Silicon Valley Watcher - Former FT journalist Tom Foremski reporting from the intersection of technology and media

Computers Want To Be Touched: Here's How It Works

Posted by Tom Foremski - July 5, 2012

(Above: Capacitive touchscreens use an electrostatic field to continuously sample the screen surface for movement and relays that information to a processor that can interpret it. )

I've long been curious about how touchscreens work and never got around to finding out. Here's a great explanation.

By Intel Free Press

Today, largely due to the proliferation of smartphones and tablets such as the Apple iPad, sophisticated touchscreens are a common fixture in our digital lives.

To understand how touch technology works, requires a look back to the basics of how touchscreens evolved from primitive interfaces to chic and powerful digital accessories.

Simple Beginnings

It might be surprising to know that touchscreens have been around for decades, although they aren't comparable to today's technology. CRT (Cathode Ray Tube) displays that react to the touch of a human finger or a stylus were developed in the late 1960s, for instance.

These early touchscreens had one significant limitation: they were only responsive to a single point of contact. This was sufficient for, say, taking money out of an ATM when you only needed to use one button touch at a time.

More complex interactions, such as typing, require multi-touch technology, which was only developed within the past 20 years. But to understand the technical details of how multi-touch screens work, let's first examine its single-touch predecessor.

Types of Touchscreen Technology

A touchscreen relies on sensors to pick up the location of a pointing device, whether it's your finger or a stylus. Early touchscreen research soon developed along two paths, with physical devices that used surface pressure to detect location, and using electrical current to translate touch into function.

Physically, touchscreens aren't meant to be used like a mechanical button -- you touch, not press the button. In order to register contact, then, some touchscreens use resistive surfaces.

Resistive technology employs narrowly separated layers of conductive material that react to the location of the contact. In a sense, then, you really are "pushing" a button, insofar as you're completing a circuit.

Because resistive sensors aren't very precise, they're cheap and easy to implement in kiosks and terminals that only require a user to tap large buttons.

Capacitive touchscreens, on the other hand, take the principle behind resistive touchscreens and add precision. Here, the surface itself is electrically charged in the form of a clear conductive coating.

When contact is made, the charge is interrupted and a precise location is registered. Because the only "moving parts" are electrons, capacitive touchscreens last longer and are more durable than their resistive counterparts.

The drawback is that a stylus or other charge-neutral writing instrument isn't going to register, which means capacitive touchscreens rely upon your body's natural electrical charge in order to function.

The actual capacitive touchscreen surface begins with a coating of anti-glare material applied to a transparent protective cover. The cover is treated to guard against scratching and is bonded to the capacitive layer(s).

The layer that does the actual touch sampling can use either self capacitance, which features an array of integrated circuits and electrodes that sense points of contact, or mutual capacitance, which uses two distinct layers to carry and sense current.

Some touchscreen systems, such as the iPhone, use both capacitive methods to detect contact. Only below this layer are the actual LCD display and any backlighting systems.

How It Works

When you use a touchscreen, the effect appears instantaneous. You can give thanks to powerful software for this illusion, for even the simplest interaction with a touchscreen is, under-the-hood, a complicated process.

First, the software must locate where you are on the screen before refining your position. This refinement looks for pressure points (not to be confused with resistive touchscreen technology), eliminates any noise surrounding the input (your finger is blunt, unlike a narrow stylus) and then plots this data to a grid to calculate where it thinks you meant to touch the screen.

More complicated movements are compared against a library of known gestures to understand your input. The device's processor then relays this information to the application level of the software and finally displays the intended result on the display. All this is done in less than a blink of the eye.

Multi-Touch and the Market

In principle, some single-point touchscreen technology is applicable to multi-touch, which allows you to interact with multiple points on the screen simultaneously and can detect complicated motions and gestures. In addition to the capacitive technology, some touchscreen devices utilize infrared, sonar and optical waves to sample surface input.

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