Schlieren photography is a technique for taking pictures of fluid flow. The basic setup consists of a spherical or parabolic mirror,
a point light source, and a sharp edge. The point light source is placed at a distance of twice the focal distance of the mirror,
and adjusted so it reflects light back to a focused point where a razor edge is placed. A camera is placed behind the razor, and adjusted
so the mirror fills the frame. See the image below. The technique is nicely described on
Wikipedia.
Optical layout of a single-mirror Schlieren system. Credit to Wikimedia Commons.
Camera and Mirror setup
I used a DSLR to take pictures and video. I only had a 105mm lens, but a 200mm would have worked better and required less cropping.
Exposure time was 1/250s, and aperture f6.3. Any cheap telephoto lens will work, as there are no real requirements for using a wide
aperture. Some form of optical zoom is required though, since the focus distance of these mirrors is typically 1 meter or more, and the mirrors themselves aren't very large. I shot the images in
a dark room, but it works equally well with the lights on.
I used a 3D printed stand to hold the mirror in place, see it on Thingiverse.
I chose to let the mirror stay stationary, and adjust the light source and razor instead. This simplified the system, compared
to others I have seen where the mirror is adjusted. The mirror I bought has these specs:
The point light source I used was an ultra-bright white 5mm LED, which had been shaved down to a flat head. I attempted to remove as
much plastic housing as possible without touching the die. Afterwards the LED was wrapped in tinfoil which will block out all light,
and finally a small pin prick was made right over the LED die. This works as an acceptable point light source, and is actually bright
enough for the camera. The LED itself is driven by a constant current source, simply so I didn't need to worry about temperature
and battery levels affecting the light intensity.
I needed to make a few attempts at covering LED in tin foil before not having any holes.
Once all the light was removed, I made a soft prick in the center of the LED with a needle. Couldn't see a hole, but it was very visible once
the LED was on. Constant current source should be able to keep the LED intensity within 10% over the full battery voltage range,
with a sharp drop once the battery is dead. Ensure the LED is sticking out of the housing, any reflections from the sides will
disturb the image.
Aligning the System
Set the light source and mirror two focal lengths apart. In a darkened room, use a piece of white paper to find where the reflected
pin point appears. Adjust the light source position, and the razor, until the reflected point of light just hits the razors edge
when perfectly focused down to a pin prick. I taped the razor to the light source housing, which made adjusting the position much
easier as I only needed to move the combined unit. Definitely use a tripod for this.
Once the light source and razor are in position, hang a piece of white paper over the camera lens, and use this to visualize where
the light is reflected back into the camera. Position the camera so the reflected light is reasonably center on the camera lens,
and then put the camera into live mode. The focal point of the camera should be set to have the mirror in focus. I'm not sure exactly
what the correct focus point is, but this works as a good starting point. Adjust the camera position forwards or backwards until the
shutter blinds are visible. Then move forward until they are gone. You should see the entire mirror in the frame. Fine adjustments
to how much of the reflected light hits the razors edge can be made to change the sensitivity or contrast of the setup.
[Left] Notice the two tripods used. [Right] The focused point of light is just visible on the razors edge.
Disclaimer:
I do not take responsibility for any injury, death, hurt ego, or other
forms of personal damage which may result from recreating these
experiments. Projects are merely presented as a source of inspiration,
and should only be conducted by responsible individuals, or under the
supervision of responsible individuals. It is your own life, so proceed
at your own risk! All projects are for noncommercial use only.
This work is licensed under a
Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License.
