This document describes several touch screen technologies: - Resistive touch screens use two conductive layers that make contact when touched to detect the touch location. - Capacitive touch screens detect touches by measuring changes in electrical charge on a capacitive layer when a finger touches the screen. - Surface acoustic wave technology uses ultrasonic waves that are partially absorbed when the screen is touched. - Infrared touch screens project an invisible light grid and detect touches by the absence of light signals. - Optical touch screens use line scanning cameras and infrared backlights to detect finger shadows and locate touches.