Mobile devices such as smartphones and tablet computers don’t have room for physical keyboards. Instead, you control these devices by using fingers or a stylus on the screen. These screens sense the presence of one or several fingers or a stylus on the surface of the screen and use varying technologies to determine what part of the screen the finger or stylus is touching. When the software has calculated the location of the touch, it executes the command corresponding to the image the finger or stylus has touched.
An inexpensive touch-screen technology uses electrodes in two separate screen layers to locate a touch. When an object such as a finger presses on the screen, it forces the two layers together at that point. The resistance of the resulting electric path lets evaluation circuits determine where the object touched the screen. The technology works for any object, including styluses and gloved fingers, but it can’t handle multitouch. The electrodes in the layers block some of the screen light, and the screen requires a slight pressure to force the two layers together. Since current flows every time there is a touch, the resistive touch screen reduces battery life.
The most common technology for mobile devices analyzes the capacitance of the screen glass to determine where you touch the screen. Some capacitive touch screens have electrodes to deliver a charge to the screen, and some have miniature capacitors under the glass. When a finger touches the glass, the capacitance changes, and software can analyze the change to determine the location of the finger. The software can identify several fingers and their motion for multitouch capability, but it is optimized for a human finger and doesn’t work for a gloved hand or an insulating stylus. Special resistive styluses work with capacitive screens.
Acoustic sensors installed over a screen can identify any object that touches the screen. This technology requires a raised bezel to house the ultrasound transducers that produce acoustic waves travelling along the screen. When an object touches the screen, it interrupts the sound waves, and the sensors determine the location of the object. This technology works with any object, and it doesn’t have electronics or wires embedded in the screen to interfere with image transmission. The raised bezel and the power consumption of the transducers have prevented use of this technology in mobile devices.
Screens with optical touch detection work similarly to acoustic screens. A raised bezel holds small LEDs that shine infrared beams of light across the glass to detectors on the other side. When an object interrupts a beam, the detectors that no longer see their beams give the coordinates of the object. A variation of this technology places the LEDs behind the screen, sending their beams through the glass. The technology detects multitouch, but it is mainly suited for larger stationary monitors.