A number of technologies are used for touch screens. Resistive screens have multiple layers of conductive material separated by thin air gaps maintained by non-conductive spacers. Touching shorts the layers and the position is computed by the resistance from the edges. Capacitive screens use a two layer capacitor whose capacitance is altered locally by a touch. LED screens use an array of LEDs on two sides and of detectors on the other two sides. Touches are detected by the interruption of a vertical and horizontal beam. Surface Acoustic Wave Screens detect reflections of sound waves off an object touching a glass screen. Pressure sensing ("Pizza Box") sensors go under a conventional monitor and detect relative pressure changes at the corners of the monitor's base when the screen is touched.
Various technologies vary in lifetime, how much they cut the light from the monitor, how rapidly they respond, how precise they are, how difficult they are to calibrate, how frequently they require calibration, and whether they are subject to false triggering.
Touch screens use either an interface card plugged into the computer or a serial interface unit that obtains power from the PC and emulates a mouse.
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