Schlieren imaging is a 150+ year-old technique for visualizing density changes in air. The technique is routine in supersonic wind tunnel laboratories, where scientists and engineers use it to view shock waves, boundary layers, turbulence, and other aerodynamic phenomena. The technique, however, is not difficult to understand and a working setup can be created at home at low cost.
This kit aims to facilitate the setup and operation of a simple desktop-sized conventional “Z-type” schlieren setup, putting all of the critical components on a single rail system crafted from readily-available ½" EMT electrical conduit.
The rail-based assembly and provided printable component modules keep most of the items fairly close to their ideal alignment, facilitate small adjustments, and allows for the whole system to collapse nicely into a compact size that can be hung on the wall or stood up neatly in a corner.
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Here are a few images created using this kit with a simple LED flashlight and an iPhone camera:
A simple schematic of the Z-type schlieren system is shown here (credit: Wikipedia, CC-BY-SA 4.0):
A small point- or slit-source of light (here, from an LED flashlight) is located at the focal point of a parabolic mirror, illuminating it. The light that hits the mirror is then reflected in a “collimated” fashion. This means that the light rays are neither diverging or converging – they travel through the test area parallel to one another – it's a constant-diameter tube of light.
Opposite the first parabolic mirror on the other side of the collimated light path is a second, identical parabolic mirror. The light is reflected off this mirror and then focused at its focal point. What results here is an exact image of the light source, in focus and at 1:1 scale. At this point, approximately half of this light is then blocked by a “knife edge”, here a safety razor blade. Past the knife edge, a camera with an appropriate lens is positioned and focused to capture the image that remains.
The basic reason this system is sensitive to density variations along the collimated path of light is that as the density of air changes, so does its index of refraction; that is, disturbances in the flow distort the path of the light rays that pass through them. Because all light through the test area (in an undisturbed state) is focused onto the knife edge and half of it is removed from the image, some of the light that would have passed to the camera sensor is blocked – these areas show as dark on the image. Similarly, some light that would have been blocked is passed over the knife edge and onto the sensor, appearing as brighter-than-normal pixels. The height of the knife edge (= the "amount of cut-off") controls both the sensitivity of the system and the brightness of the image, and the orientation of the knife edge selects the directions of density gradient to which the system is sensitive.
For authoritative background on the topic, the reader should consult the work of Dr. Gary Settles, Professor Emeritus at Penn State, in particular his textbook [1] on schlieren and shadowgraphy and his 2018 journal article [2] on implementing these techniques using modern smartphone cameras. Veritasium also has an excellent video:
This kit is based around a particular mirror design common to entry-level DIY Newtonian telescope builds. Specifically, you will need two “first surface” spherical mirrors with a 114 mm diameter and 900 mm focal length. (Technically, it is not ideal that these mirrors are spherical and not parabolic in profile, but it is minor for an optic of this f/ number. The cost benefit to using this type of mirror is substantial (10x).)
QTY 2 x 114mm-diameter, 900mm focal length first-surface spherical mirrors
I purchased my mirrors from SurplusShed (model #T1663), a great vendor with excellent pricing (just $19 each). You can find similar-spec'd mirrors at similar prices from Amazon, AliExpress, etc.
For a light source, I used a compact LED flashlight from OLight, the S1R Baton II. Any similar LED flashlight can be made to fit in the provided holder or the LED can be supported in position otherwise. Note this particular light is tremendous overkill – you don't need a lot of brightness – even a single, simple 5V LED emitter can suffice.
The other items are trivial to source:
For this system, the knife-edge functionality can be provided either by an actual safety-razor blade or by a surrogate fully-printed part (for safety).
Imaging can be performed using a smartphone camera or projected directly onto a simple screen (white paper or posterboard). If you choose to use a smartphone camera, the following is recommended:
A print profile is provided with all recommended settings. Nothing fancy here, and no supports needed. PETG recommended.
Print all provided components
Keep the mirrors covered when not in use to prevent damage in the form of scratches, cracks, or dust. The provided mirror covers push onto the mirror cases and twist slightly to secure.
Use M5 SHCS to secure the mirror case to the tip-tilt bases. Between the mirror case and threaded insert, use either 2x M5 nuts or an appropriate spring such that turning the SHCS hex key precisely moves that corner of the mirror in and out. Ideally, the middle corner will be set to a fixed position and not used often – the opposite two will then control tip (vertical) and tilt (pan) of the mirror.
Carefully snap it into the pocket on the manipulator as shown.
With the components roughly positioned on the rail, position the light source emitter approximately 900 mm away from the face of the mirror it is facing.
Check the collimation again and iterate until the second mirror is fully illuminated with minimal spill.
Adjust the resting position of the knife edge to be at the same height off the table as your light source. This should expand the accordion of the compliant mechanism by 2-3 mm, which puts tension on the screw enabling adjustment in both directions.
Adjust the tip and tilt of the second mirror together with the knife-edge position. Your goal here is to arrive at an in-focus, 1:1-scale image of your light source in the plane of the knife edge, centered on the blade.
Finally, once you have an in-focus image of your LED light source on the knife-edge, reduce the light source down to a small point or slit. You can use the provided caps in the LED holder, or use foil with a pinhole or slit. The smaller the point-source of light, the better – if you can properly expose and see your image on the screen/camera.
The simplest way to perform a first test is to just then allow the image behind the knife edge to project onto a wall or piece of paper/posterboard. Adjust the knife-edge to taste. If your knife edge is properly located, the image should uniformly dim and brighten when adjusting the amount of cut-off.
If you then wish to capture images on your camera/smartphone, you'll need a relatively long-focal-length lens (5x+) and you'll need to position the front element just past the knife edge. Use manual focus and exposure (i.e., with a third-party app like Halide).
In the absence of a supersonic wind tunnel for visualizing shock waves, there are still plenty of things you can do to create fun density-gradients at home. Most of these are thermally-dominated.
Note that the strongest of these are easy to visualize – others are trickier and may require a well-aligned system with careful adjustment of the cut-off amount and orientation.
Have fun doing science! Be safe!
I'd love to hear your feedback on the design and see what you do with it. I will be revising this incrementally over the next couple of weeks:
[1] G. S. Settles. Schlieren and Shadowgraph Techniques. Springer Berlin Heidelberg, first edition, 2001.
[2] G. S. Settles. Smartphone Schlieren and Shadowgraph Imaging. Optics and Lasers in Engineering 104 (2018) 9-21.
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