A touchscreen is a visual display that can detect the presence and location of a touch to its surface, either from a finger or an object such as a stylus. A number of different technologies are in use on touchscreens, with the technology to a large extent determining suitability for use in particular environments and on what type of device.
Two thin conductive layers are separated by a narrow gap. When the outer coversheet layer is touched, it comes into contact with the inner layer – typically a coating on the glass panel. The contact between the two conductive layers is detected and processed to determine the exact location and type of touch (e.g. quick ‘mouse-click’ or prolonged hold). Resistive technology is highly resistant to liquids but the surface layer is vulnerable to damage by sharp objects. Best used with a finger, gloved hand or stylus.
Surface Wave / Surface Acoustic Wave Technology
Ultrasonic waves pass across the surface of the touchscreen panel, with a portion of the wave being absorbed when the screen surface is touched. The change in the ultrasonic waves is registered and the position determined. Not suited to wet environments, since drops of liquid can give false reads. Best used with a finger, gloved hand or soft stylus.
The glass touchscreen panel is coated with a conductive layer. The screen’s electrostatic field is distorted when touched by a finger (since the body is an electrical conductor), allowing the location of the touch to be determined. Various technologies can be used for detection, but they are beyond the scope of this article (e.g. surface capacitance, projected capacitance, self-capacitance, mutual capacitance). Since dependent on the conductivity of the human body, this technology does not work if wearing gloves. This technology is used in many tablet PCs and smartphones, such as the iPad and iPhone.
Acoustic Pulse Recognition Technology
A recent technology, a unique sound is generated based on the position touched on the screen. The sound is picked up by transducers at the corners of the screen and processed to determine the position, by comparison with a pre-recorded sound profile. Background noise does not cause disruption, since it doesn’t match the pre-recorded profiles. The touchscreen surface is glass, so it is durable and is not usually disrupted by scratches or objects such as dust on the screen.
Optical Imaging Technology
With this fairly modern technology, image sensors around the screen detect touching objects and then calculate the position based on readings from multiple sensors. This technology is relatively cheap and works well with larger screens.
A frame around the screen contains LEDs with light receptors opposite, with the LEDs being pulsed sequentially to create a scanning grid of infrared light beams. Touching the screen obscures one or more beams, enabling a position to be calculated. Suitable for use with finger, gloved hand or stylus. Dispersive Signal Technology Complex algorithms interpret information from sensors that detect piezoelectricity in the glass panel resulting from a touch of the screen. Since detection is based on pressure, this technology is suitable for use with finger, gloved hand or stylus.