The absorbed radiation allows the sensor to map the temperature profile of a body, similarly to a thermal infrared image, Helistö explained.
"The good point of terahertz is that it penetrates most clothing material," he said.
Since the device is also sensitive to the differing properties of materials, a metal object—even if it is the same temperature as the body—will show up in the image, Helistö added.
The sensor itself is a superconducting bolometer.
"A bolometer is a device that heats up when it absorbs power—in this case terahertz power—and the change in temperature is detected with a sensitive thermometer," he said.
The primary challenge is to keep the bolometer a frigid 5 Kelvin, which is -451° Fahrenheit (-268° Celsius).
The chilly temperature decreases interference from other heat sources and allows the sensor to detect minute temperature changes, making for more detailed images, according to Helistö.
The device maintains this temperature with a commercially available, low-temperature refrigerator known as a cryocooler.
Currently the sensor takes a few minutes to make a single image, which is too long for busy security checkpoints like airports, Helistö said.
He and colleagues at VTT and the National Institute of Standards and Technology in Boulder, Colorado, are working on a device that can make an image in one-thirtieth of a second. They anticipate completion within two years.
At that speed the sensor would be sufficient for security screening. It will also be able to image objects 100 to 330 feet (30 to 100 meters) away, making possible covert surveillance at places such as subway and train stations, Helistö said.
But Melissa Ngo doubts whether the superconducting bolometer is a security improvement over simple pat-down searches.
Ngo is director of the Identification and Surveillance Project with the Electronic Privacy Information Center in Washington, D.C., which has voiced privacy concerns related to security screening technologies.
If the technology works by its sensitivity to temperature, she asked, would it detect liquid explosives that people had held next to their bodies long enough to reach body temperature?
"If you lightly pat someone down—which is what they do now—you can find the bottled liquid next to a person's leg," she said.
"It doesn't have to be a very intrusive search in order to be a thorough search."
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