The utility of these millimeter-wavelength/terahertz systems lies primarily in their ability to penetrate materials that might shield concealed weapons or explosives from detection. Typically these materials may be normal clothing, but they could also include luggage. For the Terahertz Imaging Focal Plane Array Technology (TIFT) program of the Defense Advanced Research Projects Agency (DARPA), Ohio State University (OSU) and the University of California at Santa Barbara (UCSB) have characterized numerous articles of clothing (Figure 2-4). The OSU data were collected on clothing provided by the U.S. Army from southwest Asia, while the data measured by UCSB were from clothing provided by university students and staff. Without making an attempt to characterize the particular articles of clothing in a parametric sense, these sets of measurements indicate that transmission through clothing is generally better than 80 percent (−1.0 dB) at frequencies below 300 GHz but decreases as the frequency of operation increases. It is also apparent that transmissivity varies greatly with different types of clothing.
The data collected by UCSB generally appear to have reduced transmissivity when compared with OSU data. The difference may be in the clothing from southwest Asia being of a lighter material than that examined at UCSB, where the climate is generally cooler. Any system that would be required to image through clothing would be expected to operate through at least two layers of clothing, perhaps a cotton shirt and a wool sweater. The one-way transmissivity through the combination could thus be less than 50 percent at 300 GHz and 25 percent at 600 GHz. Whether the system is active or passive, the worst-case transmission, for the purpose of discerning concealed items,