A touch screen computer typically has one of several types of touch screens as its primary input device. Some touch screen systems also include a keyboard and mouse while others do not. A touch screen monitor can usually detect the presence and location of a finger, fingernail or special stylus. Tablet computers, mobile phones and video games often use a touch screen computer monitor for input. Personal digital assistants (PDAs), automated bank teller machines and information kiosks may use touch screens as well.
A mutual-capacitance screen is sometimes used in a touch screen computer tablet. This type of screen can detect multiple touches simultaneously, allowing more complex input. A grid of etched capacitive sensors behind a protected glass screen can detect the presence of fingers near any row-column intersection. It can also detect other conductive objects, such as a specially-designed stylus or the palm of a hand. Its multi-touch ability makes it well-suited for drawing applications such as the input of handwriting or Asian characters.
Two other types of capacitive displays are also used as touch screen computer monitors. A self-capacitive screen measures the current induced by the presence of a finger near its grid. Like a mutual-capacitance screen, it can also detect other conductive objects, but it can only sense one at a time. A surface-capacitance screen has lower resolution and can also detect only one object at a time. Its glass screen has a solid conductive coating on the back which can be used to approximate the location of a touch.
Resistive technology is often used in some types of touch screen computer monitors as well. When a finger, fingernail or stylus presses on a resistive screen, the display's flexible metallic layers touch at that point. Circuitry measuring the current flowing in the screen can determine this location. Like most other types of touch screens, it can only detect one touch at a time. Its physical nature requires pressure to register a contact and makes it more susceptible to damage than other types of touch screens.
Sound waves and light waves are also used to detect and locate touching motions by some screens. An acoustic display transmits ultrasonic sound waves across the screen and measures the response. A finger or other pointing object near the screen absorbs some of the sound waves, revealing its location to the control circuitry. Interrupted light waves can also be used for this purpose. In these displays, a grid of Light Emitting Diodes (LEDs) and matching photo detectors monitor the screen area.