Touch screen:-

A touch screen is a display 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 stylus. However, if the object sensed is active, as with a light pen, the term touch screen is generally not applicable. The ability to interact directly with a display 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 screen, where it is displayed, rather than indirectly 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. Building touch screens there are several principal ways to build a touch screen. The key goals are to recognize one or more fingers touching a display, to interpret the command that this represents, and to communicate the command to the appropriate application. In the most popular techniques called the capacitive or resistive approach, manufactures coat the screen with a thin, transparent metallic layer. When a user touches the surface, the system records the change in the electrical current that flows through the display.
Dispersive-signal technology which 3M created in 2002, measures the piezoelectric effect- the voltage generated when mechanical force is applied to a material- that occurs chemically when a strengthened glass substrate is touched. There are two infrared-based approaches. In one, any array of sensors detects finger touching or almost touching the display, there by interrupting light beams projected over the screen. In the other, bottom-mounted infrared cameras record screen touches. In each case, the system determines the intended command based on the controls showing on the screen at the time and the location of the touch.