Single-Mirror Coincident Schlieren Camera Setup (double pass)

Yes I know I seem to go on and on about Schlieren Photography on my blog... but hey it's my blog and it's about the things that I am doing! As many people know I have been messing around with Schlieren Photography for some time now and mostly it's been the camera setup itself and not the things I am photographing that I have been playing with. My latest adventure in camera setups has yielded the best results so far. If you are a avid reader of my blog you have already seen the results of this with last weeks post HERE.

  

What I have been looking for is an even illumination of the subject matter and a high degree of sensitivity. The sensitivity issue isn't something that I feel I have optimized but the setup that I have now seems to be the best so far. Up to now what I have been doing is putting the light source off to one side of the camera. The mirror then reflects the light from the light source back into the camera but because the light and the camera are side by side the light path going out to the mirror and coming back isn't 'lined up'. From what I have read this isn't the best way to do it. A better way is to have the light going out and coming back be 'lined up' in the same optical path. This is called a Coincident Double Pass setup and that is what I built. Note: I'm not an optics person so forgive my layman's explanation of all this!

Double Pass Coincident Schlieren Setup (click for bigger view)

The tricky thing about doing it this way (other than building it) is getting everything lined up just right so the light source and the camera are both coaxial and looking at the mirror. This involves getting the beam splitter, the light source, the camera and the mirror in just the right places. The mirror I am using has a focal point of 118 inches so it's really far away from the camera and beam splitter. That means that if the positioning of anything is off by even just a little bit it wont work. Usually before building something I would try a 'hack' and hold everything in place with tape and toothpicks just to see if it worked before I took the time to build a proper setup. But because of the sensitivity of the positioning I decided that I had just better build a solid and adjustable setup right from the start (and how it works!). Below is a picture of what I ended up with. If you are interested in an explanation of Schlieren photography HERE is a short Wikipedia link explaining what it is (but not how to do it).

My Double Pass Schlieren Setup

The neat thing about the above setup is the entire thing cost me nothing but time to build. I had all the individual pieces laying around in the garage or I took them out of gizmo's I had in various junk piles.

The beam splitter is a half inch square and mounted to a optical rotational base with a couple of standoffs. That allows for the mirror to be rotated to exactly a 45 degree angle to the camera and the light source. The rotational base is mounted to a pan and tilt base that allows the mirror to be panned and tilted! Not too exciting but very important to get everything just right. Both the rotational adjustment and the pan and tilt are 'optical quality' so the adjustments are done with very finely threaded screws. The entire beam splitter assembly is mounted on a linear slide bearing rail so I can move it toward or away from the mirror to get it in the right place.

On another linear slide bearing I mounted a plate with a 1/2" hole in it to hold the light source. The plate has a  spring loaded set screw to grab the light source (a white LED) and it's mounted to a vernier adjusted X-Y platform. The X-Y platform allows for fine adjustment up and down as well as left to right. The linear slide bearing allows in and out adjustment relative to the mirror.

  

On the same linear slide bearing as the beam splitter assembly I mounted the video camera. It has no adjustment other than rotating around the mounting screw and the in-out of the slide. I'm still using a piece of colored plastic on a dial indicator stand as a 'Schlieren stop' to add some color to the refracted light.

  

Picture of 28,000 volt electric arc using double pass Schlieren setup.

As I mentioned the alignment of everything is really sensitive and important but as it turned out with all the adjustments it didn't take more than 5 minutes to get it all working. I started by turning on the LED with a variable voltage power supply and turning it up to full brightness. I took the camera off and put a piece of white plastic where the camera sits. I could see the light reflected back from the mirror as a half moon shaped spot on the plastic. I adjusted the beam splitter and LED position to make that spot round and by moving them both on the linear slide rails the round spot became a point of light.

  

Once I had a point of light I put the camera on and moved it up until the mirror filled up the view finder on the camera. The image was really overexposed so I turned the LED down by lowering the voltage and after a couple of fine tuning adjustments everything was ready to go.

  

I think that this setup works so much better than all my previous ones for a couple of reasons. One, this is the first setup where everything is solidly mounted to adjustable bases and doesn't move all over the place every time I sneeze. Previously the light source was on dial indicator bases, tape and paper clips and it was almost impossible to get everything lined up and keep it there. Second, the double pass setup optically (I think) makes the image clearer and more sensitive to the refraction of the light because the light is travelling twice over the exact same path.
  
As always if you want more info email me at ottobelden@yahoo.com or leave a comment (or both!). Here is a video I made showing the setup I made and a couple of clips - Enjoy!