Those blinking lights on a computer modem indicate the device is transmitting and receiving digital data. But those lights can also let a nosy neighbor read your e-mail, one blink at a time.

So say a pair of computer scientists who have intercepted computer data by analyzing the light pulses from modems and other networking devices.

Joe Loughry, a computer programmer at Lockheed Martin Space Systems in Denver, and David Umphress, an associate professor of computer science at Auburn University in Alabama, say the technique works with everything from a home telephone modem to a high-speed network router and presents an unlikely but technically feasible way for spies and computer crackers to steal information.

Loughry and Umphress have written a technical paper on their exploit, for presentation later this year at a meeting of the Association for Computing Machinery.

Most home Internet users rely on telephone modems. Many are built into the computer and do not have external light displays. But many others sit on a desk alongside the computer and feature a bank of indicator lights that flash as data is transmitted or received.

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These are actually light-emitting diodes, electrical devices that respond quickly to changes in voltage. Even a slight fluctuation causes a change in the intensity of the light.

Often these changes are too subtle for the eye to detect. But Loughry and Umphress used a highly sensitive light detector, connected to a computer and aided by a 100mm lens.

The detector caught every flash of light and fed the data to the computer, which translated it into the data being sent over the network.

Loughry and Umphress tested a variety of computer devices with LED displays and found that in many of them, flashing lights revealed the data passing through the system. They discovered that with the aid of the lens, they could read data from as far as 100 feet away.

Loughry had the idea for the experiment while he was attending graduate school at Seattle University and working at a bank crammed with computer gear. It occurred to him that all the flashing lights might be revealing sensitive financial information.

Loughry doubts that individual computer users are at risk. But he sees it as a significant threat to businesses and government agencies. A network router facing an open window could be an open book for spies. A tiny hidden camera could collect huge amounts of data by focusing on the flashing lights in an FBI computer room.

One year ago, after he and Umphress were convinced the idea would work, Loughry said they contacted the federal government about it. Loughry would not wouldn't say which federal agency they contacted.