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While airport security has become an obvious priority since the September 11 attacks, relatively little change has been seen in the technology used to achieve higher levels of security. Now, though, several initiatives are emerging for development and introduction of new techniques based on optical technology that might help prevent future attacks on aircraft. Efforts are under way to use lasers to detect explosives, for holographic scanning to check for hidden weapons, and to implement already mature iris-scanning technology to help identify travellers and key personnel. Some of these measures raise serious privacy concerns, however, so it remains unclear whether passengers will, for instance, accept technology that allows them to be seen ‘naked’ in exchange for a promise of safer skies.
Explosives detection is one of the key objectives of airport security. This is a difficult task because explosives do not necessarily show up on x-rays. But work now being done by The Canadian Federal Aviation Administration and Intelligent Detection Systems (IDS) indicates that traces of explosives left on the outside of luggage can be detected by lasers.
It is very difficult to avoid leaving microscopic traces of explosive on the outside of luggage if those handling the luggage have themselves packed the explosives. Conventional detection techniques, which involve wiping the luggage with solvent and then feeding the dissolved material through a gas chromatograph, are sensitive, but require that the luggage already be selected for further examination. Laser-based techniques could, in principle, scan all items of luggage. The IDS method looks for the light produced when tiny amounts of explosives are detonated by a laser pulse.
Laser inspection of luggage doesn’t raise privacy issues, because luggage is x-rayed anyway and the laser only inspects the outside of the luggage. But privacy does become an issue with efforts to use millimeter-wave radiation, which penetrates clothing to search for concealed weapons.
Millimetric radiation is generated by methods similar to those used at microwave frequencies but, because of its short wavelength, can use technology like holography, which is based on optical techniques. For detecting weapons hidden on a person’s body the radiation has the dual advantage that it penetrates clothing easily but is reflected by metal, plastic and any other material from which a weapon could be made. Scanners operating at about 100 GHz exhibit high resolution, near 3 mm, which is adequate for detecting even small weapons and distinguishing them from innocuous objects like cell phones.
A broadband rather than a single-frequency source and an original algorithm are used to reconstruct a fully three-dimensional image with good depth-of-field. The algorithm is a combination of holographic techniques derived from optical holography and synthetic aperture radar techniques which have been used to form images of terrain from aircraft.
The privacy concerns about millimeter-wave scanning are clear from the images produced. Since millimeter waves are reflected by the human body as well as by concealed objects, the scanned image shows the subject as naked. With 3 mm resolution the view is about as detailed as one would get with ordinary vision of an unclothed person 15 ft away. It seems unlikely that airport passengers will put up with being seen naked by a stranger in return for enhanced security.
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