Security Screening

Security screening is a large potential market for millimetre wave technology, because in recent years the demand for screening individuals in public places and at building or venue entrances for illegal items has risen dramatically, and shows no sign of abating.

Millimetre wave screening systems are now emerging with enhanced performances, by using Artificial Intelligence (AI) to recognise the interaction between millimetre waves, concealed threats, the human body and clothing.

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 unobtrusive and fully automated using AI, to detect and identify metal and non-metallic threats. The technology offers a touchless screening capability, deployable in any scenario where there are people. 

Systems research and development over the last decade has led to deployment scenarios for millimetre-wave technology falling naturally into two categories, full-body scanning and stand-off screening:

1) Full-body (or surround imaging) portal scanners need to scrutinise closely all surfaces of the human body for concealed metal and non-metallic threats. Used mainly at airport departure lounge entrances, the requirement is to detect threats such as (sheet) explosives, contraband and smaller threats that cannot be detected using metal detectors.

The screening protocol requires an individual divested of false alarm objects from pockets, and other areas, to stand motionless for a few seconds inside a two-metre sized portal, with legs apart and arms stretched out. Within a few seconds, analysis flags any potential threat items on the body with a high probability of detection and a low false alarm rate. Should purchase prices fall and space permit, these systems may be deployable at entrances to other areas and buildings.

2) Stand-off screening scanners provide an initial first layer of defence out to ranges of tens of metres, detecting larger threats on the body, such as knives, guns, pure explosive materials, bombs, improvised explosive devices and narcotics. They are less well developed than full-body portals. Being physically smaller (20 cm), they are wall-mountable, systematically screening individuals as they approach entrances.

Having greater flexibility and costing less, they are potentially usable for protection at transport networks, shopping centres, arenas, schools, corporate headquarters, office complexes, places of worship, asylum centres and public servant buildings. Hand-held, drone or tripod mounted systems would enable rapid deployment for crowd screening. Wide spread use of these systems represents a huge market for these emerging systems.

Cooperative and non-cooperative are two further categories of screening. The former is overt, where the subject is aware of being screened and may have to divest items in pockets, remove shoes or remain stationary for a few moments while being scanned. The latter is covert, where the subject is unaware of the screening process, respecting the legalities of the particular deployment scenarios.

Sensor systems may be active (as in radar) whereby man-made millimetre wave radiation is reflected off a subject and analysed or passive (like a long-wavelength infrared camera) whereby thermal (Planck) radiation emitted by a subject is measured. Both methods have complementary attributes, discussed more fully in Sensor Science.

Knife and gun crime is an area now being addressed for non-cooperative screening of people in public places. Evolving millimetre wave sensors are becoming more effective at recognising these types of weapons concealed on persons. Security services are immediately alerted about the positive results of non-invasive pat-down searches.

Rucksack bomb detection is also enabled by the transparency of fabric to millimetre waves. Should a device be wrapped in an opaque material, the sensor will detect this as an anomaly.

Body screening videos from a 30 GHz millimetre wave imager are shown:
1) Metal pot in a rucksack
2)  Empty rucksack
2) Beeswax disk - one centimetre thick 
3) Polarimetric millimetre wave radar simulation

Recognising threats on the body (particularly in areas difficult to screen using existing methods) is key to the newest methods of screening. The methods need to be unobtrusive and protect personal privacy. Machines running algorithms process sensor data to recognise threats. Human operators cannot assimilate the throughput data fast enough at entrances, so machine learning takes over and passes cues to security officers, so potential suspects can be engaged at an appropriate location. In future, these systems will be omnipresent, fully automated, screening adults, children and all groups of people without discrimination or prejudice. 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.

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. This varies with gender ethnicity, body location and age. The skin-model for humans is a key element of machine anomaly detection algorithms.

Recognising threats inside packages and concealed behind walls, under floors and ceilings is another capability of millimetre-wave sensors. Since many materials (paper, cardboard, plastic, thin plasterboard, carpet, thin dry wood) are transparent to millimetre-waves radar sensors can categorise types on objects concealed in enclosures.

Combatting human trafficking by screening fibre glass (refrigerated) and canvas sided vehicles for stowaways at road and ferry port (border) checkpoints is a current application of MMW sensors. The technique uses passive (or radiometric) imaging. The capability is enabled by the transparency of the fibre glass and canvas, and the opaqueness of the human body. Stowaways have high contrast against haulage vehicle contents and the canvas and fibre glass sides.

Health & Safety: Our environment continually bathes us in millimetre wave radiation wherever we are, as a natural phenomenon; it is non-ionising and extremely low in intensity, therefore completely non-dangerous to life. Active systems like radar deliver similar dose levels, lower than that from mobile phones. Hence millimetre wave security screening systems are safe to use.

Receiver Operating Characteristics (ROC) of detection-probability and false-alarm-rate are the performance metrics of security screening sensors. A good sensor has a high detection-probability and a low false-alarm-rate, 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 lowered at periods of heightened security, to detect more potential threats. However, this raises the false alarm rate, which slows the personnel throughput rate. For this reason End Users, equipment manufacturers and governments demand the highest detection-probabilities and the lowest false-alarm-rates for emerging personnel security screening sensors.

The ‘grudge purchase’ is the biggest problem facing security screening technology development; those buying the technology don’t profit from it, even though it stops bad things happening. This means it's difficult to get development funding from industry or government. Mandating the use of this technology may go some way to solving this problem. Having cost-effective security screening solutions will force this mandate, making mm-wave technology commonplace in society.