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What Is Schlieren Photography?
Schlieren photography is essentially a method of photographing the flow of fluids of varying densities. It's an old technique, developed all the way back in 1864 to study supersonic motion and the shock waves created by it, otherwise invisible to the human eye. Now, it is used not only for scientific purposes anywhere from engineering better airplanes to the thermodynamics of candles, but also for beautiful photography that anyone can immediately appreciate.
The physics behind how this works is a bit complicated in the details, but basically, as light travels through different densities, it refracts. This goes for air that is of different densities as well, which can happen either through heating (expansion and thus less dense) or cooling (contraction and thus more dense) or through the normal expansion and contraction of solid objects moving through air. This makes Schlieren photography a potent tool for studying fluid dynamics.
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How To Do Schlieren Photography
So, how do you practically take these sorts of photographs? Now, the set up is a bit more complicated than what you're used to, but it's well worth it:
You need a parabolic mirror that is large enough to reflect your subject matter back into your camera. Most people don't have human-sized parabolic mirrors in their home, so you might want to ask around your local physics departments if they have a set up you could use. For small demonstrations, many cosmetics mirror can do our purposes. Telescope mirrors are preferable, however, and can be purchased for around 50$ at a decent size online. Spherical is preferable, but those are hard to come by at friendly prices.
Now, every parabolic mirror has a “focal distance”, at which the light from it focuses: you'll want your camera twice that away from the mirror. This is exactly the same as that magic distance between a magnifying glass and whatever you're trying to magnify where everything snaps into focus.
Your next piece of equipment is a point light source. This is easy enough to make: take some sort of bulb, LED or otherwise, poke a hole in a piece of tinfoil or some other reflective material that won't light on fire, and cover it up! The brighter the light, the better: after all, all of your light will be coming out from this single point. Make sure you brush up on your usual low-lighting techniques, such knowing your aperture and ISO settings, though be mindful of your exposure as you'll need it to be short.
You have two options from here: you can use either the edge of a knife to force the light to diffract, or a special color filter. It's hard to get the knife edge set up to work, but basically the end of the knife edge needs to be at the focal point of the camera. As far as the special colour filter method is concerned, you need to have this pattern printed on a transparency and placed in front of the camera. Try both methods!
Now, it'll take a fair amount of adjusting to get the set up just right, so be patient. As far as subjects are concerned, nothing fancy is really necessary—a lit candle, a friend with a cold, anything that creates temperature or pressure differences can be used.
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Synthetic Schlieren: Doing It Digitally
In the digital age, there are also methods of creating Schlieren photography, well, digitally. This is done by placing the target fluid dynamics in front of some sort of evenly patterned backdrop. Because of those differences in how the light refracts, as the light bounces off the patterned backdrop and goes through the fluids and into the camera. Mind you, you need to same point light source as before, and it needs to be placed next to the camera.
However, these differences will be incredibly slight, possibly not even visible to the human eye. Here's where the software comes in: highly specialized, it picks up on the subtle differences and can perform calculations to not only bring out the contrast of the refractions of the image, but also do qualitative calculations on the density of the flow. Quite useful for science.
A bit heavy on the set up and software, so maybe not the most practical thing to do at home. Still, it's pretty cool what you can do with photography and a little software!
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Schlieren Photography Resources
For more information, Photojojo has an excellent, if somewhat interestingly phrased, guide to Schlieren photography for laymen. Here's the original DIY Schlieren photography article upon which it was based on.
The example photos used throughout this article came from the website of Gary Settles, who is in charge of Pennsylvania State's Gas Dynamics Laboratory. His textbook on Schlieren photography is another excellent resource, though a bit on the pricey side—check them out at a library near you if you want to know the technical details of all this.
The New York Times has an excellent slideshow of Schlieren photography as well, including several of the photos featured in this article.