Security screening is likely to be a very large market for millimetre wave technology. This is because over the last few decades the demand for individuals to be screened for illegal or forbidden items has risen dramatically. The demand shows no sign of abating.
The capability for screening individuals for items concealed under clothing is enabled by the almost complete transparency of clothing to millimetre waves. It will be the security screening technology of the future, being fully automated and capable of detecting a very wide range of both metal and non-metallic threat objects.
Systems research over the last decade indicates deployment scenarios for this technology fall naturally into two categories, full-body scanning and stand-off screening:
1) Full-body (or surround imaging) scanners need to closely scrutinise all surfaces of the human body with millimetre scale resolution. At airport departure lounge entrances, the market requirement is to do this for threats such as (sheet) explosives and contraband within about one second, in a walk-through system, with a high probability of detection and low false alarm rate. At entrances to other areas (transport networks, shopping centres, arenas, schools, corporate headquarters, office complexes, government buildings and churches) the requirements will be similar, to detect a diversity of weapons and threats, but with slightly lower specifications than that of the airport.
2) Stand-off screening systems provide an initial first layer of screening of individuals for larger threats (knives, guns, bombs, improvised explosive devices), at ranges out to tens of metres. Typical scenarios here are approaches to checkpoints and in public places where people congregate.
Recognising threats on the body, particularly in areas difficult to screen using existing technology, is key to the newest methods of screening. The methods need to be unobtrusive and protect personal privacy. Machines running algorithms to process sensor data recognise threats; human operators cannot assimilate the throughput data fast enough, so imagery is not viewed by operators; only cues are sent to security, so a potential suspect can be engaged. In future these systems will be omnipresent, fully automated and screen adults and children alike, and all groups of people without discrimination. The technology will be accepted as a necessary part of society and blend seamlessly into building infrastructure and street furniture of future towns and cities.
Performance metrics of security screening sensors are centred on the Receiver Operating Characteristics (ROC) of detection-probability and false-alarm-rate. A good sensor has a high detection-probability and a low-false-alarm, meaning it can detect many threats without generating false alarms. Specific ROC characteristics have been set for the different screening scenarios based on the estimated costs of the risks. For a given sensor, the detection-probability threshold may be raised at times of heightened threat levels, so more potential threats can be identified. However, this raises the false alarm rate, which slows the personnel throughput rate. For this reason End Users, equipment manufacturers and governments are demanding the highest detection-probabilities and the lowest false-alarm-rates for emerging personnel security screening sensors.
Clothing penetration of millimetre waves varies considerably over the band and this dictates the screening capabilities. Greater penetration at lower frequencies enables screening through thicker clothing and greater numbers of layers, whilst at the high frequencies screening is guaranteed only through thinner clothing. Thicker materials, such as (shoe) leather and rubber, and materials which contain moisture, can be penetrated at the lower frequencies.
Penetration into human skin of millimetre waves is only a fraction of a millimetre, with 10% to 40% of this radiation being reflected from the body, the precise percentage being dependent on the thickness and moisture content of the skin. The skin-model of the human is a key element of machine anomaly detection algorithms.
Security screening of fibre glass and canvas sided vehicles for stowaways at road and ferry port (border) checkpints is an application where millimetre wave sensors are currently combatting human trafficking.