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touch screen

A display screen that is sensitive to the touch of a finger or stylus. Used in myriad applications, including ATM machines, retail point-of-sale terminals, car navigation and industrial controls,...

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Definition: touch screen

A display screen that is sensitive to the touch of a finger or stylus. Used in myriad applications, including ATM machines, retail point-of-sale terminals, car navigation and industrial controls, the touch screen became wildly popular for smartphones and tablets after Apple introduced the iPhone and iPad.

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 "virtual" 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), and they can be made resistant to harsh environments.

All touch screens "digitize" the point of contact on screen into an X-Y coordinate. Following are the major categories.

Resistive
Resistive screens are pressure sensitive and can be operated with a finger or stylus. 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 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 on laptop touchpads.

Projected Capacitive
A capacitive variation, this method 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 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. Typically used for interfaces with larger buttons, such as found on kiosks, when the screen is touched, receivers at opposite ends sense the blocked signals. Since these methods do not use active layers over the screen, they do not block any light. For protection, the screens can also be overlaid with clear, heavy-duty glass.


touch screen

Pressure Sensitive
This Keytec resistive screen 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)



touch screen

Touch Screens Enable Flexibility
Although there are a few physical buttons on this Pioneer in-dash head unit, the touch screen offers infinite flexibility in the design of the user interface without requiring a mouse.





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