Passive imaging systems in the “lower THz range”

International terrorism has reached a level where adequate countermeasures to protect the population have to be provided by the authorities. Similarly, the improved surveillance and protection of sensitive infrastructures, like for instance nuclear power plants or hazardous materials industries, attracts increased attention. The efficient inspection of people with respect to weapons and explosives is of increasing interest particularly for transportation services or public events. Hence, research on these new challenges had been animated through various organizations. In addition, the requirements for considerably extended and improved security on the civilian sector have blurred the former frontiers between exclusively military and civilian use and development of high-tech systems. Driven by this evolution the so-called THz range of the electromagnetic spectrum came into the focus of recent research activities. In the past mainly the radio astronomy community and some atmospheric researchers were using these wavelengths already for many years. Nowadays the upper microwave range, i.e. the millimetre-waves, is often assigned to the lower THz range, since frequencies above 100 GHz are not widely used. Microwaves in the range of 1-100 GHz are already used in many respects for remote sensing applications. Besides radar sensors particularly passive measurement methods are used for two-dimensional imaging. Microwaves can penetrate clothing and a multitude of other materials and allow the detection of hidden objects by monitoring dielectric anomalies. Passive microwave remote sensing allows a daytime independent non-destructive observation and examination of the objects of interest under nearly all weather conditions without artificial exposure of persons or areas. In this paper the application of some “lower THz” imaging radiometer systems for security, military, and defence applications is illustrated