Resistive Touch Screen Technology

• A resistive touch screen is made up of several layers of material. The two key layers in the system are electrically conductive films with a small gap between them. When the screen is touched it deforms slightly, allowing current to flow between the conductive layers in the area where pressure is applied. This current is then interpreted by a central processor that determines which area of the screen was pressed. A resistive touch screen has the advantage that you can use it with any stylus; one disadvantage is that you generally cannot rest your hand against a resistive screen while you write or draw.
  
  

Capacitive Touch Screen Technology

• Capacitive touch screens are made by coating an insulative material such as glass with a transparent conductor, typically indium tin oxide. A small voltage is applied to the conductive layer, creating an electrostatic field. The surface of the skin is also conductive, so when the screen is touched with a hand or finger, a change in the screen's electrostatic field results. This change can be measured as a variation in capacitance, which in turn is interpreted by a processor to determine what area of the screen was touched. Because a change in capacitance is necessary for the screen to work, you cannot operate a capacitive touch screen while wearing gloves or using an ordinary stylus.
  
  

Surface Acoustic Wave and Acoustic Pulse Recognition

• Surface acoustic wave touch screens uses sound-waves to detect touch. A high-frequency (ultrasonic) wave passes over the surface of the display. When the display is touched the wave is disrupted. The position of this disruption can be measured and converted into data. Surface wave technology is not as widely used as capacitive or resistive touch screen technology as it is more advanced and more expensive. Acoustic Pulse Recognition uses piezoelectric transducers -- which vibrate when an electrical current is passed through them -- to create a standing wave over the touch screen. This is interrupted by touch; the transducers register the change in vibration and convert it into variations in output current.
  
  

Optical Imaging and Infra-Red

• Optical imaging uses image sensors to read the position of a touch. Two -- sometimes more -- image sensors are placed at the edges of the screen, generally in the corners. On the opposite sides of the screen there are infra-red lights. When you touch the screen, the touch casts a shadow that the image sensors can pick up and localise. Optical imaging is becoming more popular as it is versatile, can be used in large units as well as small ones and is relatively cheap to implement. Infra-red imaging works in a similar way but uses an X-Y grid array of infra-red LEDs and sensors, locating touch by which combination of beams is broken.