New-wave detectors

By: 
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

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