Smart Phone Schlieren Imaging System - Heat

Schlieren imaging visualizes some of the smallest changes in air density we interact with everyday, like heat, air jets, or even sound.

Air of different densities slightly bends light (refraction). A Schlieren setup converts these tiny deflections into visible brightness differences that we can capture either with our eyes or with our cameras!

Schlierens require only 5 things things: pin-hole light, a mirror, knife edge, a camera, and patience for adjustments. These systems are super precise and because of that it is important that we have methods to easily adjust the positions of each of our components.

Lighting:

1. Strong Flashlight and Tape
2. Floor Lamp with a lampshade that can be pointed upwards and cardboard/tape

Mirror:

1. The Best mirrors are going to be parabolic telescope mirrors. The bigger the better but for the cost of the project it is best to get a mirror around 110mm in diameter. I found mine on amazon here!

Tripod:

1. We will be primarily adjusting three things in this project, the mirror the camera and the razor edge. To hold the mirror without interfering with any of the light around it, it is best to use a tripod: found here! Depending on your set up you might also want to use a tripod to hold your camera.

Razors Edge:

1. Knife edge: using books and clay should hold the knife edge in place for the purposes of our imaging.
2. Razor: using cardboard and clay to hold our knife edge in place should be enough for the purposes of our imaging.

Camera:

1. It is recommended to use a second tripod to hold the camera/phone. However, you can also find a surface to hold your camera and adjust your razor and mirror to compensate.

1. Light Source → Parallel Rays of light

1. You start with a small light source, like a pinhole or the reflection from a tiny bright LED that is far away so we can approximate parallel rays of light.
2. The parabolic mirror collects that light and focuses it.

2. Why Parallel rays are important

1. The parallel beams of light that comes from our light source and prior to hitting our mirror is where all the magic happens.
2. Normally, the rays would go straight to the mirror unchanged.
3. But if the air has variations in refractive index (hot vs cold air), some rays get slightly bent up or down making these interesting density patterns where some areas get brighter and some darker.

3. Mirror

1. The mirror will capture all these light beams and focus them for us on our camera

4. Knife Edge

1. The knifes edge is an old photography trick that increases the contrast in an image and allows us to better see the contrast between higher and lower density areas in our air field.

This converts tiny angle deflections (from air density changes) into brightness variations you can see and record with your camera.

When choosing a location for set up. Always keep the light source as far away from your mirror as possible as the further away it is, the better the light approximates parallel rays. Look at the set up above and you should be good.

The light source needs to be two things, a pin hole light source (where it comes from a singular points) and bright enough to aluminate our image.

Option 1: Lamp & Lampshade

1. Make sure your lampshade can be directed towards your mirror
2. Once you have made sure this is safe, tape cardboard with a hole in it to the lampshade

Option 2: Flash light

1. Use a flashlight! I covered my flashlight in enough paper that you couldn't see light when you turned it on
2. Fix the paper to the flashlight with rubber bands
3. Use a compass or a thumbtack to make a hole for the light to come through

Use the adjustable clamp on the tripod to fix the mirror in place!

Remember, we need our Razor edge to be in between our camera and our mirror.

Option 1:

1. Razor edges can be approximated with knife edges. Just make sure you find the knife with the straightest and sharpest edge. The sharper the edge the better. Be careful but use clay to hold the knife edge up.
2. Adjust height using paper and books

option 2:

1. Put the knife through cardboard so you can adjust the left-right position of the knife instead of the up-down position of it.

I've set up a physical diagram with all the tools we will use. The idea is we keep the light and the mirror as far away as possible and we focus the light towards the knife and the camera.

In this step we need to find the focal point of the mirror. This will be the point where the light is made into the smallest point.

Use a piece of cardboard or a book to use as a gauge as to where the point is in space. Make sure your knifes edge gets placed near the position of this focal point.

Once we have found the focal point we will want to cover half of the focal point of the mirror with the Knife. It can cover it in the vertical or horizontal direction.

Once it is in the general area of where the focal point is, we turn off the lights in the room to better see where the light is hitting and how much of the focal point we are covering.

In the video all of the focus is on the knife, that is primarily for demonstration purposes. The idea is move the knife a few millimeters to the right to only cover half of the light's focal point.

Place the camera behind the focal point and zoom in on the mirror. The mirror will look dark until it is really bright. All I can say is when you finally see the light you will know.

You did it, do some cool experiments! I even point the heat off my own palm!