Seeing the Invisible

"The whole idea is to see things that are invisible, things that happen in the air that we look right through."

Grilling as seen in a schliren camera

Ten years ago, Penn State mechanical engineer Gary Settles set up a 40-inch-diameter mirror and started seeing – and catching on film – such invisibilities as breath, heat, a cough, a bullet's wake, propane leaking from a hose, music. He's since expanded his horizons to a nine-by-seven-foot frame, making visible the cascade of cold air falling from an open refrigerator, the eruption of heat above a barbeque grill or, a set of daintier plumes, a candlelit dinner. He can see why the ventilator over a restaurant's range is inefficient – or worse, why it suffocates the cook. He can see the turbulent air above a propane torch and the volatiles leaking out of a storage drum. He can see the warmth rising from the seat just vacated by his research assistant, J. D. Miller.

"Now, tell me when that seat is warm, J. D.," he calls.

Miller is sitting in profile on a metal stool atop a five-foot-high scaffold in the middle of an almost empty warehouse.

A stack of wooden crates with curious markings ("Kudu," says one, and "Water Buck") fill one corner; these belong to the Horticulture Department, which rents Settles the space. To Miller's right is a 16-by-18-foot panel of retroreflective material (the shiny stuff highway signs are made of) striped at precisely 0.2-inch intervals with black paint. To Miller's left, beyond two spotlights, Settles stands on another scaffold, surrounded by black-draped boxes like a Victorian-era photographer with a bevy of cameras. Among those boxes is a TV monitor.

"It should be hot enough now," says Miller. He stands up.

On the TV screen his image looks real as life, yet surrounded by tendrils and plumes that twist and mingle as he turns around. The empty seat seems to smoke.

"If you want to look at really big stuff like an automobile," Settles says, "you can't do it in a 40-inch-diameter mirror. This whole room is a camera. We can see all sorts of equipment that involves air flow, gas flow, heat transfer. And we can look at the full-size thing." The only requirements for this "Full-Scale Schlieren System" (schlieren is German for "streaks") is that the car or kitchen or whatever is to be imaged fit through the warehouse doors and sit on the scaffold.

"Here's the simple principle of how this works," says Settles, who made his first schlieren camera at age 14 to see why his balsa airplanes refused to fly. "All you need to know is how a camera works." A lens produces an image of the striped reflective wall, or source grid, on a photographic film. This film is developed, and the eight-by-ten-inch negative is placed in front of the lens so that the dark lines of the negative, or cut-off grid, match up with the light lines of the source grid. "So no light gets through," Settles explains. "Now, the source grid is quite large and the lens is quite small. Any disturbance causes light rays to bend and lets the light through. So if you put a person in the middle, that person gets imaged. You don't see the image on the negative, you see it behind the negative. What you're seeing is what gets past the negative – if the grids were completely aligned, you'd get a uniform gray." A TV camera focused on a ground-glass screen captures the light that sneaks past the cut-off grid and images the cause of the disturbance: the person's heat plume.

"This system's not as sensitive as the one you reported on ten years ago," Settles says, adjusting the cut-off grid to sharpen the image. "We've compromised sensitivity for size." The 40-inch-diameter mirror system could capture the heat plume of a small girl (it looked like rain-bows flowing from her fingers as she twirled a pirouette), but could see only the head and shoulders of a full-grown person. "To get an idea where size is important–" He turns to Miller, still on the scaffold. "Go ahead and light the torch and sit it down on the floor so we get a nice plume off it."

In real life, the propane torch is a blaze topped by a blur; on the TV screen, it's a bright spot below a huge coiling cloud.

"You can see this stuff mixing with the surrounding air," Settles says. "At six to seven feet up it cools off enough that you can't see it any more. Previously, to visualize things like this you had to either build a model or look at a small piece of the problem or not do it at all."

Kitchen ventilation was the first issue Settles and Miller addressed with their new full-scale system. Onto their scaffold went a restaurant kitchen range complete with hood. Miller (in chef's hat and apron), turned it on and the video camera started rolling. "Kitchen ventilation is a very big energy conservation issue," Settles explains. "With the ventilating fan, you're dumping cooking effluents" – smoke, and worse – "but you're also throwing away conditioned air, either heated or cooled. A good ventilation system makes a compromise between dumping too much and not enough. You want the effluents to be almost falling out of the hood. You don't want to be drawing too much air out of the room."

On the video you can see the air drifting across the room toward the fan, actually see it being sucked out of the room. Then Miller adjusts the fan. The swirls dip and realign and their direction changes. Says Miller, "Not only is the heat from the stove now going out into kitchen, the smoke is in the cook's face."

Modelling such an air flow, says Settles, "is really complicated. It's very difficult to scale down. In textbooks you'll see drawings of things like this. Some of them come very close. Some of them are from the wrong planet.

"With this system, we can actually see that balance point" – between drawing out too much room air and getting a smoky kitchen. "We're now talking to the ventilation hood maker – as you can see, the position of the secondary vent on the hood was a mistake."

Settles hopes to apply this same type of seeing-is-understanding to improving the design of industrial clean rooms, hospital operating rooms, gas stations ("as you pump gas, the fumes roll right out at you"), supermarket refrigerator cases, cars and motorcycles, even houses:

"The EPA just did a study," Settles explains, "in which they put monitors in rooms and on people to find out what pollutants came into a house." The results surprised them. "The personal monitors on the occupants showed a lot more outdoor pollution. The stand-alone monitors didn't pick it up." Rationalizing their results, the EPA scientists called it the Pig-Pen effect, after the character in the Peanuts comic strip who brings a cloud of dirt with him wherever he goes. "But that isn't it," Settles says. The dirt's not on the people, it's in the carpets and on the floor. When a person walks by and dislodges it, he explains, "It gets entrained in that person's thermal plume and rises up along the body and gets into their breathing zone. The stand-alone monitors don't have a heat plume, so they didn't pick up the particulates.

"The advantage to us is that we're looking at the body's actual thermal flow. Few people have ever seen it before."

Gary S. 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; J.D. Miller is senior research technologist in the gas dynamics lab. The researchers thank the Penn State Horticulture Department for providing the building space that made this work possible. Funding sources include the Electric Power Research Institute (EPRI) and the 3M Corporation; the researchers credit Jeff Lamens of D. W. Miller Industries, Huntingdon, PA, with developing the grid silkscreening technique. Settles's early work was reported in Research/Penn State in March 1984.

Last Updated May 01, 1997