New-wave detectors

September 04, 2000

Safety in the air depends on security technology on the ground. The widely used metal detectors we currently pass through in airports are sufficient for detecting metal weapons, but they have limitations.

Single- and multiple-zone metal detectors use low-intensity magnetic fields. When a transmitted magnetic field passes through a metal object, eddy currents appear on the surface of the object. The eddy currents produce magnetic flux. A receiver detects the disturbance in the transmitter field.

Metal detectors respond to anything metal, such as keys, change, belt buckles, and metal implants. But, this non-discriminating, low sensitivity technology slows down traffic through security checkpoints, resulting in delays for passengers. And, it doesn't detect low-metal and non-metal weapons, including plastic explosives.

High-frequency, millimeter-length radio waves provide a possible means of overcoming the limitations of metal detectors in airport security systems.

Millimeter-wave systems under development are capable of more accurate imaging of low-metal and non-metal weapons, and explosives.

The FAA is considering two approaches to weapons detection using millimeter wave systems. One system uses active millimeter waves, the other uses passive millimeter waves. The challenge in either case is to make the wave technology both fast and affordable.

Typical readings of objects using millimeter waves
Every object emits distinct electromagnetic waves that are dependent on physical temperature and emissivity. Following are the readings of common objects that an airport security system operator might encounter.

Object Reflectivity %
Human skin 5
Plastics 30-70, depending upon type
Paper 30-70, depending upon moisture content
Ceramics 30-70
Water 50
Metal gun 100

Passive millimeter wave imaging

Millivision, a developer of security products (Northhampton, MA) is developing a system based on passive millimeter waves. It's passive because the wave-imaging camera emits no signal. The technology measures naturally occurring electromagnetic waves produced by the objects being viewed.

"Every object generates electromagnetic emissions at millimeter wavelengths with an intensity proportional to the object's physical temperature times its emissivity," says G. Richard Huguenin, the executive vice president of Millivision. Their approach uses a camera for contrasting the differing electromagnetic wavelengths of an object. "The human body is highly emissive, which presents a 'warm' background on the monitor," Huguenin says. "Metal objects have a near zero emissivity, so they appear cold against the body's warm background. For a reference point, we use a black body, which represents zero reflectivity."

Plastics and ceramics have emissivities higher than metal, but lower than human flesh, so they also contrast against the body, he says. At millimeter wavelengths, the object's emissivity doesn't change over time, so metals that oxidize maintain their brightness at millimeter wavelengths even though they may appear dull to the naked eye.

The Millivision camera uses focal-plane-array technology, an imaging process that uses receiver elements positioned along the focal plane of the optical system. "Like a camera, the imaging lens transfers the electromagnetic image onto a plane where you would normally have a piece of film," says Huguenin. "We have an array of electronic sensors in place of the film."

In addition to a primary lens, optic filters, wave plates, and other elements, the Millivision imagers use a new class of optics employing arrays of wave processing MMIC (monolithic millimeter-wave integrated circuit) chips from HRL (Malibu, CA). The chips, which are made from indium phosphide, amplify the weak signals and perform the signal processing functions on the electromagnetic waves.

Huguenin explains that there is a trade-off with the new technology-cost. "The Millivision sensors are more expensive than the metal detectors they replace," he says. "To a certain extent, we've had to wait for development of lower cost chips." A key to making these systems affordable was going to the MMIC chips," he explains. "We've crossed a threshold where the cost is no longer prohibitive."

Two approaches to millimeter wave technology

PNNL Scanner Millivision Scanner
Technology Active millimeter wave Passive millimeter wave
Footprint 5x5x8 ft 3x5x8 ft
Operating Frequency 12-18 GHz 100 GHz
Image Data 3-D 2-D
Imaging Aperture 45 inchesx6.5 ft 18 in
Scan Time 1-2 sec 4 sec
Illumination Active, extremely low power Passive, no emissions
f number 1 1.1

Holographic radar using active millimeter waves

In contrast to passive millimeter waves, the millimeter wave holographic radar developed at Pacific Northwest National Lab (PNNL) uses active millimeter waves for detecting metal, plastics, and other objects. It essentially bounces waves off the object being scanned, then reads and images the reflected waves. "Think of the image processor as the lens," says Doug McMakin, a staff engineer with PNNL. "It focuses the millimeter waves and helps form the image," he says.

A vertical scanner and a 112-element horizontal linear array of antennae scans a person in one second. "Its movement is similar to the way a photocopy machine moves a scanner across the paper," he says. A high-speed computer processes the data obtained from the scan and reconstructs it into high-resolution images.

The radar array distributes the illuminating source from the transmitter over the length of the imaging aperture. At the same time, the array receives reflected millimeter wave signals from the person under surveillance. Unlike the passive millimeter wave system that operates at 100 GHz, the PNNL scanner operates between 12 and 18 GHz.

A high-speed parallel processor using digital signal processing (DSP) boards from Alacron (Nashua, NH) reconstructs the holographic data from the scanner into images. "The DSP boards are a key element in managing system costs," says McMakin. "These new DSP boards have the processing power that we need for high-speed imaging, without costing an arm and a leg," he says.

Detection methods at a glance

Metal detector Millimeter wave technology
Principle of operation Magnetic field
disturbance detected
Electromagnetic waves imaged
on screen
Screens people x x
Detects plastic weapons x
Detects explosives x
Detects narcotics x
Detects metal weapons x x
Multiple configurations
(e.g. booth, hand wand, etc.)
x x
Poses no known health threats x x
FAA endorsed x

The DSP boards are configured into parallel processing compute cells. The host computer system is an Intel-based MS Windows 95/98/NT Pentium II PCI platform. "Processing power translates into passenger throughput, says McMakin.

Current metal detectors process passengers through in approximately four to six seconds. Any new system needs to be at least this fast, if not faster. The cycle time per passenger for both systems described above is less than four seconds.

All approaches to weapons detection in airports must pass rigorous tests, according to FAA spokesperson Rebecca Trexler. "We have an integrated team of FAA officials, airline representatives, and security experts who determine what we'll purchase for airport security."

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