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. Millimetric sensors are capable of detecting both metal and non-metallic objects. Deployment scenarios for these systems fall naturally into two categories, full-body and stand-off:
1) Full-body (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 (public transport networks, arenas and buildings) the requirements will be similar, to detect weapons in general, but with a slightly lower specification 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.
Key is to be able to recognise threats on the body in areas difficult to screen using existing methods. The methods need to be unobstusive and protect personal privacy. In future these systems will be omnipresent, fully automated and part of building infrastructure or street furniture.
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 of millimetre waves into the human body is only a fraction of a millimetre into the skin, 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.
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.