BOTH HOMELAND SECURITY and military agencies have been seeking technologies that detect guns, knives and improvised explosive devices (IEDs). Using the millimeter and sub-millimeter wavebands, techniques to image people when they are close to a sensor have already been developed. Yet research by Roger Appleby from QinetiQ (Malvern, UK) and H. Bruce Wallace of MMW Concepts LLC (Havre De Grace, MD) examines standoff ranges above 5 m. In this range and in the open, one could survive a person-borne IED with no shrapnel.

The researchers review the techniques and technologies that are currently being investigated. At 100 GHz, equipment is relatively mature and can form a baseline for performance comparisons. At 1 THz, however, the combination of atmospheric absorption and scattering from clothing can severely attenuate the signatures available. To sensor developers in the millimeter-wave, submillimeter-wave, and terahertz regions, two primary concerns exist: atmospheric attenuation and scattering from atmospheric particulates like rain and fog.

From 100 GHz to 1 THz, there are differences in the optical properties of skin, explosives and metal. As the frequency rises, the attenuation in clothing and the atmosphere tends to increase. At 100 GHz, for example, it will be dominated by the reflectivity of the items and their geometrical relationship to the body. At and above 500 GHz, attenuation will be dominated by the emissivity of the items and their physical temperature. To exploit these signatures, systems must have the correct parameters in terms of aperture size, sensitivity and signal-to-noise ratio. See "Standoff Detection of Weapons and Contraband in the 100 GHz to 1 THz Region," IEEE Transactions on Antennas and Propagation, Nov. 2007, p. 2944.