THz Absorption in Fabric and Its Impact on Body Scanning for Security Application

Abstract

In recent years, body scanner technologies based on millimeter-wave and terahertz technologies have been shown to improve security in areas sensitive to terrorist attacks by detecting hazardous objects hidden underneath the clothing of people. Inevitably, useful devices have to provide an adequate compromise between spatial resolution limited due to diffraction and penetration through clothing. Within this context, the spectral absorption of a wide range of fabric, interference caused by the texture, and the effect of moisture within the fabric have to be considered. We have studied these effects by time-domain and Fourier transformation spectroscopy. The experimental findings are verified using a passive THz security camera operating in two frequency bands at 0.85 and 0.35 THz. The latter band was proven to be of superior use for security checks as it provides useful results even in cases of wet clothes.

Microstructure and broadband dielectric properties of Zn2SiO4 ceramics with nano-sized TiO2 addition

“…In the THz frequency range, from 0.2 THz to 2 THz, the dielectric properties of ZST ceramics have opposite changing tendency compared with that in the kHz and microwave bands. The permittivity firstly is decreased from 9.0 at 1200 0 C to 6.8 at 1250 0 C, and then increased to 8.5 at 1275 0 C. At microwave frequencies, the porosity, density and grain size are dominant factors in determining the permittivities, while 13 defects related dipoles and lattice vibrations are more important in the THz band. In our research, the sintering temperatures are from 1200 0 C to 1275 0 C, which are much higher than the sintering temperature of TiO2, typically 1100 0 C. This high temperature can induce the formation of oxygen vacancies as Ti 4+ tends to transform into Ti 3+ at high temperature.…”

“…In recent year, the technologies have been expanded to millimeter wavelength and terahertz (THz) bands due to the shortage of available frequency regions [12]. THz waves, an electromagnetic wavelength from 0.1 THz to 10 THz, are now applied in many practical fields such as passive body scanning [13], fingerprinting and chemical identification [14], and are highly potential for next generation communication system [15]. Various kinds of materials have been explored to make devices operating at THz frequencies.…”

Analysis and design of efficient passive components for the millimeter-wave and THz bands

“…Coherent techniques could provide noticeable improvement of the image contrast in the case of active imaging [113]. [118]. The results in that work indicate that current performance suggest better using lower frequencies in order to achieve the required penetration in the weaves.…”

“…Therefore, inasmuch as resolution is a suitable feature, but penetration may result mandatory, leaving apart the friendly environments of molecular characterization applications, nowadays technology suggest as more suitable to employ, for the moment, the lower frequencies offered by the mm-wave band. A good example of this comparison is found in [118] (consult again Fig. 2.13(b)).…”

“…Although transmission is strongly limited by the atmospheric absorption, several interesting transmission windows can be found, and short-range imaging below 1 THz is today a reality (although several hoped technological advances should drive it to the desired performance yet) [1], [106]. A big amount of research work has been done [111]- [118], including passive imaging, active imaging, and video; and several patents proposed [119], [120].…”