for defeating terrorist threats. The committee judged that this could best be done by issuing a series of short reports, of which this report is the third.

This report focuses on maturing millimeter-wave and terahertz imaging and spectroscopy technologies that may offer promise in meeting aviation security requirements. The committee considered the spectrum often mistakenly referred to as the terahertz region to encompass the radio frequencies from 10 GHz to 10,000 GHz, the latter being the top of the true terahertz region (10,000 GHz or 30 micron wavelength). When referring to this entire spectrum, the committee has chosen the nomenclature “millimeter wavelength/terahertz.”

To assess the potential for millimeter-wavelength/terahertz technologies to detect weapons and explosives in an airport screening environment, the committee examined four aspects of the problem:

  1. The currently available phenomenology associated with the atmosphere, concealing materials, and materials to be detected;

  2. The maturity of electronic and electro-optic components;

  3. The suite of millimeter-wavelength/terahertz scanning systems currently undergoing development; and

  4. A potential implementation strategy for the Transportation Security Administration (TSA).

BACKGROUND

The sense of urgency about addressing emerging terrorist threats and the availability of funds to develop potential technologies to address these threats have combined to elicit a plethora of proposals for funding. However, the committee believes that there has been significant overselling of the potential of these technologies to address screening requirements. Proposals that are not well founded on the principles of physics or that are driven by those who lack a sound understanding of the technology and its strengths and limitations appear to exaggerate the potential benefits of millimeter-wavelength/terahertz technology as being more widely applicable to security screening than it is.

The electromagnetic spectrum from submillimeter wave through terahertz can be used both to create an image of an object by measuring the intensity of reflected or emitted energy and to gather information on the chemical makeup of an object by measuring the absorption of electromagnetic energy. There are two general classes of millimeter-wavelength/terahertz imaging techniques examined in this report: passive and active.

Passive imaging detection techniques rely on collecting naturally occurring radiation and using the contrast between apparently “warmer” and “colder” objects, which usually results from contrasts between the emissivities of different materials. For example, millimeter-wavelength/terahertz technologies are being examined for their ability to detect metal guns concealed underneath clothing by detecting the contrast between the warmer human body and the apparently cooler metal weapon.



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