How Touch Screens work – Lee Zi Yang

All of us are familiar with electronic devices which utilize touch screens. Even if we don’t own one, we see it every day in places like MRT stations and NTUC check-out counters. We find that some screens, like Apple’s, are particularly sensitive to touch, albeit only if contact is made with bare skin. On the other hand, some phones require more pressure to be exerted, but can be used with a stylus (or just about anything that can exert pressure on the screen). Then there are those that support multi-touch functions, and some which don’t.

These various permutations are the result of the different mechanisms through which these touch screens work. Let’s explore a few more common mechanisms here. Note that each mechanism has many different permutations, but their main concepts are explained below.

Disclaimer: Everything in italics is based on my own experience and observations.

Resistive Touch Screens

Resistive touch screens are one of the cheaper mechanisms, and can be found in lower-end touch screen devices like the Nokia 5800 Xpress. It’s usually easy to identify a device utilizing resistive touch screen technology – its screen looks “flexible”, and the reflections on it are not sharp.

Figure 1 Picture of Nokia 5800 Music Xpress and iphone. Note the difference in reflections. You can even see the divisions of the ceiling boards on the iphone, while the nokia barely reflects any light, and images are not sharp. Taken from http://dailymobile.se/2008/12/02/pictures-nokia-5800-xpressmusic-vs-apple-iphone-3g/nokia-5800-xpressmusic-vs-iphone-3g-03/

The flexible screen is the key feature enabling such devices to function. Underneath the screen lie two layers separated by a very tiny gap. One’s a conductive layer, and the other, a resistive layer. (see Figure 2)  A current is passed through the conductive layer all the while. When the user presses down on the screen, it pushes the conductive layer down to make contact with the resistive layer (in some devices the resistive layer may be the top layer instead). This causes a disruption of the electrical current at that specific point. The device then registers the coordinates of that point.

Figure 2 Taken from http://www.i-techcompany.com/touchscreenr.html

If you look closely, you can actually see a depression on the screen when you press down. You can borrow my phone to observe its screen.

Pros

  • The fact that resistive touch screens are pressure-based means that you can use anything to make contact with the screen. It also means that you can keep your gloves on when using your phone. This could be a significant advantage in countries with colder climates.
  • Cheap

Cons

  • Can only recognize one touch at a time
  • Sensitive to pressure, not “touch”. So you might need to apply a fair amount of pressure for the device to register anything
  • The three layers (resistive, conductive and air) between the screen and monitor result in poor transmission of light from monitor to screen
  • Less durable than other common touch screen technologies
  • The necessary soft screen is very prone to scratches
  • Scratches on the screen can hamper effectiveness

Capacitive Touch Screens

Capacitive touch screens are most commonly found on high-end touch screen handphones/devices, most notably iPhones/iPads/iPods. They are distinguished from resistive touch screens by their inflexible screen.

Like resistive touch screens, capacitive touch screens work by registering disruptions in electrical phenomena, in this case, capacitance! This time, there is no resistive layer. The screen is just coated with a conductive layer. A charge is maintained on the conductive layer. When a human (who is a conductor) touches the screen with his bare skin, some charges flow into him. Sensors at the four corners of the screen then register the various changes in capacitance and hence determine exact location of the charge.

Pros

  • Much higher transmission of light from monitor to screen
  • Can support multi-touch capabilities
  • Very accurate
  • Durable

Cons

  • Sensitive only to bare human skin. So gloves have to be removed
  • Expensive
  • Scratches on the screen can hamper effectiveness

Optical Touch Screens

Optical touch screens are more commonly found in big devices where a lot of people are expected to use them. They are easily distinguished from the other two technologies by a necessary protruding border around the screen.

Unlike the technologies mentioned above, optical touch screens do not utilize electrical phenomenon. Instead, they utilize infrared waves.

Figure 3 In a typical device, the top and right borders may be infrared sources, while the left and bottom borders contain detectors. Taken from http://www.planartouch.com/101/select/

When someone touches the screen, infrared waves passing through that particular location are absorbed. The relevant detectors detect the drop in intensity of infrared light and the x and y coordinates of the touch are then determined.

Due to the way optical touch screens work, you do not actually need to make contact with the screen for a “touch” to be detected. You just have to block the relevant pair of detectors. I’ve actually tried this out myself, on some exhibition display.

The need for the infrared sources and detectors is the reason why there must be protruding borders around the screen.

Pros

  • Anything can be used to register a touch
  • Durable, because people can abuse the screen as much as they like and the device will still work (as long as the detectors and infrared sources are well-protected)
  • Can be used for very large devices

Cons

  • Cannot support multi-touch capabilities
  • Has to have a protruding border
  • Can be terribly inaccurate
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