"The schlieren optical system was invented in the 17th century and then forgotten for two hundred years. A classic example of an idea before its time," says Gary Settles, a professor of mechanical engineering at Penn State.
He draws a stick figure, with steam rising off of it, in front of a mirror; a light source pointing at the mirror; and a camera next to the light source. The schleiren system, he explains, lets us see changes of density in the air (schlieren, in German, means streaks'). "The irregularities in the air cause the light to bend," he explains, drawing the rays out from the light source so that they hit the steam. "With the right lens, an image of proper size can be projected onto the 35-millimeter film in the camera."
Settles has spent the last 18 years developing large-scale schlieren cameras and using them to photograph heat currents, spreading gases, and biological processes such as a breath or a cough. In 1997, he developed the world's largest: At 7 by 9 feet, it is ten times larger than any other schlieren system. Earlier, he had been limited by the size of the mirror; for his 7-by-9 system he used retroreflective material, the same shiny material on highway signs. With this new system, Settles began photographing just about anything that could fit into his lab: automobiles, motorcycles, gas grills, refrigerators, a candlelit dinner—many of them research projects not traditionally associated with his field of gas dynamics.
"I take the advice of Mike Wallace of 60 Minutes," Settles says. "I read a lot of different journals outside my area of expertise to get ideas about problems in other areas, and I look for ways that my work can help solve these problems. Then I create a list of target areas for research opportunities and select from those."
Currently, he is working with the Federal Aviation Administration (FAA) to improve airplane security through advanced bomb detection equipment. Using schlieren imaging, Settles had observed the "human thermal plume," a layer of warm air that rises naturally from a person's body and moves in a constant upward flow off the head and shoulders, rising up to 6 feet or more above the head. The air contains microscopic flakes of skin and other particles, and small amounts of the materials the person has been in contact with. Settles has designed a portal to trap these microscopic particles and test them for traces of explosives. When a person steps into the portal, a blower pulls the thermal plume up to be captured in a special trap that collects and analyzes the biological particles.
Settles's portal system is both more sensitive and faster than alternatives such as passenger profiling, which was signed into law by President Clinton in 1996. Profiling determines how closely each passenger's profile matches that of a terrorist. People of certain ethnic backgrounds, traveling to certain destinations, traveling alone, and those paying for their ticket in cash are subject to more extensive questioning and baggage searches. A profiling system is part of the extensive security system of El Al Airlines, Israel's national carrier. The airline's security techniques are extremely expensive and inconvenient, but they are generally acknowledged to result in safer air travel. However, critics say the method violates people's civil rights, institutionalizes racism, and encourages stereotyping. But portal screening of explosive traces puts all passengers on an equal footing.
Settles's portal can be combined with the standard metal detectors, allowing airport personnel to check passengers for weapons at the same time they are being checked for explosives, thus keeping the process relatively simple. It is sensitive enough to detect faint traces of explosives on the human body. Settles explains, "Take a polished metal disc, put the tiniest drop of a solution of explosive, like RDX, the main component of plastic explosives—we're talking a nanogram maybe—and let that dry. Then you rub it on your shirt, walk through, and bang, a detection alarm goes off: It's very, very sensitive." The portal also has the potential to test for drugs, which could significantly reduce the amounts smuggled through U.S. airports.
Not everyone is enthusiastic about the potential of Settles's portal. Critics believe that testing a person's plume is an invasion of privacy. "Some people are calling it Big Brother, but I believe for the most part people want to feel safe when they board an airplane, and they want to know they are protected from terrorism."
Settles's portal will also be able to detect chemical and biological agents, which the government believes will be the next wave in terrorism. In 1995 Japan experienced chemical terrorism in the Tokyo subway when a cult group released the nerve gas Sarin in commuter trains on three different subway lines. Sarin, colorless, odorless, and highly lethal, was concealed in lunch boxes and soft-drink containers and placed on train floors. Terrorists punctured the containers with umbrellas before leaving the trains. The incident was timed to coincide with rush hour, when trains were packed with commuters. Over 5,500 people were injured in the attack. Settles's portal could be a valuable tool in detecting chemical agents like Sarin.
Settles believes his portal technology will eventually be used in fields beyond security, for instance, to screen for disease. "The medical field is looking for non-invasive ways of testing as an alternative to having blood drawn." Signs of diabetes, gangrene, some skin disorders, tuberculosis, some cancers, and many other diseases appear in a person's thermal plume, as does DNA. "We're releasing skin all the time, so it's not like being requested to give a DNA sample. You can't keep your skin from coming off. And it is possible to sample DNA from the airborne skin flakes. I think that could end up being a valuable instrument for medical diagnosis."
Gary Settles, Ph. D., is professor of mechanical engineering and director of the gas dynamics laboratory, College of Engineering, 301D Reber Bldg., University Park, PA 16802; 814-863-1504; gss2@psu.edu. His prototype portal is awaiting approval from the FAA; a commercial portal, based on a patent licensing agreement with Penn State, has just been released by Ion Track Instruments of Wilmington, MA. See other reports on Settles's work in Research/Penn State, March 1984, May 1997, and September 1999, at www.rps.psu.edu.