Feb 152010
 
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THE popularity of touch-screens on mobile phones means that a swipe, tap or a flick comes as naturally these days as the click of a mouse. But existing touch-screens have their limits. Those relying on changes in electrical resistance tend to have poorer resolution than is needed for modern applications, while those that rely on capacitance require an ungloved finger.

Consequently, a new generation of touch-screens, known as optical liquid crystal displays, is emerging. Optical LCDs embed tiny light sensors next to many of the screen’s pixels. In the brief moments between each successive screen image, the backlight is turned off. In these periods of darkness, undetectable to the human eye, sensors are able to pick up light coming from outside the device.

Although such sensors are designed to detect only the presence or absence of a finger touching the screen, Ramesh Raskar, a researcher at the Massachusetts Institute of Technology, wondered whether this new type of device could be turned into not a touch, but a touchless screen by using the sensors to detect more distant objects as well.

His idea was to treat each sensor as if it were a pinhole camera. He (or, rather, his software) would then stitch the two-dimensional images from each pinhole together to obtain a three-dimensional picture. This could then be used to determine which bit of the screen a distant finger is pointing at.

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