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Researchers at MIT's Computer Science and Artificial Intelligence Laboratory have come up with a way to create a low-power, portable device that can see through walls using Wi-Fi signals.

Technology of this sort, similar in concept to radar or sonar, has existed for years and relies on radio waves and other parts of the electromagnetic spectrum. It is used mainly in law enforcement and military applications, ideally when the law allows.

Wall penetration systems have become common enough that the U.S. Department of Justice last year funded a market survey of what's known as "through-the-wall sensors," or TTWS.

[ What does that mean for security? Read Google Shares Advice On Wi-Fi Security. ]

Security products maker Camero-Tech, for example, offers its Xaver line of through-wall imaging devices for defense and law enforcement applications. But with prices at about $9,000 for the handheld Xaver 100 and $47,500 for the 7 lb. Xaver 400, these aren't consumer products.

The legality of TTWS technology is sufficiently unclear that ManTech Advanced Systems International, the company that prepared the market survey, recommends those planning to use TTWS equipment seek legal advice in advance.

In 2001, the U.S. Supreme Court ruled that the use of thermal imaging to monitor what's going on inside a private home violates Fourth Amendment protection against unreasonable searches. But as the ability to see through walls reaches the civilian market, this legal boundary is likely to be tested again.

There is at least one consumer TTWS device on the market already, STI's Rex Plus, an $80+ device that can be placed against a wall/door in order to sound an alarm when someone approaches the opposite side of the wall/door.

Dina Katabi, a professor in MIT's Department of Electrical Engineering and Computer Science, and graduate student Fadel Adib propose wider civilian use of the technology through a simple, affordable device like a mobile phone, equipped with two antennas and a receiver.

In an email, Katabi suggested the technology, which she calls WiVi, can be used for virtual reality and gaming, without requiring the user to remain in a specific area in front of a sensor. She also says the technology could be used for personal safety.

"For example, if I am walking at night in an isolated area and suspect that someone is following me, hiding behind a fence or around a corner, I can then use WiVi to detect that person and alert myself to the person's movement," she said.

Katabi says WiVi can be used for "privacy-preserving monitoring," such as tracking the movements of elderly people or children without actually having them on camera.

In time, however, improvements in Wi-Fi-based sensing may require a reexamination of the privacy implications of making walls effectively transparent.

"Today the technology does not show body parts or the face of the person," said Katabi. "Hence it is naturally anonymized. However, as we improve the technology it will start giving higher resolution images for things that one cannot see because they are behind a wall. This will raise privacy related questions. As a society, we still have time to look at these issues and ensure our society has the right policies by the time such high-resolution version of the technology becomes available."

That future already has been contemplated: University of Tokyo researchers have developed paint that blocks Wi-Fi signals.

Wi-Fi-based sensing appears to be a particularly active area of research at the moment. At the University of Washington, researchers have developed a related technology, WiSee, a Wi-Fi-based gesture sensing system that duplicates the functioning of sensor-based motion detection systems like Leap Motion and Microsoft Kinect without the sensing area limitations.