Plasma screens [11]

At the moment this is the main way of achieving very large screens. Samsung have developed a 63-inch plasma screen, far bigger than that achievable with conventional technology [12]. The advantage of a large viewable area combined with a very small depth makes plasma screens both desirable and practical due to the space-saving factors. However a relatively high voltage, of about 160-200V, is required to cause emission, heat is often given off and burn out can occur after as little as 10 years.

How a plasma screen works [13 ]

The production of light is achieved by passing an electric current through a mixture of gases, often helium and xenon, which causes ionisation, the knocking out of resultant emitted light, has wavelengths in the ultraviolet region, which excites a chemical coating of phosphors, which glow red, green and blue respectively. Plasma emission can occur due to both DC and AC currents but AC dominates, as the design is easier to manufacture. This was pioneered in the 1970s by H.G. Slottow and D.L Bitzer in Illinois [14]. Each screen is made up of smaller pixels each containing three smaller cells. The phosphor coated cells consists of a pair of electrodes between which a constant current flows. There is a dielectric medium, which insulates the film from the gas and acts as a capacitor storing charges. To produce an image a sequence of alternating polarity voltage pulses are applied just below the threshold of discharge, this is the sustain voltage. To turn on an individual cell a higher voltage is applied to electrodes corresponding to the relevant row and column of cells. This causes a plasma discharge and a flow of current building up a layer of charge on the capacitor. When an opposing voltage is applied it adds to the next pulse of applied voltage triggering further discharge. Each pulse has duration of one microsecond and is cut off by applying a pulse to reduce the stored charge on the capacitor. Changes in brightness are achieved by controlling the fraction of time the cell is on. As the pixels emit every time the voltage reverses they remain on for a greater proportion of time compared to other screen technologies. There are a very large number of circuits necessary to produce an image, in a typical computer monitor there are 480 rows and 640 columns of pixels and as such 2400 drive circuits. This is in contrast with cathode ray technology where only 2 deflection circuits (horizontal and vertical) and 3 modulating circuits are needed. This significantly increases the cost of plasma screens but this is balanced by manufacturing advantages such as low temperature, direct printing manufacture without the need for ultra clean conditions. Also the screens have wider viewing angles and no susceptibility to magnetic fields unlike LCDs. The displays do however use a lot of power and are suitable mainly for applications where energy efficiency and portability are not critical.