Millimeter-wave Imaging Sensor

Electromagnetic waves in the millimeter-wave band have attractive characteristics. One of their features is the wider usable frequency band compared with waves in the microwave band or lower bands. Another feature of using the millimeter-wave band is the fact that it becomes possible to design smaller and lighter equipment that utilizes that band. So it is useful to adapt millimeter waves for short-range broadband communication systems, highresolution sensing systems and radio astronomy. In this chapter, the authors describe imaging sensors using the millimeter-wave band. In simple terms, a millimeter-wave imaging sensor is a camera that uses millimeter waves. Receivers detect the millimeter-wave energy on the imaging plane, and record the relative intensity at each pixel. Then the millimeter-wave image is reconstructed using computers as shown in Fig. 1. The most attractive feature of millimeter waves, when compared with optical, infrared, and terahertz waves, is their ability to penetrate obstacles. And a spatial resolution higher than that achievable with microwave imaging sensors is possible. Therefore, they can be used under low-visibility conditions such as in fog, rain, dust, or fire, where optical or infrared cameras cannot be used (Mizuno et al., 2005). Their most promising application is for security. As the radiometric temperatures of an object are different depending on its metallic or dielectric properties (r) and its temperature, the sensors can detect concealed weapons or explosive materials. Consequently, these sensors have undergone test installation for use in security cameras at the entrances of airports (TSA, 2009) and buildings. In addition, these sensors are also useful for finding landmines, for offering all-weather vision, for detecting cracks in exterior walls, and for screening people for skin cancer.