A display screen that is sensitive to the touch of a finger or stylus. Widely used on ATM machines, retail point-of-sale terminals, car navigation systems, medical monitors and industrial control panels, the touch screen became wildly popular on handhelds after Apple introduced the iPhone in 2007.
Touch screens offer several advantages, the primary one being the infinite ways the user interface can be designed and changed compared to a fixed set of physical buttons. If there is no hardware keyboard on the unit, a "soft" keyboard can be displayed on screen whenever text must be typed in. Touch screens are also able to accept hand printing, handwriting, graphics and finger movements (see multitouch). Touch screens can also be made resistant to harsh environments, compared to ordinary computer monitors.
All touch screens "digitize" the point of contact on screen into an X-Y coordinate. They fall into two major categories: active digitizer and passive touch screen.
Active Digitizer Tablets
Although graphics tablets have been used in computer-aided design (CAD) and other graphics applications for years, the digitizer technology was embedded behind the screen of tablet PCs in order to operate the computer with the precision of a mouse.
A stylus is used to transmit an electromagnetic signal to an X-Y grid, which has a very high resolution. Either by being wired to the unit or from a tiny battery, the stylus may be self powered, or it may obtain its power from the tablet surface, which alternates between transmitting and receiving. Tablet PCs may use both active digitizer and passive touch screen technologies, offering both stylus-based precision and finger-based simplicity. See digitizer tablet.
Passive Touch Screens
Resistive
Resistive screens are pressure sensitive and can be touched with a finger, stylus or just about any pointed object. They use two active layers: a flexible plastic layer on top of a rigid plastic or glass layer, with insulated spacers in between. The layers are coated with indium tin oxide, and different voltages are applied across the coatings, typically alternating between the layers. When touched, the front layer picks up the voltage from the back, and the back layer picks up the voltage from the front, enabling the controller to determine the X-Y location.
Although the least expensive, the resistive method blocks up to 30% of the light from the CRT or LCD screen due to the multiple layers and coatings.
Capacitive
The capacitive method uses only one active layer: a metallic coated glass panel, thus allowing more light to come through. Voltage is applied to the corners of the screen, and when a finger touches the screen, it draws a tiny amount of current. The controller computes the X-Y location from the change in capacitance caused by that touch point. Because the human body absorbs current, either the finger or a "touch pen" that transfers electricity must be used, but not an ordinary plastic stylus. This method is also commonly used by touchpads on laptops.
A variation called "projected capacitive" uses a sensor grid sandwiched between two glass layers. When the screen is touched, the controller computes the X-Y location from the change in capacitance in the grid. The grid also enables two-finger touching like Apple's multitouch iPhone. Although the grid is embedded and protected, the screen can be overlaid with a clear, heavy-duty glass layer for more protection. See multitouch.
Acoustic Waves and Infrared
Acoustic waves or infrared signals are transmitted across the screen's external surface from the top and side. When the screen is touched, receivers at the opposite ends form an X-Y matrix and sense the blocked signals. Since these methods do not use active layers over the screen, they do not block any light. The screens can also be overlaid with a clear, heavy-duty glass layer for protection.
Keytec makes a wide variety of resistive touch screens that can be touched with the finger or any stylus-like object. When the plastic layer is pushed into the glass layer, it diverts the current. (Image courtesy of Keytec, Inc., www.magictouch.com)
Widely used for both industrial and consumer applications, touch screens provide complete flexibility in the design of the user interface. To implement the same number of controls with physical buttons on this automobile GPS/entertainment system would be much more cumbersome.
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