Touch-screen monitors have become more and more commonplace as their price has steadily dropped over the past decade. There are three basic systems that are used to recognize a person's touch:
- Resistive
- Capacitive
- Surface acoustic wave
In the capacitive system, a layer that stores electrical charge is placed on the glass panel of the monitor. When a user touches the monitor with his or her finger, some of the charge is transferred to the user, so the charge on the capacitive layer decreases. This decrease is measured in circuits located at each corner of the monitor. The computer calculates, from the relative differences in charge at each corner, exactly where the touch event took place and then relays that information to the touch-screen driver software. One advantage that the capacitive system has over the resistive system is that it transmits almost 90 percent of the light from the monitor, whereas the resistive system only transmits about 75 percent. This gives the capacitive system a much clearer picture than the resistive system.
On the monitor of a surface acoustic wave system, two transducers (one receiving and one sending) are placed along the x and y axes of the monitor's glass plate. Also placed on the glass are reflectors -- they reflect an electrical signal sent from one transducer to the other. The receiving transducer is able to tell if the wave has been disturbed by a touch event at any instant, and can locate it accordingly. The wave setup has no metallic layers on the screen, allowing for 100-percent light throughput and perfect image clarity. This makes the surface acoustic wave system best for displaying detailed graphics (both other systems have significant degradation in clarity).
Another area in which the systems differ is in which stimuli will register as a touch event. A resistive system registers a touch as long as the two layers make contact, which means that it doesn't matter if you touch it with your finger or a rubber ball. A capacitive system, on the other hand, must have a conductive input, usually your finger, in order to register a touch. The surface acoustic wave system works much like the resistive system, allowing a touch with almost any object -- except hard and small objects like a pen tip.
As far as price, the resistive system is the cheapest; its clarity is the lowest of the three, and its layers can be damaged by sharp objects. The surface acoustic wave setup is usually the most expensive.
A) INDIUM TIN OXIDE
Capacitive sensors found in most existing touchscreens, track pads, OLED displays, and other electronics use indium tin oxide (ITO) as an electrode. The conductive material transmits an electric current to a user’s fingertips. ITO is also transparent, so light from the underlying LCD screen shines through.
B) SENSOR BOARD
The sensor board injects tiny electric currents into the ITO layer. When a user touches the screen (and thus the ITO layer), current flows from the sensor board through the ITO to the person’s body. The sensor on the board measures the body’s unique impedance at multiple frequencies. The most recent prototype takes up to a second to recalibrate for each new user. Once calibrated, it can recognize a familiar body in 500 milliseconds.
C) LCD PANEL
An LCD provides the touchscreen's graphical interface.
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