TouCH ScReeN

A touch screen is an electronic visual output that can detect the presence and location of a touch within the display area. The term generally refers to touch or contact to the display of the device by a finger or hand. Touch screens can also sense other passive objects, such as a pen. However, if the object sensed is active, as with a light pen, the term touch screen is generally not applicable. The ability to interact physically with what is shown on a display (a form of "direct manipulation") typically indicates the presence of a touch screen. The touch screen has two main attributes. First, it enables one to interact with what is displayed directly on the hand, where it is displayed, rather than indirect with a mouse or touchpad. Secondly, it lets one do so without requiring any intermediate device, again, such as a stylus that needs to be held in the hand. Such displays can be attached to computers or, as terminals, to networks. They also play a prominent role in the design of digital appliances such as the personal digital assistant (PDA), satellite navigation devices, mobile phones, and video games. 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, the finger must be used and not a plastic stylus. This method is also commonly used by touch pads 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 multi-touch iPhone. Although the grid is embedded and protected, the screen can be overlaid with a clear, heavy-duty glass layer for more protection. See multi-touch.

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.