Review of Backscatter X-Ray And Millimeter Wave Airport Scanning Technology

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Visually Detecting Deception

The next ten years may well become known to future engineering generations as the “decade of the whole body scanner.“ These airport body scanners are intended to detect anything hidden on the body by clothing, non-body personal effects, and other forms of deception. This includes visual enhancement of metal, plastic, glass, liquids, and other materials not considered to be part of a human body. The images they produce are quite detailed and allow visual and computational identification of potentially dangerous items. Two of the most recently mentioned technologies are the backscatter x-ray and millimeter wave scanners.

Backscatter X-Ray Scanners

Backscatter x-ray technology has been around for quite some time. It operates on the principle of the Compton scattering effect, which applies to x-rays and gamma rays as they are deflected off certain kinds of materials. When an x-ray or gamma ray photon collides with an orbiting electron, part of the ray’s energy may be transferred to the electron in an inelastic deflection. This results in the electron being ejected from its orbit while increasing the wavelength of the now less energetic ray. For x-ray backscatter images, a source of high energy x-rays are focused into a rapidly scanning beam and the patterns of deflected rays are collected at one or more detectors. These high energy rays do not have the penetrating power of the lower energy rays medical imagers use, so while they can penetrate clothing, they are deflected by soft body tissues as opposed to passing through them. By electronically filtering and processing tuned detector signals, a detailed 2D surface image of these tissues can be produced, in this case the gray scaled image of human skin. All other materials show up as anomalous objects which may be associated with weaponry or explosives. Additionally, since these rays are surface deflected, the interaction with human tissues is very minimal, resulting in less radiation exposure than would be experienced from the constant bombardment of radiation through the atmosphere in course of a few minutes.

Millimeter Wave Scanners

Millimeter wave whole body scanning devices operate by utilizing extremely high frequency radio waves. These waves have a frequency above that of microwaves to just below the long infrared light band. Consequently the associated wavelengths are on the order of 1mm, giving rise to the “millimeter wave” name. Some materials appear semi-transparent to these radio frequencies, most notably organics like paper, plastics, wood, and clothing. Other materials are more reflective, like human skin and metals. By rotating two directional high frequency radio antennas around an object, like a clothed human body, and detecting the reflected waves a characteristic signal is developed. This signal can be electronically processed and filtered to produce a detailed 3D image of the body sans clothing. Objects like those associated with explosives which do not belong in a typical 3D body map can be flagged for further investigation. These images are also gray scaled, to the point of looking almost metallic. Interestingly, this technology has other scanning applications like accurately measuring body shapes in 3D for various manufacturing and simulation purposes. Additionally, these waves are much less energetic than other forms of radiation and consequently are not capable of ionization or chemical bond disruption. Presumably this means frequent exposure to the millimeter wave scanner will not be harmful to human tissues.

Improved Hybrid Systems and Other Alternatives

Of course improving methods to conceal and smuggle potentially harmful objects and materials is an ongoing endeavor by those who intend harm. Several obvious methods to defeat the current backscatter x-ray and millimeter wave scanning technologies could be deduced from reading just this basic article, let alone after performing an intense Internet research. Fortunately development of other advanced detection and interdiction methods are also ongoing. Certain other forms of radio frequency detection, for example, can produce characteristic signatures or “fingerprints” of materials being scanned even at a distance, which also mitigates the current concerns over privacy. Hybrid systems using ion mobility spectrometry, quadrapole resonance, computer tomography, strong electric fields, and other technologies are being investigated as well. In the meantime while the current slate of whole body scanning technologies may be an electronic invasion of privacy for some, whole body pat-downs, strip searches, or experiencing an explosives detonation up close and personal are probably alternatives most would rather avoid.