If you’ve ever seen the wavering air above a hot road, or the heat shimmer over a hot kettle you’re already half-way to making a Schlieren image.
In physics you very rarely ever directly measure the thing you are interested in. To measure the spacing between atoms in a crystal structure we don’t use a ruler to determine the spacing, we bounce electrons or x-rays off of the atoms and look at the diffraction pattern. To measure forces, we don’t hook systems up to a spring scale, instead we stick a tiny piece of piezo-electric material in place and measure the voltage created by squeezing the piezo. The secret to being good at physics is learning how to sneak up on what you want to measure, but to get at it by measuring something else all together, something that allows you to deduce what it is you really want to know. I always imagine its like trying to see something clearly without looking directly at it, only using the corner of your eye to see. Schlieren imaging is another example of this.
Schlieren imaging is a tool for measuring changes in the density of composition of a fluid, but what we actually measure is a deflection of light. The heat shimmers you see in the hot air over a road or another hot object is observable when the light from the background behind the warm air is deflected, making the background appear to flutter like a curtain in the wind. The reason for the fluttering is that the hot air coming off of the road has a different density from the surrounding air, resulting in a different index of refraction. In order to see this effect the temperature differences need to be pretty great, and it would be nice to see more subtle variations.
What I’m going to do next is propose a series of “improvement” on a system for detecting these variations, pointing out the benefits and pitfalls, ultimately culminating in a setup for Schlieren imaging. Our starting point will be with the shimmering hot air over a candle flame.
To aid in seeing the shimmering we might consider putting a grid behind the candle so we can see the shimmering more easily. We set up a camera on the right side to record the image. The grid helps us see the wavering air, but it isn’t much better than the road on a hot afternoon.
Maybe we need more light, so we break out a lamp and set it near the camera pointing above the candle flame. What you notice is a slight shadow on the wall, fluttering and curling along with the heated air rising from the flame.
What this tells you is that, while the light doesn’t reflect back towards the camera off of the air, when the light passes through the air it gets bent. In other words the air acts like a weak lens. Maybe the thing to do is shine the light through the air towards the camera.
So we move the light to the other side of the candle, and we do kind of see some wavering, but the camera and our eyes are so blinded by the light we can’t make out any details.
Unfortunately the light still blinds the camera, but looking at the wall behind the camera, we can clearly see an image of hot air coming off of the candle in fairly sharp detail. Congratulations! We’ve just created a shadowgraph, which is the sister of Schlieren.
We can clearly see the column of hot air rising above the candle, and dark and light lines indicate regions where the density of the air is changing. But this isn’t enough for us. We want to take things one step further.
We get out something thin, like a razor blade, and place it right where the image of the light bulb forms. We position the blade so it just barely blocks out the light bulb, but any light bent by the hot air will just miss the blade and hit the camera. This….this is a Schlieren image. We can see much more subtle variations in the density of air as light and dark bands that slowly dance with the candle flame. Only the light bent around the blade by the air will actually be seen.
This is a bit of a simplified picture of what is going on, and in my next post I’ll go into a bit more detail of some of the subtleties. Future posts will deal with more complex aspects, as well as how to set up your own system for less than $100.