A device capable of detecting concealed weapons in a crowd is being developed by US researchers.

Scientists at the Electromagnetic Technology Division at the National Institute of Standards and Technology (NIST) in Colorado have tested components of the technology. They say a prototype, which will fit in a van and have a range of five to ten metres, will be ready by the end of the year.

Police could use the device to identify potential assassins in a crowd. It could also be used by customs officials to help them recognise smugglers. The research has been funded by the US Justice department and the Federal Aviation Administration.

The apparatus uses electromagnetic waves to identify concealed objects. Clothes do not obstruct the waves, but hard objects reflect them back. Reflected radiation is focused on silicon wafers fitted with millimetre-wave antennas. The concentrated electromagnetic radiation is then converted into images that appear on a connected laptop computer.

"It penetrates clothing quite well," Erich Grossman, who is leading the research, told New Scientist. "Some clothing, like leather jackets, are more difficult, but there's not much difference. We're really very confident that it will produce recognisable images."

Signal and noise

The prototype device will be able to pick up objects longer than 20 centimetres.

Grossman says that exceptionally sensitive antennas let the equipment efficiently distinguish between electromagnetic signals and noise. The system can produce an image on the computer screen in one thirtieth of a second. Other researchers have previously attempted to develop similar technology. But these systems could produce images of objects only after several minutes of scanning, Grossman says.

Grossman hopes that the device will eventually be sensitive enough to detect plastic explosives in bags.

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1845 GMT, 1 June 2001