

Schlieren Optics – See small changes in the index of refraction in air - jetskindo
http://www.youtube.com/watch?v=mLp_rSBzteI

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pierrec
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.

~~~
rtb
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.

~~~
pierrec
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.

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nsajko
Invented 1864!
[http://en.wikipedia.org/wiki/Schlieren_photography](http://en.wikipedia.org/wiki/Schlieren_photography)

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

~~~
kr0
There was a yellow version shown on Time Warp.

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sah88
The NPR has a good explanation of how it works in their video:

[https://www.youtube.com/watch?v=px3oVGXr4mo](https://www.youtube.com/watch?v=px3oVGXr4mo)

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

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Osmium
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](http://dx.doi.org/10.1121/1.4784877)

~~~
RobGR
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.

~~~
Osmium
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](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.

~~~
RobGR
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
?

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jleader
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...](http://www.scientificamerican.com/article/the-amateur-
scientist-1971-05/) and [http://www.scientificamerican.com/article/the-
amateur-scient...](http://www.scientificamerican.com/article/the-amateur-
scientist-1974-08/)).

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](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...](http://www.amazon.com/exec/obidos/ISBN%3D0970347626/sciencehobbyist/)

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

~~~
daniel_reetz
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](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.

~~~
deutronium
This paper is really nice
[http://www.ualberta.ca/~bsuther/papers/procsorrento0998/repr...](http://www.ualberta.ca/~bsuther/papers/procsorrento0998/reprint_style.pdf)
on the synthetic method.

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degio
And this is the sonic boom visualized by Schlieren photography:
[http://youtu.be/lbomsOPSSII](http://youtu.be/lbomsOPSSII)

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ajcarpy2005
I'm super-curious about how something similar might be happening in the
Double-Slit Experiment.

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

~~~
pflanze
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](https://www.youtube.com/watch?v=wOUuj1LQRyk)

[1] it could use some image stabilisation though.

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leeoniya
related at all to?

[http://en.m.wikipedia.org/wiki/Adaptive_optics](http://en.m.wikipedia.org/wiki/Adaptive_optics)

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nodata
Is a city-scale version of this possible?

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miningold
What are those circle shapes?

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

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gus_massa
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._

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

