Using a camera to detect blood moving under the fingernail, a new system can make almost any object touch sensitive.

Pressure-sensitive touch interfaces are common, but they usually require a smooth surface, with sensors on or beneath its surface. This is fine for simple and robust devices such as laptop touchpads, but it makes adding touch-sensitivity to irregular or fragile objects practically impossible.

However, a new system developed at the University of Nottingham, UK, can do just that. Unlike other systems, it detects the effect of touch on the finger, not the surface being touched. It works by using a simple video camera to observe the movement of blood under a person's fingernails.

Contrasting colours

When a finger is pressed onto a surface blood shifts away from the top of the nail, making it lighter. That blood concentrates near the nail's base, which gets darker.

The camera monitors that change in contrast and can detect the fleeting change that indicates a single touch. It can also gauge how hard a finger is pressed from the intensity of the colour change.

"When you touch a surface, even lightly, the blood under the nail concentrates in one place. Surprisingly, this is relatively easy to spot with a camera," says Joe Marshall, a researcher in the Nottingham team.

Using an external camera to make objects touch sensitive could allow tactile interaction with new objects, says Marshall. "For example, historic tools in a countryside museum could be touched in order to trigger audiovisual material about their use," says Marshall.

Pet rock

The simplicity and low cost of the technology is another advantage, he adds, because all that is required is an inexpensive webcam and desktop computer.

A video (see right) shows a demonstration of the system – a touch-sensitive rock, which purrs when stroked and growls when grabbed. A webcam inside the rock's cage watches people's fingernails to cause it to react to their touch.

Marshall was impressed by the emotional response that the rock evoked in people who touched it. "It was presented as 'my pet rock' and people seemed to really engage with that," he says. "One person actually asked me if they could take it home."

Making curved or irregular 3D objects respond to touch opens up a whole new area for the way people use touch interfaces, Marshall says. "A lot of what is out there is just taking a computer desktop and saying 'let's put it on a wall or a table,'" he says. "I’m much more interested in interactive projects using objects."

Colour-blind system

Although anything that obscures the user's nails – such as gloves or nail varnish – can cause the system to fail, variations in skin colour between different users do not seem to cause problems. "Surprisingly, people's fingernails are pretty consistent. So it's colour-blind in that sense," says Marshall.

Ming Yang, at Cardiff University, UK, developed a way to make any table touch sensitive using a few microphones (with video). "This is a very interesting method which has never been explored for touch-sensitive applications," he told New Scientist.

Yang sees the simplicity of the system as its main advantage, but adds that simplicity also has drawbacks. "There are a lot of limitations that are inherited from the technology that was adapted for this system," he explains, "I think that reliability may be a big issue due to the low-cost camera that was used."

A paper on the nail-watching system will be presented in May at the Pervasive 2008 conference in Sydney, Australia.

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