By Alex Shaw
Who, What, Where, When and Why
How do touch screens work?
Electronic devices can use lots of different methods to detect a person's input on a touch screen. Most of them use sensors and circuitry to monitor changes in a particular state. Many, including the iPhone, monitor changes in electrical current. Others monitor changes in the reflection of waves. These can be sound waves or beams of near-infrared light. A few systems use transducers to measure changes in vibration caused when your finger hits the screen's surface or cameras to monitor changes in light and shadow.
The basic idea is pretty simple -- when you place your finger or a stylus on the screen, it changes the state that the device is monitoring. In screens that rely on sound or light waves, your finger physically blocks or reflects some of the waves. Capacitive touch screens use a layer of capacitive material to hold an electrical charge; touching the screen changes the amount of charge at a specific point of contact. In resistive screens, the pressure from your finger causes conductive and resistive layers of circuitry to touch each other, changing the circuits' resistance.
Most of the time, these systems are good at detecting the location of exactly one touch. If you try to touch the screen in several places at once, the results can be erratic. Some screens simply disregard all touches after the first one. Others can detect simultaneous touches, but their software can't calculate the location of each one accurately.