Tuesday, October 11, 2016

Schlieren imaging system

Hello and welcome to another blog post. And it is not going to be neither about genetic algorithms or programming nor about space related stuff. Today's blog post is going to be about a nice physics phenomenon called schlieren effect and it's application for visualising air flows. There is going to be a lot of pictures and if you get to the end, a video is waiting for you. Pleasant reading!

What is the schlieren effect?

The word "schlieren" comes from the German language and is, in fact, a plural of a word "schliere" which means "streak", so schlieren means streaks. And streaks are what all this is about, but let's call it schlieren. The schlieren in question are schlieren in gas. For example, when there is a heat source, the air around this source heatens up which makes it less dense which means that it also refracts light differently than the surrounding cold air. That leads to schlieren produced by light passing thourgh this non-homogeneous environment. You can see this effect every summer - look over a roof of a car that has been standing in sunlight for some time. You are going to see the image from behind the car subtly shaking as the hot air rises from the roof.

What is the schlieren photography, how does it work?

Except for such extreme cases like hot air rising from the black car in a summer day, the schlieren are often next to invisible for naked eye. For example, when you light a candle, you can't see much more than the flame itself. Another example might be an air stream from a haidryer. However, with a proper schlieren photography setup, you can visualise these subtle differences.

There are several ways of visualising the tiny differences in air (gas) density. Here I'm going to focus on the probably simplest and cheapest one which is the single mirror setup. This setup requires three key components:

  • a point light source,
  • a concave spherical mirror (parabolic can also work) and
  • a sharp edge.
The mirror must have a focal length (half its curvature radius in case of spherical mirror) in the order of meter(s).

Now, the best way to illustrate how does it actually work is to show it with a picture:

As you can see, the point light source is placed at double the focal length from the mirror and a little bit off-axis. Double the focal length is the radius of the mirror's curvature. If you put something in the center of a reflective sphere, its image gets reflected back to itself. In this case the point gets reflected to the opposite side of the optical axis back to a point (technically, because it is off-axis, it does not focus exactly into a point but the difference is so small that it can be ignored). The sharp edge, e.g. a razor's edge, is placed so that it goes through exactly the focused point of light. A camera is placed behind the edge so that it looks straight into the mirror through the focused point at the edge. The testing area is in front of the mirror.

Normally, when there is no disturbance in the path of the light, it is just focused back to a point and the camera captures a uniform brightness across the mirror. However, when there is even a small disturbance in the air (gas) density, it very slightly bends the light. It gets reflected from the mirror and the slight disturbance gets magnified as it travels towards the focus point. Since the light is diverted from its original path it's going to be out of focus. Some of this distubed light will get blocked by the razor, causing a dark spot in the image, and the other part will "dodge" the razor, causing a bright spot in the image.

When everything is set up and you light a candle in front of the mirror, it might look like below. This is the very first image I captured on my schlieren imaging setup:

First light #schlieren

Fotka zveřejněná uživatelem Jan Žegklitz (@gandalvv),

My schlieren setup

My schlieren setup and the making of it is heavily inspired by this great guide, so everything that is said there more or less applies to my setup as well.

The mirror

The most expensive and the most important component is the mirror. I have a primary telescope mirror, spherical, 160 mm in diameter and with a focal length of 1400 mm. That means that the light source and the edge is 2800 mm from the mirror. I got my mirror at eBay from optics21406 for 69.99 $. I can say that the mirror is great, very precise.

The mirror looks like this:

New toy has arrived. 160 mm diameter, 1400 mm focal length, spherical.

Fotka zveřejněná uživatelem Jan Žegklitz (@gandalvv),

It's a nice and robust piece of glass. It weighs about a kilogram! Quite a lot for such, one would say, a small mirror :).

Regarding the mirror, if you want to build your own schlieren system, forget about using your magnifying bathroom mirror. These are VERY imprecise. You need to get a telescope grade mirror. The focal length must not be too short. Otherwise the light might not get enough space to be diverted enough to be blocked by the razor. Also, the size matters. Much. The bigger the mirror, the bigger the testing area is going to be. So forget about laser optics - these are very small (since laser beams tend to be very narrow).

The mirror mount is very homemade :). Sticking to the guide I mentioned above, it's made of cardboard, specifically from the box the mirror arrived in:

Homemade mirror mount for #schlieren. Made from the box the mirror arrived in.

Fotka zveřejněná uživatelem Jan Žegklitz (@gandalvv),

The light source and the edge

I use a super-bright white LED (3.4 V, 20 mA, I forgot the luminosity), shining though a piece of aluminium foil with a small hole in it. I power it by a 9V battery through a 270Ω resistor. The edge is just a simple razor edge. I have it all "mounted" and taped on a piece of polystyrene which itself is mounted (taped) on a tripod. It looks like this:

#schlieren light source. 3.4 V, 20 mA super bright LED.

Fotka zveřejněná uživatelem Jan Žegklitz (@gandalvv),

To be honest, this image was taken before I set the camera up. After I had set it up, I had to cover the LED from behind so that it hasn't produced parasite light.

The result

Finally, I recorded a video of some stuff in front of the mirror. To be specific, you will see a candle being lit up and moved around, a hairdryer blowing on its own and also on an obstacle (my hand), and also the famous chemical reaction of baking soda and vinegar, producing some carbon dioxide (which is heavier than air). Enjoy!

What can be improved

My current setup is far from perfect and there are some things that can certainly be improved.

The light source

The super-bright LED is very fine, but the other stuff is not. I think I could make the hole in the aluminium foil even smaller so I would get better resolution of the disturbances. Also, as you saw it in the photo above, the backwards reflection from the aluminium foil shined a lot into my camera's lens and I had to additionally cover it with a spare lens cover. The best would be to just make some nice box with a switch, proper place for battery, a tripod thread for secure mounting and maybe some additional screws for precise alignment.

The mirror

The mirror is amazing! I could, of course, use bigger but that would be at a completely different budget level... But back to the troubles. It is terribly easy to get it dirty and it was. For telescope usage it is not a big deal as there the whole mirror makes up the whole image "collectively", so even with quite a significant amount of dust (judging by eye) it can still produce crystal clear images. Here, however, every spot on the mirror makes its own part of the image. And cleaning a telescope mirror is not an easy task if you really don't want to damage the protective and reflective coating. I tried somewhat and kind of failed. Luckily, it was the good type of failure - I failed at removing the dirt, not scratching the mirror.

The razor

In the video you could have seen two phenomena. The first one is that when the hairdryer was blowing horizontally it was much less visible than when it was blowing vertically. This has to do with the orientation of the razor. If the razor was horizontally aligned it would have been vice versa. The second one is that sometimes, e.g. when I put something in front of the mirror and it shaked, some of the stuff in the mirror that seemed like dirt shaked too. That means that it was not a dirt on the mirror but some other abberation somewhere else. I suspect that it could be the razor.

Another thing I would like to try is to replace the opaque razor with some color filter. The effect would then be colored instead of black and white. It could be quite nice :). Another idea that occurred to me was to replace the razor with yet another pinhole (about the same size as the one in front of the LED) or with two razors perpendicular so that any disturbancea in all/both directions would be pronounced. But I'm not very sure about this.

Other stuff

As you could have seen, the colors in the video are wildly red. That's because of me screwing up the white balance. I also had a lot of troubles with stability of the setup as I have set it up on carpets so the razor was sliding to one side all the time. Also, my tripods are far from ideal. The one with the light source is no big deal but my camera tripod is a mess. It's very old, some pieces are broken and it's not very stiff, which is kind of a problem when you need to align the camera precisely :). Some intermediate piece between the tripod and the camera, with extra collimation screws for precise alignment would be awesome but that is a pie in the sky for now.

I hope you enjoyed this first experiment. I will do more of them and better and I will report about it here. If you have any comments or questions, feel free to leave them below.

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