Visualizing the changes in air density with the Schlieren effect

Shawn Knight

Posts: 12,297   +120
Staff member

Odds are, you’ve probably never heard of the Schlieren effect (I hadn’t until today). In layman’s terms, it’s simply the ability to see changes in air density, like when you turn on a hair dryer or open a can of soda (we know air is rapidly moving, we just can't see it). That’s probably not a very helpful description but you’ll see what I mean in the clip above from YouTube user brusspup.

Using a concave mirror, a light source, a razor blade and a camera, brusspup was able to visualize these invisible changes in air density. It almost looks like special effects or camera tricks but the user claims it is 100 percent science so I’ll take their word for it. All I know is that it’s incredibly impressive and I want to try it for myself.

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mbrowne5061

Posts: 1,480   +820
I used to work in my Uni's supersonic wind tunnel. Schlieren lighting was how you capture the super-sonic air flow in its various stages. By focusing the light into the point, and then placing the razor blade right at that focused point, you can 'split' the entire image, allowing only one half to continue on into the camera lens. The result is that all the light distortions are visible.
 

Adhmuz

Posts: 2,003   +791
It's been features on several episodes of Mythbusters over the years, 100% a real thing and not simply camera trickery.
 

stewi0001

Posts: 2,387   +1,879
TechSpot Elite
I'm still trying to figure out the purpose of the razor blade. All I can figure is that it is cutting down the amount of light coming to the lens.
 
D

davislane1

I'm still trying to figure out the purpose of the razor blade. All I can figure is that it is cutting down the amount of light coming to the lens.
It shaves down unwanted light to produce a silky smooth stream of visual distortions.
 
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Uncle Al

Posts: 7,078   +5,439
LOL ..... we were capturing it on still camera's back in the mid-70's and it was nearly as incredible as trying to pump our high speed photographs up with some of the first electronic flashes .....
 
R

Raoul Duke

So...why don't we just get a picture with a razor blade in the middle?
 

mbrowne5061

Posts: 1,480   +820
I'm still trying to figure out the purpose of the razor blade. All I can figure is that it is cutting down the amount of light coming to the lens.
By placing the razor blade right at the narrowest point of the beam, you 'block' half of the information contained in the shadows. This leaves the light distortions 'unbalanced'. Think of the razor blade here like a diode in an AC-DC transformer. Looking at a normal AC wave, with an AC meter, and you will see a steady voltage. Place a diode in the circuit, and look at it with an AC meter, and you'll see just half of the Vpp - giving you an idea about the waveform itself.
http://photron.com/wp-content/uploads/2014/04/schlieren1.jpg
 
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Uncle Al

Posts: 7,078   +5,439
By placing the razor blade right at the narrowest point of the beam, you 'block' half of the information contained in the shadows. This leaves the light distortions 'unbalanced'. Think of the razor blade here like a diode in an AC-DC transformer. Looking at a normal AC wave, with an AC meter, and you will see a steady voltage. Place a diode in the circuit, and look at it with an AC meter, and you'll see just half of the Vpp - giving you an idea about the waveform itself.
http://photron.com/wp-content/uploads/2014/04/schlieren1.jpg
Exactly right, good answer .... glad you got to him before me, your answer was much more clear and avoided a 60 minute physics lecture! ROFL