How Touchscreens Work

How Touchscreens Work

Joelle Alcaidinho
Jan 18, 2011

Once a niche technology, touchscreen devices are now mainstream, which is no huge surprise since their tactile interface make it easy and inviting to use. Whether in the form of tablets, smartphones, or ATM machines it is difficult to go 24 hours without interacting with at least one touchscreen. All touchscreens are not created equal and we know some of this is due to the software, but what about the hardware? What hardware makes touchscreens different from one another? It was with these questions ringing in our ears that we decided to take a look at touchscreen technology.

Anyone who has ever been told to tap harder when paying in a cab knows that not all touchscreens have the same responsiveness. Why do some need a stylus but not others? Wanting to get the scoop on just how touchscreens work, we turned to the helpful folks at HowStuffWorks and obtained the information below about the three basic systems used in touchscreens, resistive, capacitive, and surface acoustic wave.

Resistive
The resistive system, aka the cheapest of the touch-screen tech, consists of a normal glass panel that is covered with a conductive and a resistive metallic layer. A scratch-resistant layer placed on top of the two layers held apart by spacers. An electrical current runs through the two layers while the monitor is operational. When you touch the screen, the two layers make contact in that exact spot. The change in the electrical field is noted and the coordinates of the point of contact are calculated by the computer. Once the coordinates are known, a special driver translates the touch into something that the operating system can understand. 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 stylus.

Capacitive
In the capacitive system, a layer that stores electrical charge is placed on the glass panel of the display. When you touch the screen with your finger, some of the charge is transferred to the you, 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. A capacitive system, must have a conductive input, usually your finger, in order to register a touch.

Surface Acoustic Wave
On the display of a surface acoustic wave system, two transducers (one receiving and one sending) are placed along the x and y axis of the display'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% light throughput and perfect image clarity. This makes the surface acoustic wave system best for displaying detailed graphics (both resistive and capacitive have significant degradation in clarity). 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. This type of technology not surprisingly is the most expensive of the three.

For more on touchscreen technology, check out the incredibly informative post on Howstuffworks.

(Images: Flickr users Hajime NAKANO, Richard Walker, and Rasmus Fritzon under license from Creative Commons .)

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