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).