Most modern computers or mobile phones are now operated using touch screen computer monitor or touch screens. The sensitive surfaces replace the computer mouse on the PC and the keyboard on telephones or tablet PCs. Behind all these simple operations is a sophisticated technique that combines hardware and software.

All touch displays have the same components – a touch-sensitive surface and a surface controller

All touchscreens have the same pattern: they have a touch-sensitive surface and a controller that measures the signals on the surface and passes them on to an operating system. The operating system then translates our finger movements into a mouse movement and transfers them to the screen. Touching the surface of the touch screen corresponds to a mouse click, if you pull two fingers apart from the middle, you can enlarge the image on the screen and scroll through pages and images with three or four fingers wiping over the surface. The hardware that generates the signals can be based on different physical principles.

The most common are resistive and capacitive touchscreen computer monitors

How resistive works

Resistive touchscreens require the pressure applied to the surface by a finger or other object. The touch-sensitive surface of the touchscreen consists of two conductive indium tin oxide (ITO) layers separated by small spacers. The lower layer is applied to a solid and stable base, while the upper layer is covered with an expandable polyester from the outside. If the polyester layer is touched, the upper ITO layer is pressed onto the lower layer. In order to determine the position of the printing position, a direct voltage is applied alternately on the one and milliseconds later on the other conductive layer. These stresses are perpendicular to one another and fall off uniformly from one edge to the opposite edge. Since the two layers are connected together at the pressure point for a short time, a current flows. The position of the pressure point can then be determined unambiguously on the basis of the voltage changes caused thereby. The controller forwards the coordinates to the operating system. In this principle, two layers are always required for the measurement: one is the voltage, the other the position in one direction.

Resistive touch screens are a pioneer in touch technology, but are generally not multi-touch capable. This means that you can not operate with several fingers. If two or more fingers are pressed onto the surface, only the contact surface of the two ITO layers is broadened and the fingers can not be detected individually. The main disadvantage of this technique, however, is that a coordinate is always detected by means of the upper, flexible layer. The constant bending and stretching leads to microscopic cracks in the ITO coating, which changes the electrical properties. As a result, the determination of the coordinate becomes less precise. However, resistive touchscreens can be produced comparatively inexpensively and can be operated with any object. This is especially important for doctors who often need to use their medical equipment with rubber gloves. Resistive touchscreens are used mainly in older mobile phones or in some tablet PCs and in medicine. Meanwhile, however, resistive touchscreens are being developed, which are also multitouch-capable and gain in importance in industry.

How capacitive works

In contrast to resistive technology, capacitive touchscreens need no pressure. They are composed of a two-layered coordinate network consisting of electrodes arranged in one layer as columns and in the other as rows. Between the electrodes is an insulating material, a so-called dielectric. On the lower side is a circuit which constantly measures the capacitance at the intersection points of the electrodes. On the upper side, an insulating protective layer, usually made of glass, ensures that the electrodes are not damaged and the finger can slide well over the surface. This feature makes the capacitive touchscreen far more robust than the resistive touchscreen. Since a finger is electrically conductive, charges can flow to it, as soon as it touches the surface of the touchscreen. This changes the electrostatic field between the electrodes and leads to a measurable change in capacitance. As the finger moves across the surface, the capacitance changes at the various electrode intersections. The changes are all captured by a microcontroller and passed to the operating system. This translates the signals into a “click” or a movement on the screen. Capacitive touchscreens are multi-touch capable, as they can continuously measure the capacity across the entire coordinate network and register the inputs of individual fingers separately. They differ from resistive touchscreens. The disadvantage of the capacitive principle, however, is that only conductive objects can be used, to operate the touchscreen. Other objects such as pens, fingernails or gloves do not have any effect. The capacitive principle is used for smartphones and tablet PCs. But also 99 percent of all laptops contain capacitive touchscreens.