This technology consists of an insulator such as glass, coated with a transparent conductor such as indium tin oxide. As the human body is also a conductor, touching the surface of the screen results in a flow of current through the body’s impedance to ground. This impedance is much less than the inherent impedance of the system. One main advantage is that unintended contact (bottles, insects etc) are not detected.
This consists of a conductive surface on the glass which is then coated with a hardened layer. The current flows from the conductive surface into the finger, and the controller can then calculate the position of this touch in relation to the four corners. Due to the single uniform surface, multi-point touch is not possible, and only conductive styluses are detected.
Rather than a single surface layer, projected capacitive has an array of embedded wires which emit a signal outwards and can thus be placed behind a toughened glass layer. These wires have an oscillating frequency passed through them, and a finger touch will change this frequency, allowing position calculation. This array also allows more precise location calculation, and also multi-point touch.
Resistive technology works on the premise of a voltage gradient across two perpendicular conductive layers separated by insulating spacers. As a touch is made the distance between the two layers is closed, and a circuit is made across the layers (like closing a switch). The position of the touch can be calculated from the amount of the voltage gradient transferred across the layers in both X and Y direction. Any stylus or object can be used with resistive technology, but this can be a disadvantage where accidental touches are detrimental. Also due to the multi layers, light transmission is reduced.
SAW touch screens use an ultrasonic wave passing across the surface of the glass, which when blocked by a finger or stylus, are absorbed. This change in the ultrasonic waves can be used to determine the position of the contact. Unfortunately any contaminants can be detected as a touch and close metal frames can affect the ultrasonic waves, so environmental consideration must be taken into account.
This is the newest technology to be developed and relies on a series of IR emitters, and photo sensors, both horizontally and vertically. When an object is placed on the screen, it blocks some of these beams, and the location can be easily determined. Because there is no surface conductors, the glass surface can be any type including anti-vandal or toughened. IR touch has also been developed to allow multi-point operation. The downside of IR technology is that it is unreliable outdoors, as the Sun also produces IR rays which can be erroneously detected by the touch controller as a touch input.