How touch screens work

A touch screen is a computer input device that allows users to make selections by touching the screen, unlike a keyboard or mouse that allows input. Computers with touch screens require little storage space, have few moving parts, and can be packaged. Touch screens are more intuitive to use than keyboards and mice, and training costs are low.

All touch screens have three main components. A sensor unit that processes user selections; and a controller that senses touch and locates it, and is driven by a software device that transmits touch signals to the computer's operating system. There are five technologies for touch screen sensors: resistive, capacitive, infrared, acoustic wave, or near-field imaging.

Resistive touch screens usually consist of a flexible top film and a glass base layer separated by insulating points. The inner surface coating of each layer is a transparent metal oxide. The voltage is differential across each layer of the diaphragm. Pressing the top film creates an electrical contact signal between the resistive layers.

Capacitive touch screens also have a transparent metal oxide coating bonded to a single layer of glass surface. Unlike resistive touch screens, where any touch creates a signal, capacitive touch screens require direct contact with a finger or contact with a conductive stylus. The capacitance of the finger, or its ability to store charge, absorbs current from each corner of the touch screen, and the current flowing through the four electrodes is proportional to the distance from the finger to the four corners, thereby determining the touch point.

Infrared touch screens are based on the interruption of light. Instead of placing a thin film layer in front of the display surface, a frame is placed around the display. The frame has light sources, or light emitting diodes (LEDs), on one side of the frame, and light detectors or photoelectric sensors on the other side, forming a horizontal and vertical cross-shaped infrared grid. When an object touches the display, the invisible light is interrupted and the photoelectric sensor cannot receive the signal, thereby determining the touch signal.

In acoustic wave sensors, the sensor is installed at the edge of the glass screen to send an ultrasonic signal. The ultrasonic wave reflects through the screen and is received by the sensor, and the received signal is weakened. In surface acoustic wave signals (SAW), light waves pass through the surface of the glass; while guided acoustic wave (GAW) technology, sound waves pass through the glass.

Near field imaging (NFI) touch screens are composed of two thin glass layers with a transparent metal oxide coating in the middle. Applying an AC signal to the conductive coating generates an electric field on the surface of the screen. When a finger, with or without a glove, or other conductive stylus touches the sensor, the electric field is disturbed, resulting in a signal.