Plasma

1.1 What is Plasma Display Panels?

A plasma display panel (PDP) is a type of flat panel display common to large TV displays (80 cm or larger). Many tiny cells between just two panels of glass hold a mixture of noble gases. The gas in the cells is electrically turned into plasma which emits ultraviolet light which then excites phosphors to emit visible light. Plasma displays should not be confused with LCDs, another lightweight flat screen display using different technology. We see this more and more on store signs and in ads promoting large-screen, flat-panel plasma TVs. The flat plasma display is a major competitor among several flat panel display technologies – all vying for the potentially enormous High Definition TV market.

For many people, the word “Plasma” has come to mean only display screens. As impressive as those displays are, the word refers much more generally to a special state of matter that resides not only inside those display panels, producing their light and images, but also in many other devices we use every day. In some cases plasma is part of a product’s manufacturing process. In addition, plasmas fill much of our surrounding universe. Plasma is a gas containing a large number of electrically charged particles, both negatively-charged electrons and positively-charged atoms, called ions. In most plasma, including those in the plasma display panel, there also remains a large number of uncharged particles (called the “background gas”). The plasma in a display panel is much like the plasma in another familiar device, the fluorescent lamp. A plasma display panel (PDP) is essentially a collection of very small fluorescent-type lamps, each a few tenths of a millimeter in size. If we look closely, it is easy to distinguish the individual PDP cells – the tiny color elements of red, green and blue light that together form what is called a pixel. As in a fluorescent lamp, the light we see does not come from the plasma directly, but rather from the phosphor coatings on the inside walls of the cells when they are exposed to ultraviolet (UV) radiation emitted by the plasma. Because each cell emits its own light, a plasma display panel is called an “emissive display.” This contrasts with the familiar liquid crystal display (LCD), a type of flat display in which the light comes from a lamp (actually a plasma lamp!) behind the liquid crystal, which has arrays of small switches controlling where light is allowed to pass through.

All plasmas require a source of energy. As in fluorescent lamps, the plasma in a PDP is produced by applying a voltage across a gap that contains gas. The plasmas used in PDPs are considered "cold" plasmas in the sense that the background gas stays relatively cold while the electrons (and ions) in the plasma are heated by the applied voltage. When the hot electrons collide with the background gas atoms and transfer energy to them, many of those atoms respond by emitting UV radiation. The operating conditions of the display (gas composition, pressure, voltage, geometry, etc.) represent a compromise, taking into account performance requirements such as low voltage operation, long life, high brightness and high contrast.

The plasma display itself is a simple device consisting of two parallel glass plates separated by a precise spacing of some tenths of a millimeter and sealed around the edges. The space between the plates is filled with a mixture of rare gases at a pressure somewhat less than one atmosphere. Parallel stripes of transparent conducting material with a width of about a tenth of a millimeter are deposited on each plate, with the stripes on one plate perpendicular to those on the other.

These stripes are the "electrodes" to which voltages are applied. The intersections of the rows of electrodes on one side and the columns of electrodes on the opposite glass plate define the individual color elements – or cells – of a PDP.

High quality color images it is important to keep the UV radiation from passing between cells. To isolate the individual cells barriers are created on the inside surface of one of the plates before sealing. Troughs, honeycomb-like structures and other shapes have been used. The red, green and blue phosphors are deposited inside these structures. An important feature of PDPs is that the plasma in each individual cell can be turned on and off rapidly enough to produce a high quality moving picture. (To help turn the individual cells on and off, there are actually two electrodes on one side and a third electrode on the opposite side of each cell.) Switching the cells on and off cheaply and efficiently is now possible because of advances over the past 20 years in the miniaturization and efficiency of electronics. A commercial panel consists of several million cells which have to be switched at a rate that will create 60 TV picture frames per second. A computer translates an image into a sequence of on and off voltage pulses which are applied to the electrode arrays line by line and row by row to select individual cells. Such control is possible because the plasma is fast and can respond to the voltage pulses in a millionth of a second. The complexity increases significantly when we consider that each small picture element, or pixel, consists of three color cells, and each color cell can display 256 intensity levels.

Thus each pixel can display over 16.7 million (or, more exactly, 256x256x256) colors. Variation in light intensity from a particular cell is not accomplished by changing the voltage or the current through the cell. Rather, it is achieved by changing the length of time that the cell is ON during one TV frame. Since the eye response is slower than the TV picture frequency, it perceives different colors depending on how long each cell is ON. Each company has made its own contribution to the switching systems to improve efficiency, speed and performance. Many years of research and development as well as major advances in electronics and manufacturing techniques have led to the plasma display panels we see on the market today. The plasma display panel itself was invented in 1964 by researchers at the University of Illinois, with the first PDPs being single-color (or “monochrome”) displays. Research on multi-color PDPs was going strong in the 1980's, and the first commercially available color displays appeared in the late 1990’s. It is now possible to manufacture PDPs with diagonals as large as 80 inches and with a thickness of only 3 to 4 inches. Considerable progress has also been made recently to reduce the power consumption and increase the efficiency and life-time of PDPs. Large screens, excellent image quality and brightness, and greater than 160° viewing angle characterize today's plasma panels, which are perfectly flat and perform well even in bright environments. Long-term commercial success of the PDP now lies in the manufacturers' ability to produce low cost displays. Significant reductions in costs have been realized over the last few years, and new PDP designs and processes are being introduced all the time, continuing to reduce production costs. The PDP is sure to be one of the predominant large-format displays of the future.




Fig1.1 Plasma Display Panel

Plasma displays are bright (1,000 lux or higher for the module), have a wide color gamut, and can be produced in fairly large sizes—up to 3.8 m (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 itself is only about 6 cm (2.5 inches) thick, generally allowing the device's total thickness (including electronics) to be 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 - this is also true of CRTs. Typical power consumption is 400 watts for a 50-inch (127 cm) screen. 20 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. Panasonic has greatly reduced power consumption by using Neo-PDP screens in their 2009 series of Viera plasma HDTVs. Panasonic claims that PDPs will consume only half the power of their previous series of plasma sets to achieve the same overall brightness for a given display size. The lifetime of the latest generation of plasma displays is estimated at 100,000 hours of actual display time, or 27 years at 10 hours per day. This is the estimated time over which maximum picture brightness degrades to half the original value.

Plasma display screens are made from glass, which reflects more light than the material used to make an LCD screen. This causes glare from reflected objects in the viewing area. Companies such as Panasonic coat their newer plasma screens with an anti-glare filter material. Currently, plasma panels cannot be economically manufactured in screen sizes smaller than 32 inches. Although a few companies have been able to make plasma EDTVs this small, even fewer have made 32in plasma HDTVs. With the trend toward larger and larger displays, the 32in screen size is rapidly disappearing. Though considered bulky and thick compared to their LCD counterparts, some sets such as Panasonic's Z1 and Samsung's B860 series are as slim as one inch thick making them comparable to LCDs in this respect.