Passive millimetre-wave imaging and how it differs from terahertz imaging: One contribution of 16 to a Discussion Meeting 'The terahertz gap: the generation of far-infrared radiation and its applications'

It is well known that millimetre-wave systems can penetrate poor weather, dust and smoke far better than infrared or visible systems. Imaging in this band offers the opportunity to be able to navigate and perform surveillance in these conditions of poor visibility. Furthermore, the ability to penetrate dielectrics such as plastic and cloth has opened up the opportunity of detecting weapons and contraband hidden under people's clothing. The optical properties of materials have a direct impact on the applicability of imaging systems. In the terahertz band solids have absorptions which can be assigned to vibrational modes. Lattice modes occur at the lowest frequencies and polythene, for example, has a lattice mode at 2.4 THz. Solids have no such absorptions in the millimetre bands (30-300 GHz) and image contrast is produced by differences in transmission, reflection and absorption. A novel, real-time, mechanically scanned, passive millimetre-wave imager has been designed. The antenna elements are based on a combination of a Schmidt camera and a conical scanner, both of which have their origins in optical systems. Polarization techniques, which were developed for operation in the centimetric band, are used to fold the optics. Both 35 GHz and 94 GHz versions have been constructed.