passengers and baggage that takes advantage of the ability to penetrate clothing and other nonmetallic coverings. This application is intended to find objects such as knives, guns, and explosives by detecting their shapes through the concealment. A second, more advanced application is intended to be able to classify materials, which also may be concealed, by observing their differential absorption or reflectance of radiation—in effect, spectroscopy. Recent data have shown that solid explosives do exhibit some repeatable spectroscopic features in the spectrum above 800 GHz that may be used to differentiate them from other solids.
This chapter describes passive and active imaging technologies, fundamental characteristics and technical limitations of these technologies, and spectroscopic technologies.
Imaging detection techniques rely on the contrast between warmer and colder objects or on the contrast between objects that have high and low emissivity of radiation or, equivalently, low and high reflectivity of radiation. 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. Active-illumination systems generate and emit radiation that is used to illuminate the detection space.
Every object generates electromagnetic emissions at all wavelengths with intensity proportional to the product of its physical temperature and its emissivity in accordance with Planck’s radiation law. Objects also reflect the radiation emanating from the environment to a degree of reflectivity which is the complement of their emissivity; the sum of the emissivity and the reflectivity is 1. Thus, an object that reflects 90 percent of the radiation striking it will have an emissivity of 10 percent. These values are generally a function of wavelength, so what might be reflecting at long wavelengths in the radio-frequency region may appear to be emissive in the infrared region. An example of this would be a metal mirror with a coating of dull black paint. An infrared sensor would sense an emissivity close to 1 and would respond to the temperature of the coating, while an RF sensor would sense the reflecting surface as the mirror, since the coating is readily penetrated by the long wavelengths.
The human body has an emissivity of about 65 percent at 100 GHz, increasing to about 95 percent at 600 GHz (Table 2-1). This would make a human body appear warm relative to a metal object, which would have a low emissivity and would thus reflect the