PilakSliurin

I need to utilize a large diameter mirror to allow for a greater sized test (imaging) area. My project also requires that I select a mirror with a suitable focal length. The longer the focal length, the greater the schlieren effect becomes visible (refractive index becomes more obvious/visible due to distance light has to travel through air from light source to mirror and back to blade/camera). However, longer focal lengths require larger test areas. Given that this project requires field set up and imaging in challenging conditions, I had to balance desired effect intensity with the need to manage space requirements. Also of significance when selecting the appropriate mirror was price (these mirrors are expensive!). 

For these and related reasons, I opted to start with a single 12.5” diameter precision parabolic mirror with a focal length of 100”.  

My initial plan involved the home production of a DIY adjustable mount for the mirror to allow for adjustments (pointing light back to a small target point). Despite not immediately having a suitable mount for field use, I was able to conduct indoor testing utilizing various support and pointing aids fashioned from material on hand. I quickly realized that the heavy, expensive and fragile mirror requires a better mounting and pointing solution for use beyond ideal indoor conditions. Demanding set up specifications, stability and fine tuning capabilities meant that a DIY solution was beyond my immediate reach for this purpose.  

Commercially available kinematic (adjustable) mirror mounts are very expensive, but extremely useful in such applications. I have been able to facilitate use of a suitable mount, with support from the Nunavut Research Institute. This specialty kinematic mount provides a solid base/foundation, a safe mirror mounting solution sized for my mirror, with capability to fine tune mirror pointing.

I conducted research and testing on a number of point light sources for use in this project. I require a portable, high intensity, easy to power light source with a clean beam. 

Initial testing was met with challenges such as distorted light (flashlight reflectors projecting shadows), difficulty in obtaining a small/tight clean beam at 2x focal length (200”/16.6’), wavelengths of certain lights that affected image banding in camera, etc. Following extensive testing with different light sources and additional research, I was able to source a portable high intensity flashlight which appears to meet needs and performs satisfactorily. 

Though I have and use others, my main light source for schlieren imaging is a Coast G90 LED flashlight.

Initial testing using different apertures obtained different effects and results. I was intrigued by how changing any little input variable resulted in such different outcomes. Still, at a distance of ~16 feet, I needed/wanted to achieve a smaller, tighter beam. 

In my research, I learned about condensers and other techniques used to achieve this. 

It continued to get complicated. 

While experimenting with different rigging set ups and looking at my assembled gear, I decided to try pointing the flashlight through the back of one of my camera lenses. It resulted in a tighter beam and also allowed for adjustment of resulting light source coming out the front end (adjustment of zoom/focus rings on lens). 

Further testing ensued. Again, any little change such as size of aperture, distance between light/aperture and back of lens, zoom/focus of lens, angle at which it light/lens pointed in relation to each other and to the mirror, proximity/closeness of the exiting light to the receiving/capturing camera…all produced varying results with different effects. So much fun making discoveries.

I have been using my Canon 5d Mark3 to capture resulting images. My kit includes various lenses, batteries, cards and other accessories. Although technology has improved and more capable cameras with high speed capabilities, greater resolution, etc. now exist, my current camera is capable of obtaining quality schlieren imagery for photo and video. 

Initially, the best lens I had and was able to use for capture (on the camera end) was a 70-200mm. It worked, but one of the key shortcomings was that resulting imagery didn’t allow me to fill the mirror in the frame at a distance of 16’ (you want to see the test subject in the foreground of the image, but the mirror is where all the action is). I later purchased a 300mm lens which allowed me to fill more of the image frame with the mirror, and this is what I use as my main camera lens for this project now.

On the light source end, I have experimented with a number of lenses but now use the 70-200mm as the main light condenser.

I utilize tripods, rods, mounts, clamps, friction arms and other hardware from my cinema kit and sourced additional components to build a custom rig for my camera, light source, condenser and light separation points.

Demanding set up requirements meant that I had to rely on research for set up parameters and options but also experimented considerably with mounting hardware, different connection points and various arrangements of gear to obtain optimum workflow results.  While I had some gear on hand to complete a suitable set up, I had to source additional components and build out custom components by repurposing items and using cinema gear, popsicle sticks, guitar strings, jar tops, coloured filters and other materials.

After testing with multiple configurations, I have settled on the following primary schlieren imaging set up:

Affixed to kinematic mirror mount. Set up atop a table (then had to set up test imaging area, camera, light source and other gear to match this height). Mirror position is then fixed on vertical and horizontal axes, with ability to point light back to camera/blade, using large fine tune knobs.

Mounted on heavy tripod via stabilizer plate. Stabilizer plate allows for mounting of 15mm rods (details below). Ability to manipulate height, head angle of tripod to point camera directly at mirror on same vertical plane. Of significance for indoor/studio use, but more so for intended field use, ability to step tripod down to a low height (with various height options available) is required.  Measured, marked and set up at 200” from mirror (testing conducted at different distances, but optimal results achieved at 2x focal point).

A second tripod with an adjustable ball head is used in this set up to mount light source and light condenser. Also mounted to this ball head is a friction arm which holds the flashlight steady and allows for adjustments in all axes. 

The light source needs to be set up as close as possible to the camera/lens/blade to minimize/avoid issues related to astigmatism (blurred images due to refractive errors). The greater the distance between light source and blade, the greater the astigmatism (get close as possible!). While optimal set ups require little or no angular variation between the light source and the recording medium, manipulating this variable also affects resulting imagery.

To manage light intensity, colour, etc., I have affixed custom apertures to the flashlight as well as optional mounting points to end of aperture to allow for a velum (translucent sheet to control light intensity) or other components such as coloured filters (to manipulate colours/shapes of the light being introduced into the system). 

While I have yet to fashion additional filters, shapes and other accessories to control light source to expand creative options, it is my intention to fashion a series of assorted shaped and coloured velum and filters to facilitate further control the properties of the light introduced.

Light source, mirror, blade/lens all positioned with minimum angular variance. While most examples of schlieren imaging I have seen utilize a static, fixed set up, I have found great benefit to having the ability to change light angle as well as position of filters on camera end while filming - altering, intensifying or diminishing schlieren effect to produce exciting moving images.

While research material provided some guidance on basic set up parameters, mounting/rigging and test shooting was, to put it simply, tedious and full of learning opportunities. Every step involved tweaking, troubleshooting and further testing. Once basic set up has been completed - measured distances, all elements on same vertical/horizontal plane, light pointed at centre of mirror, pointed back to camera, etc. - minor vibrations or small movements must be controlled and limited/avoided to minimize disruption to the sensitive set up.  

Manipulating any one of the many variables - light intensity/colour/shape, distance of light from camera, distance of light from mirror, angle of light, positioning of blade/filter in front of lens…every little change greatly effects the resulting image.  

In addition to the physical components (placement/positioning of each of the pieces), ambient lighting and temperatures, in-camera settings and other factors have a significant impact also. 

Bright light, no velum? Try moving the light, or if you like the effect where the light is, but need to control image brightness, adjust the ISO and/or stop the shutter or aperture settings on the camera down. F-stop/aperture are also used to control light and effect. Banding in video? Use a different light source or adjust shutter speed and/or frame rate. Found a cool effect but want to see if you can intensify it? Play with the camera settings, play with positioning of equipment. See what you get. You see the variance in ambient temperature and a flame but learn that not running a wood stove for space heating for a few hours lowers the ambient temperature, thus the difference between temperatures of flame and surrounding air, thus intensifying visual effect…  

Additionally complicating things, any time a change is made to - shutter speed, for example, additional adjustments often need to be made in camera or at one or more of the physical set up components to achieve optimum or desired effects.  

Each of these factors - light intensity, style and placement/pointing, use of filters or extent of blade/filter light blocking, shutter speed, aperture, ISO, frame rate… - all have significant impacts on resulting imagery and much time is spent playing around and figuring things out, depending on desired outcomes. 

While I have taken down and set up the system many times in multiple different configurations, I have a good workflow for set up, which gets me at target ranges, but allows for efficient and effective manipulation and experimentation of the various factors to test the system overall and tweak the resulting schlieren images.