Assessment of Millimeter-Wave and Terahertz Technology for Detection and Identification of Concealed Explosives and Weapons (2007) / Chapter Skim
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2 Basic Operation of Systems and Phenomenology
2 Basic Operation of Systems and Phenomenology
Pages 15-26
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From page 15... ...
These investigations have been directed toward two applications of importance to aviation security. The first is to produce imagery of 1 D.T.
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For example, these technologies are being used to detect guns concealed underneath clothing by the detection of the contrast between the warmer human body and the apparently cooler metal weapon.2 Imaging technologies can be either passive or active. Passive systems are not designed to generate or emit radiation but use natural background radiation for the illumination of the detection space.
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Figure 1-2 in Chapter 1 has a curve labeled "300 K Blackbody Curve" that shows the amount of radiation versus wavelength for a perfectly emissive body at 300 kelvin (K) , approximately room temperature.
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From page 18... ...
A literature search will yield hundreds of recent citations covering all aspects of terahertz research. In order to form a terahertz image using time domain spectroscopy, the terahertz beam is brought to an intermediate focus using a pair of lenses or parabolic reflectors, which are inserted into the region where the terahertz beam is collimated.
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From page 19... ...
While it should be understood that eyeball resolution is not necessary for the detection of concealed objects or the identification of explosives, it is a convenient metric for examining relative aperture sizes of millimeter-wavelength/
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FIGURE 2-2 Antenna diameter required for an imaging system to achieve the resolution of the human eye. Atmospheric Propagation The atmosphere attenuates millimeter-wave radiation at frequencies determined by molecular absorption by water vapor, oxygen, and other atmospheric molecules.
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From page 21... ...
With active systems, if transmitter power could be increased an unlimited amount, contrast could still be maintained, assuming the atmosphere does not contain particulate matter that also might reflect energy. To a first order, it can be stated that as the atmospheric attenuation increases, the range at which good image quality can be maintained will decrease.
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From page 22... ...
For the Terahertz Imaging Focal Plane Array Technology (TIFT) program of the Defense Advanced Research Projects Agency (DARPA)
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By gating these transducers with ultrafast optical pulses, one can generate subpicosecond bursts of electromagnetic radiation, and subsequently detect them with high signal to noise using gated detectors. These transients consist of only one or two cycles of the electromagnetic field, and they consequently span a very broad bandwidth.
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From page 24... ...
While there is some information to show that some materials may exhibit "colors," there is little evidence to support an analysis of broadband versus narrowband techniques. Any investigation of materials properties needs to examine emissivities across the broad bandwidth, not just at single frequencies.
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From page 25... ...
SOURCE: Courtesy of Ohio State University. Unpublished.
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