Schermo al plasma: differenze tra le versioni

Versione delle 10:09, 3 gen 2007

Versione delle 01:53, 21 gen 2007

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{{T|lingua inglese}}

Uno '''schermo al plasma''' è uno schermo dove la luce viene creata grazie a dei [[fosforo | fosfori]] eccitati da una scarica di [[Plasma (fisica) | plasma]]. La scarica di plasma contiene una mistura di [[gas nobili]] ([[neon]] e [[xeno]]); questi gas sono contenuti in centinaia di migliaia di piccole celle all'interno di un guscio formato da due elettrodi e rivestito da uno strato di [[fosforo]]. Quando gli elettrodi inducono un campo elettrico i gas contenuti nella cella [[ione | ionizzano]], emettendo una scarica di plasma che, reagendo col fosforo della cella, produce luce.

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Gli svantaggi più grandi di questa tecnologia sono dati dal prezzo e dal progressivo degrado dell'immagine; soprattutto la persistenza di immagini statiche tende a creare fastidiose ombreggiature e zone scure, che, nei casi più gravi, si notano anche a schermo spento, il degrado dell'immagine è conseguente alla progressiva alterazione cromatica della superficie interna della singola cella contenente il gas, e dal progressivo ridursi della sua luminosità, analogalmente come avviene nelle lampade al neon.

<!--The plasma display panel was invented at the [[University of Illinois at Urbana-Champaign]] by [[Donald Bitzer|Donald L. Bitzer]], H. Gene Slottow, and graduate student Robert Willson in 1964 for the [[PLATO|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-inch 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 [http://www.plasmatvscience.org/plasmatv-history1.html 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 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|Pioneer]] started selling the first plasma television to the public.

Screen sizes have increased since the 21-inch display in 1992. The largest plasma video display in the world was shown at the ([[Consumer Electronics Show]]) in Las Vegas, Nevada, U.S.A., in 2006, measuring 103 inches and was made by [[Matsushita]].

Until quite recently, the superior brightness, greater color spectrum, and wider [[viewing angle]] of color plasma video displays, when compared with [[Liquid crystal display television|LCD televisions]], made them one of the most popular forms of display for [[HDTV]] [[Flat panel display]]s. 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. As of late 2006, Matsushita's flat-panel lineup for 40" screens or larger is composed only of plasmas, while their smaller to mid-sized line is LCDs.

However, since then, improvements in LCD technology have narrowed the technological gap. The lower weight, falling prices, higher available resolution which is crucial for HDTV, and 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" and above segment where plasma have previous enjoyed strong dominance a couple years before. [http://news.zdnet.com/2100-9584_22-6138290.html][http://www.msnbc.msn.com/id/15916808/]

== General characteristics ==

Plasma displays are bright (1000 [[lux|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 [[Electric power|power]] per square meter as a [[Cathode ray tube|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. {{fact}}

The lifetime of the latest generation of plasma displays is estimated at 60,000 hours 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 [[Cathode ray tube|CRT]], [[OLED]], [[AMLCD]], [[DLP]], [[SED-tv]] and [[field emission display|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.

== Pros and cons ==

===Pros===

* Slim, wall-mountable design

* Wider viewing angle than [[Liquid crystal display television|LCD]] screens and better color consistency throughout this range

* Until late 2006, plasma was the best suited and most affordable for [[flat panel display|flat-panel televisions]] of large sizes, particularly 40" and above, while LCD technology was mainly for smaller sized televisions.

* Currently cheaper than LCDs per unit of size at larger sizes.

* 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, though LCDs are improving (upcoming Dynamic Backlight Switching and OLED technologies)

* Contains no [[Mercury (element)|mercury]], unlike the back light of LCDs

===Cons===

* Often heavier than [[Liquid crystal display television|LCD]] panels.

* Generally has a lower resolution than LCDs, which is becoming crucial for [[HDTV]].

* 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 [[Phosphor burn-in|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 aficionados 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|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 (2003 movie)|Underworld]].

== 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 [[ion]]ize and form a [[plasma (physics)|plasma]]; as the gas ions rush to the electrodes and collide, [[photon]]s 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. 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 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<ref>Da-Lite, Angles of View vol. III, "Contrast - From Dark to Light"[http://www.da-lite.com/education/angles_of_view.php?action=details&issueid=56]</ref>.

Plasma displays achieve their high contrast ratios mostly thanks to their brightness, which is typically much higher than other display technologies, permitting 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).

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[[categoria:Hardware]]