By David Herres
A previous article discussed the cathode-ray tube, for a long time the only game in town for displaying waveforms. It provided a clear display and was capable of superb color rendition. But it has been almost totally supplanted in TVs, computer monitors, and oscilloscopes by newer flat-screen technologies.
The two primary flat-screen variations are plasma and liquid crystal displays (LCD). LCDs are further defined by the way in which they are backlit, either by fluorescent or more advanced light-emitting diode (LED) lighting. (To be clear, we are only referring to LED backlighting, not LED displays.)
Plasma displays employ an array of pixels that are in a grid configuration. Within each pixel are three fluorescent lights, red, green and blue. Each light contains plasma, which is a gas made up of electrons and atoms that are electrically charged, i.e. ions. When there is no external voltage applied, the gas itself is not charged because the constituent protons and electrons are in a balanced state.
Electrical charge may be applied from outside, through electrodes that are attached to the cells. When this happens the free electrons strike the atoms, causing them to lose electrons and go to a different energy state. The gas is now ionized, just as in the familiar fluorescent lamp. As long as the external charge is present, both in fluorescent tube lighting and in flat-screen pixels, ultraviolet (UV) radiation is emitted continually. If this radiation strikes a phosphor surface, the result is visible light.
Xenon and neon are the two gases in each cell, which sits between a pair of glass plates. Display electrodes are located horizontally across the screen, on the viewer side. Address electrodes sit at the rear, and they are arranged vertically. Together, the two sets of electrodes comprise the grid.
The plasma display’s central processing unit (CPU) energizes the appropriate vertical and horizontal electrodes to heat up the cell located at the specified intersection, and UV radiation is released. The picture is formed as these ongoing pulses appear at the electrodes connected to the cells, corresponding to the video information provided by the broadcaster. Each pixel is made up of the three color sub-pixels, each with the intended color phosphor. Of course the emitted UV radiation is not in itself red, green or blue. The picture is formed as the UV radiation strikes specific bits of phosphor in the overall array.
Next, we’ll take a look at liquid crystal display (LCD) flat-screen technology and see how it is different and better than plasma.
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