Flow Visualization with Schlieren Photography

The word “Schlieren” is German, and translates roughly to “streaks”. What is streaky photography, and why might you want to use it in a project? And where did this funny term come from?

Think of the heat shimmer you can see on a hot day. From the ideal gas law, we know that hot air is less dense than cold air. Because of that density difference, it has a slightly lower refractive index. A light ray passing through a density gradient faces a gradient of refractive index, so is bent, hence the shimmer.

German lens-makers started talking about “Schelieren” sometime in the 19^th century, if not before. Put yourself in the shoes of an early lensmaker: you’ve spent countless hours laboriously grinding away at a glass blank until it achieves the perfect curvature. Washing it clean of grit, you hold it to the light and you see aberration — maybe spatial, maybe chromatic. Schliere is the least colourful word you might say, but a schliere is at fault. Any wonder lens makers started to develop techniques to detect the invisible flaws they called schlieren?

When we talk of schlieren imagery today, we generally aren’t talking about inspecting glass blanks. Most of the time, we’re talking about a family of fluid-visualization techniques. We owe that nomenclature to German physicist August Toepler, who applied these optical techniques to visualizing fluid flow in the middle of the 19^th century. There is now a whole family of schlieren imaging techniques, but at the core, they all rely on one simple fact: in a fluid like air, refractive index varies by density.

Toepler’s pioneering setup is the one we usually see in hacks nowadays. It is based on the Foucault Knife Edge Test for telescope mirrors. In Foucault’s test, a point source shines upon a concave mirror, and a razor blade is placed where the rays focus down to a point. The sensor, or Foucault’s eye, is behind the knife edge such that the returning light from the pinhole is interrupted. This has the effect of magnifying any flaws in the lens, because rays that deviate from the perfect return path will be blocked by the knife-edge and miss the eye.

Toepler’s photographic setup worked the same way, save for the replacement of the eye with a photographic camera, and the use of a known-good mirror. Any density changes in the air will refract the returning rays, and cause the characteristic light and dark patterns of a schlieren photograph. That’s the “classic” schlieren we’ve covered before, but it’s not the only game in town.

Fun Schlieren Tricks

For example, a small tweak that makes a big aesthetic difference is to replace the knife edge with a colour filter. The refracted rays then take on the colour of the filter. Indeed, with a couple of colour filters you can colour-code density variations: light that passes through high-density areas can be diverted through two different colored filters on either side, and the unbent rays can pass through a third. Not only is it very pretty, the human eye has an easier time picking up on variations in colour than value. Alternatively, the light from the point source can be passed through a prism. The linear spread of the frequencies from the prism has a similar effect to a line of colour filters: distortion gets color-coded.

A bigger tweak uses two convex mirrors, in two-mirror or Z-path schlieren. This has two main advantages: one, the parallel rays between the mirrors mean the test area can be behind glass, useful for keeping sensitive optics outside of a high-speed wind tunnel. (This is the technique NASA used to use.) Parallel rays also ensure that the shadow of both any objects and the fluid flow are no issue; having the light source off-centre in the classic schrilien can cause artifacts from shadows. Of course you pay for these advantages: literally, in the sense that you have to buy two mirrors, and figuratively in that alignment is twice as tricky. The same colour tricks work just as well, though, and was in often use at NASA.

There’s absolutely no reason that you could not substitute lenses for mirrors, in either the Z-path or classical version, and people have to good effect in both cases. Indeed, Robert Hooke’s first experiment involved visualizing the flow of air above a candle using a converging lens, which was optically equivalent to Toepler’s classic single-mirror setup. Generally speaking, mirrors are preferred for the same reason you never see an 8” refracting telescope at a star party: big mirrors are way easier to make than large lenses.

What if you want to visualize something that doesn’t fit in front of a mirror? There are actually several options. One is background-oriented schrilien, which we’ve covered here. With a known background, deviations from it can be extracted using digital signal processing techniques. We showed it working with a smart phone and a printed page, but you can use any non-uniform background. NASA uses the ground: by looking down, Airborn Background Oriented Schlieren (AirBOS)  can provide flow visualization of shockwaves and vortices around an airplane in flight.

In the days before we all had supercomputers in our pockets, large-scale flow-visualization was still possible; it just needed an optical trick. A pair of matching grids is needed: one before the lamp, creating a projection of light and dark, and a second one before the lens. Rays deflected by density variations will run into the camera grid. This was used to good effect by Gary S. Styles to visualize HVAC airflows in 1997

Which gets us to another application, separate from aerospace. Wind tunnel photos are very cool, but let’s be honest: most of us are not working on supersonic drones or rocket nozzles. Of course air flow does not have to be supersonic to create density variations; subsonic wind tunnels can be equipped with schlieren optics as well.

Or maybe you are more concerned with airflow around components? To ID a hotspot on a board, IR photography is much easier. On the other hand, if your hotspot is due to insufficient cooling rather than component failure? Schlieren imagery can help you visualize the flow of air around the board, letting you optimize the cooling paths.

That’s probably going to be easiest with the background-oriented version: you can just stick the background on one side of your project’s enclosure and go to work. I think that if any of you start using schlieren imaging in your projects, this might be the killer app that will inspire you to do so.

Another place we use air? In the maker space. I have yet to see someone use schlieren photography to tweak the cooling ducts on their 3D printer, but you certainly could. (It has been used to see shielding gasses in welding, for example.) For that matter, depending what you print, proper exhaust of the fumes is a major health concern. Those fumes will show up easily, given the temperature difference, and possibly even the chemical composition changing the density of the air.