Touch Screen

A touch screen has two main advantages. First, it allows users to interact directly with what is displayed, rather than indirectly with a pointer controlled by a mouse or touchpad. Secondly, it does not require the use of an intermediate device. Touch screens can be attached to computers or to networks as terminals. They also play a prominent role in the design of digital appliances such as the personal digital assistant (PDAs), satellite navigation devices, mobile phones and video games.

How a touch screen event is registered depends on the touch screen's inherent technology. The three main touch screen technologies are:

• Resistive: This screen has a thin metallic layer that is conductive and resistive, so that touching results in a change in the electrical current sent to the controller. Pros: More affordable, not damaged by dust or water, responds to finger or stylus. Cons: Only 75% clarity and susceptible to damage by sharp objects.
• Surface Acoustic Wave (SAW): Ultrasonic waves pass over this screen. Touching it results in absorption of part of the wave, registering the position of the touch, which is sent to the controller. Pros: Responds to finger or stylus. Cons: May be damaged by dust or water.
• Capacitive: This screen is coated with an electrically-charged material. Touching it causes a change in capacitance, which allows the location to be determined and sent to the controller. Pros: Not damaged by dust or water and has high clarity. Cons: Must be touched with a finger only - a stylus cannot be used.

There are other, less-common touch screen technologies. These include:

• Dispersive Signal Technology: Introduced in 2002 by 3M, sensors detect mechanical energy during a touch. Complex algorithms interpret the data to determine the location, and data is sent to controllers. Pros: Durability, not affected by elements, excellent clarity and finger or stylus may be used. Cons: After the initial touch, the system is not able to detect a motionless finger or stylus.
• Acoustic Pulse Recognition: This system was released in 2006 by Tyco International's Elo division. It uses transducers located around the screen to transform vibration into electrical energy via algorithms that determine location. Pros: Good durability and clarity, resistant to the elements and well-suited to large displays. Cons: Cannot detect a motionless finger.
• Infrared: Detects touch via an array of light-emitting diodes (LEDs) and photo-detector pairs at the edge of the screen. Pros: No damage by elements, a finger or stylus may be used, highly durable with high clarity.
• Optical Imaging: Image sensors (cameras) placed at the edge of the screen pick up infrared black lights on the opposite side of the screen. Pros: Scalable, versatile, affordable and can be used for large displays.