Touchscreens

Last updated: June 3, 2008.

Once upon a time, the way to get a computer to do something useful was to feed it a stack of cards with holes punched into them. Thankfully, things have moved on a lot since then. Now we can get our computers to do things simply by pointing and clicking with a mouse—or even by speaking ordinary commands with voice recognition software. But there's a revolution coming that will make computers even easier to use—with touch-sensitive screens. Cellphones like Apple's iPhone and some MP3 players already work with simple, touch controls—and computers are starting to work that way too. Touchscreens are intuitively easy to use, but how exactly do they work?

Photo: Left: Touchscreens are commonplace in busy railroad stations. They're popular with customers, since you can often buy your train ticket more quickly without waiting in line. They're also good news for the station operator, since machines like this work out cheaper than paying a human sales person.

Keyboards and switches

Photo: Right: Electronic voting systems use touchscreen technology. Simple instructions on the screen tell you exactly what to do. And there are no buttons to wear out either!

A touchscreen is a bit like an invisible keyboard glued to the front of your computer monitor. To understand how it works, it helps if you know something about how an ordinary keyboard works first. You can find out about that in our article on computer keyboards, but here's a quick reminder. Essentially, every key on a keyboard is an electrical switch. When you push a key down, you complete an electric circuit and a current flows. The current varies according to the key you press and that's how your computer figures out what you're typing.

In a bit more detail, here's what happens. Inside a keyboard, you'll find there are two layers of electrically conducting plastic separated by an insulating plastic membrane with holes in it. In fact, there's one hole underneath each key. When you press a key, you push the top conductor layer down towards the bottom layer so the two layers meet and touch through the hole. A current flows between the layers and the computer knows you've pressed a key. Little springy pieces of rubber underneath each key make them bounce back to their original position, breaking the circuit when you release them.

Touchscreens have to achieve something similar to this on the surface on your computer screen. Obviously they can't use switches, membranes, and bits of plastic or they'd block the view of the screen below. So they have to use more cunning tricks for sensing your touch—completely invisibly!

Photo: This is the sensitive, switch layer from inside a typical PC keyboard. It rests under the keys and detects when you press them. There are three separate layers of plastic here. Two of them are covered in electrically conducting metal tracks and there's an insulating layer between them with holes in it. The dots you can see are places where the keys press the two conducting layers together. The lines are electrical connections that allow tiny electric currents to flow when the layers are pressed tightly together.

How touchscreens work

Different kinds of touchscreen work in different ways. Some can sense only one finger at a time and get extremely confused if you try to press in two places at once. Others can easily detect and distinguish more than one key press at once. These are some of the main technologies:

Resistive

Resistive touchscreens (currently the most popular technology) work a bit like "transparent keyboards" overlaid on top of the screen you see. There's an upper layer of conducting polyester plastic bonded to a lower layer of conducting glass and separated by an insulating membrane. When you press on the screen, you force the polyester to touch the glass and complete a circuit—just like pressing the key on a keyboard. A chip inside the screen figures out the coordinates of the place you touched.

Capactive

These screens are made from multiple layers of glass. The inner layer conducts electricity and so does the outer layer, so effectively the screen behaves like two electrical conductors separated by an insulator—in other words, a capacitor. When you bring your finger up to the screen, you alter the electrical field by a certain amount that varies according to where your hand is. Capacitive screens can be touched in more than one place at once.

Infrared

Just like the magic eye beams in an intruder alarm, an infrared touchscreen uses a grid pattern of LEDs and light-detector photocells arranged on opposite sides of the screen. The LEDs shine infrared light in front of the screen—a bit like an invisible spider's web. If you touch the screen at a certain point, you interrupt two or more beams. A microchip inside the screen can calculate where you touched by seeing which beams you interrupted.

Surface Acoustic Wave

Surprisingly, this touchscreen technology detects your fingers using sound instead of light. Ultrasonic sound waves (too high pitched for humans to hear) are generated at the edges of the screen and reflected back and forth across its surface. When you touch the screen, you interrupt the sound beams and absorb some of their energy. The screen's microchip controller figures out from this where exactly you touched the screen.

Near field imaging

Have you noticed how an old-style radio can buzz and whistle if you move your hand toward it? That's because your body affects the electromagnetic field that incoming radio waves create in and around the antenna. The closer you get, the more effect you have. Near field imaging (NFI) touchscreens work a similar way. As you move your finger up close, you change the electric field on the glass screen, which instantly registers your touch. Much more robust than some of the other technologies, NFI screens are suitable for rough-and-tough environments (like military use). Unlike most of the other technologies, they can also detect touches from pens, styluses, or hands wearing gloves.

Photo: Touch-sensitive cockpit screens in a NASA flight simulator. Picture courtesy of NASA Langley Research Center (NASA-LaRC).

Some of us are lucky enough to own the latest touch phones, which have multi-touch screens. The big advantage here is that the display can show you a screen geared to exactly what you're trying to do with it. If you want to make a phone call, it can display the ordinary digits 0-9 so you can dial. If you want to send an SMS text message, it can display a keyboard (in alphabetical order or typewriter-style QWERTY order, if you prefer). If you want to play games, the display can change yet again. Touchscreen displays like this are incredibly versatile: minute by minute, they change to meet your expectations.

So far, we've had fairly limited exposure to touchscreens—but all that could be about to change. Microsoft has anounced that future versions of its Windows operating system will feature touchscreen technologies—potentially making computer mice obsolete. Though it could be a while before we're all prodding and poking our computers into action, touchscreen technology is certainly something to watch out for!