Millimeter-wave sensors

The NYT has an article on millimeter-wave imaging technologies applied to the detection of concealed weapons. The human body has a high emissivity and emits a great deal of millimeter-wave energy (between 30 and 300 GHz)- it shows up as hot on a millimeter imaging system. By contrast, a concealed gun, for instance, has a low emissivity and a high reflectivity - it reflects the ambient energy (at the temperature of the surroundings) and shows up as cold on the scan. The temperature differential with respect to the surroundings allows for the discrimination of the weapon being carried.

The article profiles three companies (Brijot Imaging Systems, Millivision Technologies and Trex Enterprises) that appear to have working imaging systems integrated with video surveillance and software that accomplishes detection and classification in a device that's about $50K.

Interestingly, these passive detection systems have an active counterpart (involving bouncing millimeter waves off the subject in a manner analogous to radar). Understandably, there are health and privacy concerns around the active imaging systems as a result of which the passive systems are likely to get better traction.

Millivision has a nice whitepaper on the technology on their website.