels (particularly those using White LED illumination can "cheat" by dynamically reducing the backlighting on darker scenes.

Screen burn-in

See also: Phosphor burn-in

An example of a plasma display that has suffered severe burn-in from stationary text

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 involving gray pillarboxes, pixel orbiters and image washing routines.

External links

• Plasma display at HowStuffWorks
• Schematic drawing and explanation of a typical color plasma display
• Plasma display panels: The colorful history of an Illinois technology by Jamie Hutchinson, Electrical and Computer Engineering Alumni News, Winter 2002-2003
• Plasma is better than LCD? according to Panasonic