It took a while to take hold effectively, but the modern touchscreen has revolutionised the contemporary interface.
There was a time, not too long ago, when the only touch-sensitive screens you could find in consumer gadgets were those on stylus-driven PDAs and tablet computers; touch-based systems were, at that point, too expensive and clunky to replace the fine-tuned accuracy of the mouse or keyboard. But then came the mass-market, stylus-driven Nintendo DS and, later, the iPhone and its multi-touchscreen – and everything changed. Touchscreens are now built into everything from mobiles to printers, GPSs, ATMs, public kiosks and coffee machines – but they aren't all the same.
Types of touchscreens
Perhaps the simplest form of touchscreen is a resistive screen, which includes two layers of electrically conductive material. The layers are separated by transparent insulating dots that keep them apart; you can see these dots as faint traces on some touchscreens.
When a person presses on the screen, the two layers touch and a circuit is completed. The position of the touch along the sensor changes the voltage being sent through the circuit, which the screen's controller uses to measure the coordinates of the touch and feed this information to the running software. Conversion is typically handled using an analogue-to-digital converter that, with typical 10-bit resolution, can differentiate up to 1,024 different locations across each axis.
Microsoft's Surface table-computing devices can register dozens of simultaneous touches thanks to a rear-mounted infrared LED and camera system.
Because they use a physical process, resistive touchscreens work even when users are wearing gloves, making them preferable for outdoors or industrial environments. They're also highly durable, which makes them common in point-of-sale terminals such as those used continuously by restaurant staff. However, resistive screens only let around 75% of the light from the screen through – making them less than ideal for touch-based devices like smartphones and tablet PCs, which use the touchscreen on top of a high-resolution screen that's a key feature of the device.
Capacitive screens work using a sheet of glass that's coated on the inside with horizontal and vertical electrodes made of a conductive material (often, the nearly-transparent indium tin oxide, or ITO). Because your body also has an electrostatic field its presence affects the capacitive potential of the ITO layers when you touch the insulating glass. This change is registered as a touch by the screen's controller, which determines the location of the touch along each axis and relays it to the controlling software.
This design makes capacitive screens more sensitive, since you don't have to physically push down on the screen to make it work; it also enables novel applications such as the touchscreen property viewers now found on the windows of many real-estate agents.
However, its design also means you can't use them with gloves on, or with any other implement; a stylus can be used as long as it carries an electrostatic charge. This has not, however, been a limitation to the use of capacitive screens in handheld devices, which are generally finger-operated and benefit from the fact that capacitive screens let through over 90% of the screen's light.
Other approaches
One issue with both resistive and capacitive touchscreens has been their reliance on indium, which is a relatively rare metal produced as a by-product of zinc mining. Increasing demand has raised the price and scarcity of indium, spurring a search for alternatives; one recently announced option is graphene, a two-dimensional structure whose regular carbon-based lattice shape underlies the structure of graphite and has promise in electroconductive applications. In 2010, Japanese and Korean researchers used a technique called chemical vapour deposition to make a touchscreen that uses graphene layered on a sheet of copper foil.
Two other primary types of touchscreen also offer alternatives in certain situations. Surface acoustic wave (SAW) touchscreens use a pair of ultrasonic transducers that send ultrasonic waves along the surface of a glass screen; when you touch the screen, it interrupts the waves, whose refraction is measured electrically by X and Y sensors and converted into co-ordinates for use within the software. SAW screens are often used in fixed applications such as public information kiosks and ATMs, but its requirement for ultrasonic transducers makes it inappropriate for portable devices. SAW can also be affected by dirt or grime on the glass, which is a consideration in public terminals.
The fourth main type of touchscreen surrounds the screen with infrared LEDs, each with a matching photodetector installed in a grid across the area of the screen. Touching the screen breaks the infrared beam, with X and Y co-ordinates derived based on the LEDs whose beams are broken. Because it works by optically interfering with a light beam, infrared systems support objects other than fingers and can be more appropriate in outside systems.
One other technology used occasionally is optical imaging, which suits large-screen devices like Microsoft's Surface table PC. Optical imaging uses infrared LEDs aimed at the screen from its inside; when you touch the surface, the infrared light bounces back and is picked up by cameras positioned around the edges of the screen. This approach supports many fingers at once and also, as Microsoft has demonstrated, supports the use of image acquisition and barcode reading using the same cameras.
I've got the multi-touch
Despite the growth of touchscreens in all kinds of devices, a major advance in Apple's iPhone was its use of multi-touch technology that allows sensors to pick up more than one fingerpress at a time. This is also possible using optical imaging, but the apparatus and image-processing required for that type of touchscreen are far too onerous for a portable device; this has led to broad use of an alternative known as 'mutual capacitance'.
A mutual-capacitance screen includes a small capacitor at the intersection of each row and column; the iPhone 4's 960 x 640 screen, for example, would include 614,400 capacitors. A voltage is run through each capacitor and a finger's touch changes the ability of the capacitor at that intersection to hold its voltage; this interruption is recorded by the screen controller and fed back to the software as an X-Y coordinate pair.
The controller can register changes in more than one capacitor at a time, enabling multi-touch or 'chord' inputs; it uses an algorithm to measure the distance between touches, and to decide whether it's detecting two fingertips independently or two (or more) held together. This allows multi-touch devices to offer actions like zooming, rotating and two-fingered scrolling.
Small devices' ability to use mutual-capacitance systems depends on manufacturing techniques that pack a high number of capacitors into small areas; some smartphones and tablets suffer from low responsiveness that stems from underpowered controllers or use of cheaper screens with lower numbers of capacitors – and a resulting lower sensitivity.
Speaking of sensitivity, traditional touchscreen techniques have prevented screens from knowing how hard you were pressing; you were either touching the screen, or were not. However, newer techniques for high-resolution screens allow touchscreen controllers to poll many capacitors at once. This measures the effective diameter of the fingertip as it's pressed against the glass and spreads out: an increasingly large touch-point indicates increasing pressure.
Manufacturers face trade-offs in deciding which type of screen to use. For example, resistive touchscreens are lower-resolution and cheaper to produce, making them favourites in devices like photocopiers where the system only needs to resolve presses on a small number of fixed "buttons". Those devices can also be operated using a fingernail – and, indeed, often work better when pressed with a fingernail to ensure clean contact. Capacitive touchscreens, however, don't work with fingernail presses – much to the chagrin of long-nailed people everywhere.
Capacitive touchscreens also face potential interruption from dirt and grease stuck on the screen, which can interrupt the electrical action of the finger and cause flaky response from the device; this is a major reason for oleophobic coatings that are now frequently added to touchscreen devices. Kept clean and designed appropriately, high-resolution touchscreens are now living up to their potential as intuitive, effective input methods.
