Imaging at millimetre wavelengths can be achieved in a variety of ways.

  1. Passive radiometric imaging is a form of thermal imaging in which a radiometric sensor collects the natural thermal emission of the scene.
  2. Active radiometric imaging uses the same type of imaging sensor as for passive radiometric imaging, but the system employs some form of active illumination of the scene.  This illumination is generally noise like and is used to increase the contrast in the scene.
  3. Active radar imaging collects radar signals in an imaging configuration to give a 3D dataset which combines angular information with range to the subject, and potentially other parameters such as reflectivity and polarisation.
  4. Spectral imaging uses an imaging sensor which collects spectral information about the scene over a range of wavelengths.

The most common forms of mm-wave imaging are 1 and 2, often just referred to as Passive MM-Wave Imaging or PMMWI, and 3, usually just called radar imaging.

Much of the early development of PMMWI was driven by the need for battlefield surveillance or aircraft landing guidance in bad weather.  However, mm-wave imaging is a very hot topic right now as a technique for concealed weapon detection (CWD) and the detection of immigrants in soft-sided vehicles.  In the current heightened state of security worldwide, ports, airports and law enforcement agencies are desperately keen to exploit this new technology in the fight against terror.

Modern mm-wave cameras can clearly resolve weapons hidden under clothing.  The penetration of mm-waves through fabric is so good that even non-metallic objects (ceramic weapons and potentially plastic explosives) can be detected.Radar imaging is used when the shape or the target or the topography of the view is required.  Due to the high range and spatial resolution achievable with compact antennas, mm-wave imaging radars are often used for terrain mapping applications in robot vehicle guidance or geoscience remote sensing.