One of the more common uses for touchscreens is in public kiosks, such as this one that lets customers edit digital photographs and order prints. (Rick Smith/Associated Press)

The hands-on approach is making its way into more technology used by consumers, from laptops to mobile phones to gaming and multimedia devices. Touching the screen with a finger is often much easier and way more comfortable than clicking a mouse or thumb-typing on a tiny keyboard. (HO-Smart Technologies/Canadian Press)

With the simple touch of a finger on a transparent screen, you can make modern high-tech gadgets do some amazing things. Touch-sensitive screens come in many different shapes and sizes and can be found just about anywhere, from self-serve kiosks in stores and airports, to bank machines, to video games and handheld gadgets such as smartphones and remote controls. But how exactly do they work?

An instructor at the University of Kentucky, Samuel Hurst, is credited with inventing the touchscreen in 1974 when he developed the first display that incorporated a transparent surface sensitive to touch. All touchscreens since then work on the same guiding principle, detecting the precise location of a touch on a screen so that it can be used as both a display and an input device.

Hurst had developed a touch sensor three years prior to that called the Elograph, a name he used when he founded Elographics (a company now known as Elo TouchSystems) to commercialize his invention. In 1977, Elographics patented a five-wire resistive touch-sensitive system, which continues to be the most widely used touchscreen technology today.

Resistive touchscreens

Resistive touchscreens are fairly durable, since outside elements like dust and water don't affect their performance, but their limitation is that they can only register one touch on the screen at a time.

They are composed of several layers bonded to the face of a display screen, the most important of which are two thin metallic layers that are electrically conductive. The two layers are separated by a very thin space, so that when an object makes contact, like a finger or stylus, the pressure causes an electrical current to run between the layers.

These types of touchscreens can typically be found in smartphones and PDAs, where touch-sensitive pressure on certain points of the screen can open up new menus. A quick touch can register in one way, while a longer and firmer touch will register in another way. An example of this is in devices that use Microsoft's Windows Mobile operating system - a light tap from a stylus will open an on-screen link, while applying a little pressure on one point of the screen opens up a menu of options.

The typical number of conductive contacts in the resistive layers of touchscreens ranges between four and eight. Four-wire resistive touch panels are still the most common in smartphones and PDAs. The contacts are placed on the left, right, top and bottom of the display.

Newer smartphones and PDAs that are more expensive use five-wire touch panels where contacts are placed in the four corners of the screen with a fifth going on a separate layer. Manufacturers tend to choose five-wire touchscreens for their devices because that fifth wire allows for better calibration, so that touches are sensed more accurately. Five-wire devices can also handle up to 35 million touches before they wear out, compared to a few hundred thousand for four-wire touchscreens. But the five-wire screen does come at a cost, because of the expense of the extra layers required on top of the display.

Eight-wire touchscreens are essentially four-wire screens "doubled up." The concept behind them is to improve calibration and shorten the response time upon touch. But like four-wire screens, they can suffer from "drift," a term used to describe when the screen needs to recalibrated because it doesn't sense the location of a touch properly. Older PDAs commonly suffered from this because users would drag their finger or stylus when scrolling, confusing the sensors.

Capacitive touchscreens

Apple CEO Steve Jobs holds the Apple iPod Touch media player, which has a capacitive touchscreen that can sense multiple touch-points simultaneously. By pinching two fingers together when viewing a photo on the Touch, for example, the image can be minimized, while spreading the fingers apart enlarges it. Brushing a finger in any direction can scroll through images, like flipping a page in a book or magazine. (Paul Sakuma/Associated Press)

A more advanced type of touchscreen technology is now rivalling the longstanding dominance of resistive touch panels. Devices like Apple's iPhone and its iPod touch multimedia player, the HTC Touch smartphone and LG Electronics' Prada cellphone use a "capacitive" technology that allows for more than one simultaneous touch to be detected.

Capacitive touchscreens are coated with a material called indium tin oxide that conducts an electrical current continuously across the sensor. In turn, the sensor achieves capacitance, meaning that it stores an electric charge. Since the human body also has stored electrons that exhibit capacitance, a simple touch of a finger alters the capacitance field in that area of the screen. Electronic circuits underneath the outer layers of the display detect that touch.

This complex signal processing makes capacitive screens more expensive to make, but they also have higher resolution and clarity than resistive screens. Capacitive screens are also more durable because they aren't affected by elements like dust and water.

Some cellphones have made use of the technology, though mostly for limited touch-sensitive controls like those that handle music playback or navigation; the external controls on phones like LG Electronics' Chocolate and Motorola's RAZR 2 are two examples. However, these particular screens aren't meant to read simultaneous touches, which is one of the advantages these more expensive screens have over resistive technology. Capacitive touchscreens are put to more advanced use in Apple's iPhone and iPod touch, offering people a more interactive and intuitive interface by sensing and differentiating between simultaneous touches. By pinching two fingers together when viewing a web page or photo, for example, the image can be minimized. Spreading the fingers apart enlarges the image. Brushing a finger in any direction can flip through multiple images, almost like quickly flipping a page in a book or magazine. The HTC Touch uses a similar method for scrolling.

The popularity of these devices and their functions may lead to broader use of capacitive screens. Considering they're also brighter and sharper than resistive touchscreens, some observers expect manufacturers could embrace capacitive technology relatively quickly as its cost falls.

Surface acoustic wave technology

Another touchscreen technology primarily used in commercial settings is surface acoustic wave, or SAW. These touchscreens absorb ultrasonic waves that pass over the panel's surface. The position of those waves is registered and then sent to the controller for processing.

Some SAW touchscreens typically use glass surfaces rather than coated layers. They're also scratch-resistant and can continue to operate even if the surface does suffer scratches.

The downside is that the screens are susceptible to outside elements like liquids, dirt and dust that can affect performance. As a result, SAW touchscreens are usually found indoors — they're typically used in public information kiosks, training systems, automated ticket booths for movie theatres, and other settings that tend to attract a lot of human traffic.

Cutting-edge hardware

From a commercial standpoint, touch-sensitive screens create some intriguing business and marketing opportunities.

Microsoft's Surface is a tabletop computer that can register 52 touches simultaneously and recognize objects placed on the screen. In this photo, a program lets people paint with their fingers. (Andrei Pungovschi/Associated Press)

Microsoft has made the biggest waves in this market in recent months with a prototype multi-touch tabletop computer called Surface that can register 52 touches simultaneously. Surface is really just a Windows Vista PC within a table that has a 30-inch capacitive surface. Underneath the screen is a projector that projects the image, while five cameras record reflections of infrared light from human fingertips that touch the screen.

It can also recognize objects that are placed on it through special tags attached to the undersides of the items, creating a range of possibilities. In one demonstration, a hotel guest could drop a credit card on the touchscreen and then use a finger to drag room service items from an onscreen menu over to the card to order and pay for them. In a demo developed with T-Mobile stores in the United States, customers could place two different phones on the screen to compare pricing, features and rate plans.

Surface is being rolled out this spring, though its application is currently planned to be limited to properties owned by Starwood Hotels and Resorts and Harrah's Entertainment, along with T-Mobile stores in the U.S. The one property in Canada that will initially offer it is the Sheraton Centre Hotel in downtown Toronto.

The price is expected to be as much as $10,000 per unit, though Microsoft is reportedly hoping that the cost will drop enough in five years to make Surface feasible for consumers as well.

The author is a Toronto-based freelance writer

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