Photo Credit: © Gary S. Settles and Jason Listak / Photo Researchers, Inc. Dr. Gary Settles shines a light on schlieren photography. An electric fan blows air from left to right. This photo was taken with a 1-microsecond exposure in order to freeze the air motion.

A cough. The warmth of a candle. The sweet scent of a flower, sniffed by a dog. We all know they're carried by air and wind—we just can't see what they look like.

But Gary Settles sees. Dr. Settles, distinguished professor of mechanical engineering at Penn State University, has spent a lifetime working in the fields of gas dynamics and flow visualization. Intimidating stuff. Yet the images Dr. Settles produces can be amazing, often colorful photos of phenomena from blow-dryers to explosions, showing details of airflows and shockwaves that everyone knows are there, but have never seen before.

"It's quite exciting," says Dr. Settles.

Sometimes his images reveal information never known before. Take the study of "canine olfaction" (how dogs sniff). Until Dr. Settles and his crew turned their magical gear on a sniffing canine, no one knew that dogs smell partly by blowing, sending out jets of air from their nostrils to disturb the environment and turn up more odors.

To get the images, Dr. Settles uses techniques that are quite old—first written about in 1665. One is the "shadowgraph." It's an effect visible if you put a candle in front of a wall in direct sunlight; you can see some of the turbulence surrounding the candle in its shadow.

A more advanced technique is schlieren photography (from the German for "streaks"). Schlieren photography requires a focused beam of light to shine on a transparent phenomenon, such as air surrounding a candle. The turbulence deflects the light in various directions. Then, on the other side of the subject from that light, a lens focuses its image.

The trick comes at the focal point of the lens, where a straight edge is placed to cut off half the light. Just as closing the aperture ring in a conventional lens doesn't make an image smaller but darkens it, the straight edge darkens the image. It also cuts off some of the deflected light rays in a way that shows different densities in the transparent subject (phenomenon).

That's the concept. In the hands of Dr. Settles and other serious practitioners it's evolved greatly. Take the Penn State Full-Scale Schlieren System, developed after somebody asked Dr. Settles if he could create an image of the heat from a full-sized tractor. He said no, but kept thinking about the problem and eventually took over a small warehouse on the Penn State campus and blacked it out, turning it into a huge schlieren camera studio.

Photo Credit: © Gary S. Settles / Photo Researchers, Inc. The air expelled by whistling forms a turbulent jet that penetrates the surrounding still air.

Along one interior wall of the building is a 16 x 18-foot screen, covered with black-and-white stripes just two-tenths of an inch wide. The white is actually the sort of super-reflective material used for license plates and street signs. At the opposite end of the warehouse is an 8 x 10 view camera with a 480mm f/8.4 Schneider lens—the biggest large-format lens the team could lay its hands on. And then the key: Inside the view camera, just in front of the focal plane, is a negative of that big striped screen at the other end of the warehouse. The stripes on the screen and the stripes on the negative repeat the schlieren knife-edge effect over and over.

Altogether the system takes the biggest schlieren photos ever seen. Dr. Settles can include people (or dogs) in his images. And he's done fascinating studies of firearms showing both the shockwave from a speeding bullet and the blasts (there can be one or two) emanating from the gun that fired it.

He and his students use smaller systems, too. He's a fan of the Nikon D90 and its digital video capabilities. Some of the systems are portable, which they take to places such as the Army's Aberdeen Proving Grounds in Maryland, for some of the explosives studies. "Certainly the graduate students like to blow things up," Dr. Settles concedes.

It's entertaining, but also extremely useful. Looking at heat convection from the human body, for instance, could have potential applications in medical imaging or security screening. In fact much of the recent work seems to involve "homeland security" issues. Take the dog's sniffing—the lab is studying how canines smell as a way to improve sensors used to detect bioterrorism or explosives.

The lab's high-speed imaging of the shockwaves from explosions also has security applications. When Pan Am Flight 103 crashed in 1988 in Lockerbie, Scotland, investigators examining the wreck thought they saw evidence of multiple blasts at locations throughout the plane. Later they realized shockwaves had traveled the fuselage and caused multiple blowouts. So Dr. Settles has used high-speed schlieren photography to examine shockwaves in planes, the sort of shockwaves that might be caused by a bomb.

Decades after he began imaging shockwaves and turbulence, new applications for his work keep turning up, Dr. Settles says. And the images are created for science, not art, he emphasizes. But in his spare time he's an oil painter, "so I have an eye for a beautiful image."

So if someone, say a commercial photographer hoping to score a client like Vicks VapoRub, wants to try this, where should he or she start? "You can't buy any commercial equipment to do this, so everyone builds their own," says Dr. Settles. He suggests starting with Schlieren and Shadowgraph Techniques: Visualizing Phenomena in Transparent Media. Yes, he wrote the book.

If you decide to give it a try and happen to get stuck, Dr. Settles says, "I've become the world's impresario for schlieren photography," explaining that he doesn't mind getting e-mails with intelligent questions from people interested in the techniques.