Schlieren Optics – See small changes in the index of refraction in air | Hacker News Reader

Schlieren Optics – See small changes in the index of refraction in air

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131 pointsjetskindo28 comments

I've long had this pet theory that soaring birds have a fine vision that allows them to see thermals (warm ascending currents) directly, through changes in the air's refraction index. I've searched a bit, but no research seems to ever have considered the question. Just a wild theory, though.

Interesting. I don't think that would be possible - the technique shown here relies on a mirror to get interferometry between the two light paths. I can't see how interferometry would be available to soaring birds (between its two eyes, perhaps?). Although nature has come up with some very surprising and impressive inventions before, so I might be wrong.

I would guess they're just very good at spotting the conditions which lead to thermals.

EDIT: of course, you can directly see thermals via the shimmering / "mirage" effect. It's normally obscured by the background noise, but maybe soaring birds are attuned to that. That's much more plausible, but a different effect to that shown here.

Given that many soaring birds have fantastic eyes to find prey or carrion, they might be able to see it.

You're right of course, it can't rely on collimated light like the normal Schlieren system. But the background-oriented Schlieren technique, mentioned elsewhere in this thread, is more reasonable to look at for inspiration, and it provides pretty much the same end result.

I suppose it can be thought of as a differential filter: only minute changes are interpreted to create a representation of the flow of air.

Invented 1864! http://en.wikipedia.org/wiki/Schlieren_photography

And there's a variant in color: https://www.youtube.com/watch?v=0g4UBeaG5fs

There was a yellow version shown on Time Warp.

The NPR has a good explanation of how it works in their video:

https://www.youtube.com/watch?v=px3oVGXr4mo

That was a beautiful video, thanks. I think I'm going to make a handsome donation to NPR right now.

Purely out of curiosity, has anyone used this in conjunction with a linguistics study to visualise sounds made during speech? I did a quick literature search but couldn't find anything. Not sure whether if features would be too subtle to see anything interesting...

Edit: Spoke too soon. Did find one paper[1] (though I can't read it) which uses it to compare the production of 's' and 'z' sounds. Would love to know if there are any more papers though.

[1] http://dx.doi.org/10.1121/1.4784877

I helped get Doc Edgerton's Schlieren appartaus working again in a lab class in the mid-90s.

We tried to image sound waves, but the density gradient for sound is much less than that produced by changes in temperature. We attempted to make a resonant chamber and use a high-intensity ultrasonic source, and make a standing wave that we might capture photographically, and while we saw something once we could not produce it. It's likely that the ultrasonic source we were using was gradually degrading.

The research paper you link to almost surely uses the heat differences to see the jets of air coming out of the mouth, and not actual sound waves.

Thanks for the information, that's really interesting, and a shame it didn't work out! I did see that the sound of a clap had been imaged, but I imagine that's a somewhat extreme case.

> The research paper you link to almost surely uses the heat differences to see the jets of air coming out of the mouth, and not actual sound waves.

Regarding the paper, while I can't read it myself, I imagine they chose to look at fricatives precisely because they're the result of turbulent airflow which is probably ideal for Schlieren imaging rather than a different speech sound which would be more wave-like. If the image is the result of heat differences, I wonder if it could be improved further by changing ambient temperature or temperature inside the mouth, or alternatively if the subject could be asked to inhale sulphur hexafluoride first to increase the density differential? (Edit: Or, perhaps, introduce a 'uniform' (laminar) thermal source along the direction you're interested in, so that when a sound wave propagates, the resulting density differences would be much more pronounced? I'm not sure if that makes any sense, as a layperson this is pure speculation on my part...)

I believe this is an image from the paper: http://www.sciencephoto.com/media/89151/view

Thanks again for the info though, really fascinating. Hopefully we haven't reached the limits of this technique yet, and it can still be taken further.

I note that in that picture there appear to be wave fronts emanating from the person's mouth. It's not obvious but there are some circular artifacts that appear to be centered on the lips / teeth. Maybe they really did capture sound ?

Scientific American used to run a really cool "Amateur Scientist" column when I was a kid, edited by C.L. Stong. In the early 70s they ran a couple articles about people who'd built their own Schlieren optics. I was fascinated by them!

The articles appear to be pay-walled now (http://www.scientificamerican.com/article/the-amateur-scient... and http://www.scientificamerican.com/article/the-amateur-scient...).

It looks like the book that collected many of Stong's columns from the 50s and 60s predates those 2 articles (https://archive.org/details/TheAmateurScientist).

There's a CD-ROM available that supposedly contains the text & images for _all_ of the Scientific American "Amateur Scientist" from the '20s to the late '90s, which would presumably also have those articles: http://www.amazon.com/exec/obidos/ISBN%3D0970347626/scienceh...

And it's kindof accessible for hobbyists, here is the website of one individual who made his own setup: http://www.ian.org/Schlieren/

The most expensive component (except the camera) is the mirror. According to that website, it costs about $100 from an optics company.

You can also do it without any mirror, using Background Oriented Schlieren. Basically, you print a bunch of random dots on paper, and use a camera to see the subpixel shifts in the dots. ( see http://en.wikipedia.org/wiki/Synthetic_schlieren ). By cross-correlation, you get the change in refractive index/density of air.

One of the main limitations on a Schlieren system is that you can only image objects that are smaller than your mirror. That makes the BOS systems are pretty neat, because at least in theory, you can just print out large backgrounds to image large areas.

This paper is really nice http://www.ualberta.ca/~bsuther/papers/procsorrento0998/repr... on the synthetic method.

Or you can grind your own mirror, which is a fun project. The Foucault tester, used to see the Schlieren image is also used for that project. It is easy to see the bump created on the mirror due to its expansion after briefly touching it with your finger.

You can also do something related with materials that are not as common as they used to be but still available. If you take a piece of photo paper and put it in the background ( in the dark or course) and then shoot a strobe through the "disturbance" you'll get a shadow picture of e.g. A bullet and its shockwave (assuming the strobe is triggered at the right time). We did this sort of thing in Doc Edgerton's lab. For many years there was a Schliieren photo that Kim Vandiver made downstairs from the lab.

And this is the sonic boom visualized by Schlieren photography: http://youtu.be/lbomsOPSSII

I'm super-curious about how something similar might be happening in the Double-Slit Experiment.

This is basically the same effect you see with shimmering above a hot road surface.

Or the effect you can see after about second 30 in the following pretty[1] video of a plane passing in front of a mountain range (the narrow band of light behind it works well enough as a collimator).

https://www.youtube.com/watch?v=wOUuj1LQRyk

[1] it could use some image stabilisation though.

related at all to?

http://en.m.wikipedia.org/wiki/Adaptive_optics

Is a city-scale version of this possible?

What are those circle shapes?

I think they are aberrations in the mirror's glass. People used to use Schlieren optics-esque techniques to test glass for defects.

The HN title is incorrect. This device doesn't show movement, it shows difference of the index of refraction. From the video description:

> Demonstration of an optical technique that allows us to see small changes in the index of refraction in air. [...] Seen here are the heated gases from a candle flame and a hair dryer, helium gas, and sulfur hexafluoride gas.

Technically true. But the different refraction indices can be created by turbulent flow - which will nearly always occur with moving air.

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