3 August 2005

Reflection sensor generates X-ray vision

A new sensor could detect concealed weapons or help pilots see better through rain and fog. But unlike X-ray machines or radar instruments, the sensor doesn’t have to generate a signal to detect objects; it can spot them based on how brightly they reflect the natural radiation around us every day.

There is always a certain amount of radiation, light, heat, and even microwaves, in the environment. Every object, the human body, a gun, or an asphalt runway, reflects this ambient radiation differently.

This reflection is like how glossy and satin-finish paints reflect light differently to the eye, said Paul Berger, professor of electrical and computer engineering and physics at Ohio State University and head of the team developing the patent pending sensor.

Once the sensor is further developed, it could scan people or luggage without subjecting them to X-rays or other radiation. If they embedded the sensor in the nose of an airplane, it might help pilots see a runway during bad weather.

The sensor isn’t the only ambient radiation sensor under development, but to date it is the only one compatible with silicon, a feature that makes it relatively inexpensive and easy to work with, Berger said.

The sensor grew out of Berger’s team’s invention of a device called a tunnel diode that transmits large amounts of electricity through silicon. He was reading about another team’s ambient radiation sensor when he realized their device worked like one of his diodes, only in reverse.

Diodes are one-way conductors that typically power amplifiers for devices such as stereo speakers. Berger’s diode is compatible with mainstream silicon, so computer chip makers could manufacture it cheaply and integrate it with existing technology easily.

The new sensor is essentially one of these tunnel diodes with a strong short circuit running backwards and very little tunneling current running forward.

Phillip E. Thompson of the Naval Research Laboratory prepared the films of layered semiconductor material, and the Ohio State team fabricated and tested the sensors.

The way engineers measure the effectiveness of such sensors is to draw a line graph charting the amount of current passing through them. Then they measure the curvature of the line at the point where the current is zero. A steep curve indicates a sensor is working well, so the higher the “curvature coefficient” is the better.

In the laboratory, prototypes of the sensor averaged a curvature coefficient of 31. While one other research team has produced a sensor with a coefficient of 39, that sensor is made of antimony, an exotic metal hard to work with and not directly compatible with the silicon circuit that surrounds the sensor element, Berger said.

“So our raw sensor performance isn’t quite as good, but our ultimate performance should be superior because you could integrate our device directly with any conventional microchip readout circuitry that you wanted to build,” he said.

The team making the antimonide sensor has succeeded in combining it with a camera system; the pictures look a lot like X-ray images, with bodies and clothing appearing as dim outlines and metal objects such as guns standing out in sharp relief.

However, the camera system has performance issues Berger thinks he can solve with his silicon-compatible design. Still, the image has inspired him to think about where his work could go in the future. Combat pilots, for instance, could potentially use this technology to stealthily identify other aircraft as friend or foe.

“If you got a fast enough response and a high-resolution image, I wonder if you might be able tell one kind of aircraft from another without revealing your location to the enemy,” he said.

For related information, go to www.isa.org/sensors.