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# Seeing the invisible

Imaging changes in gas density

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This simple setup allows the users to visually capture the movement of different density gasses.

The image is created through careful lighting control and fine tuning. The basic idea is that parallel light passing through a lens will converge to an exact point. When a gas of differing density is introduced the gas acts as a lens which bends a parallel beam and any light that is bent will miss the point the of convergence and can be blocked, causing dark spots. This is used to visualized density cahnges.

• 1 × Wooden base A solid base is a good place to start
• 1 × Aluminum C channel
• 8 × Aluminum bars
• 3 × Aluminum angle
• 2 × Lenses
• 1
Step 1

1) Make the point source.

The easiest way of getting a point source is to use a clear LED. These are the ones where you can see the silicon die within the LED. To make a point source, simply remove the dome from the LED and try to remove as much plastic as possible with out damaging the LED. So I just has a yellow LED which I sanded down and polished by rubbing it on a piece of paper. It worked well enough.

• 2
Step 2

2) Acquire the lenses.

There are quite specific requirements for the lenses.
First find the focal length of the lens. This could be done a number of different ways. You could go outside and focus the light from the sun to a point (It will burn things), or focus light from a lamp until you can see the shape of the light source (eg. the shape of filament or fluorescent bulb).

Once you have the focal length, place your point source facing the lens at the focal point of the lens. The resultant beam should be the same size no matter how far away from the lens it is. It should also have uniform intensity. If anything is placed in the beam it should cast a sharp shadow of the same size as the object. Check this for both the lenses that you will be using.

• 3
Step 3

3) Making the pinhole aperture

So this could be done a number of ways.
Place aluminum foil on a hard surface and poke it with a needle.
I used a thicker material to make it easier to mount. To make an aperture in sheet steel, I placed a dent in it using a nail that has been ground to have cone tip. Then I sanded the dome formed on the opposite side until there was a hole.

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## Discussions

PointyOintment wrote 06/08/2014 at 01:52 point
Schlieren or shadowgraphy? From the description I'm thinking schlieren but I'm not sure.

Are you sure? yes | no

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