One of the things that has always been tricky for me to set up and adjust in my Schlieren setup is the "Schlieren Stop" sometimes called (and physically is) a razor edge. Everything else related to getting good images I have pretty much nailed down and figured out to the point that I can set everything up in about 10 minutes - except the Schlieren stop.
The Schlieren stop is sometimes a colored filter, a slide projector slide, a razor edge or almost anything placed at the focal point of the mirror that will either block the refracted light or color it. There is a short and somewhat descriptive explanation of what is going on in THIS Wikipedia article. According to that Wikipedia source depending on what kind of 'Block', 'Filter' or 'Schlieren Stop' you use (if any) determines if it's a Shadowgraph, Schlieren, Rainbow Schlieren or something else. I use the term Schlieren for all of them because in my mind they are all basically similar. HERE is another good (and short) article explaining the Schlieren setup and how the knife edge stop is used.
Below is a diagram of the Double Pass Schlieren setup that I am currently using. I wrote a post about this setup HERE and made a exciting video about it HERE if you are interested.
|Home Made Double Pass Coincident Schlieren System|
On the right side of the picture you can see the box labelled 'camera' and right before that is a line labelled 'colored filter' click on the picture to get a better look at it. That colored filter is what I am calling the 'Schlieren Stop'. It doesn't matter what I am using, a razor edge, colored filter or slide projector slide that thing is always difficult to get positioned correctly. Most of the problems that I have with it are associated with keeping the object that I am viewing in focus after placing it or having placed it the image of refracted light isn't uniform or is degraded.
My problem in using colored filters is the only real filters that I have are uniform in color. Same issue with slide projector slides, I haven't been able to get a slide that has a color gradient or the gradient isn't very uniform. When the light is refracted by the change in density it either goes into the colored region of the filter or it misses it. If it goes into the colored area it's given one color because the filter is one color. If there is a color gradient in the filter then the light will be colored a different amount depending on where on the filter it hits. So the problem that I seem to be having is I either use a solid colored filter that gives the refracted light all one color regardless on how much refraction there is or a slide that has different colored regions and the light is either one color or another depending on what region of the slide it hits. Either way I can't tell how much refraction there is by the color seen in the image. Slide projector slides that have one color that slowly and gradually changes to another color are hard to come by these days. Just getting a slide projector slide is hard to get since LCD overhead projectors seem to have replaced them. The other problem with this is the colored filters seem to cause double images and out of focus regions in the image.
The idea that I am going to try is to color the light coming out of the Light Source before it is sent down to the mirror instead of coloring it afterwards near the camera. There are examples of this approach in literature using colored filters and slides. I have even seen knife edge's placed in this area right after the light to do this. I have tried this already with filters and slides and it solves the focus and double image problems that I mentioned above but the single color issue is still there.
The approach that I am going to try is to use a prism to break the white light from the light source into the primary colors of a rainbow. This should create a 'infinitely graded' colored light from one side of the light beam to the other and any refraction will cause a different color to be produced (I hope!).
|Wikipedia Commons Image of Dispersion Prism LINK|
Above is a picture from Wikipedia Commons of a Dispersion Prism to give you an idea of what I am talking about. One issue that I am concerned about is the light source that I am using is a Super Bright LED from Radio Shack and the white light isn't made up of all the colors in the spectrum evenly. Below in an image from Luminosdiy.com of the typical spectrum of light from a typical white LED. The white line is how much of each color in the spectrum is typically present.
|Typical Emitted LED Spectrum, White LED|
Image From: Luminousdiy.com LINK
So I can expect the white LED light coming out of the prism to have a bunch of colors in it but not all the colors will be of the same intensity. Is that going to be a problem?
The first thing that I had to do in constructing this setup was to get a prism. As I have mentioned before there is a commonly available and inexpensive source for prisms from most hardware stores in those super wide angle front door peep hole scopes.
|Wide Angle Door Scope "peep hole" Viewer|
There are usually two decent quality prisms in those and you can usually get them from the hardware store for under $20.00 - on line even cheaper.
|White LED with Prism for Schlieren System|
Above is a picture of what I have built so far. The white LED is mounted to the aluminum bracket on the left. The prism from the door viewer is on the right and the entire thing is being held temporarily in a table vise. Both the LED and the prism and attached to brackets that can be rotated to change the angles and there are screws from the bottom to lock the brackets down.
The LED is filed down and polished flat to remove the lens that is formed by the bullet nose of a typical LED. Then the LED was wrapped in foil tape with a pin hole poked in it to make a pin point light source. Below is a picture of what I am talking about.
|LED modified to be used as a pin point light source|
That is about as much progress as I have made on this experiment. The LED and the prism work as designed and I am getting a thin band of colored light coming out of it. By rotating the prism on the mount I can make the band thinner and wider. The next step is going to be adding a flat plate with a hole in it to block the unwanted light coming out of the prism and a lens (or several) to focus the light onto the mirror in the Schlieren setup. As always email me at firstname.lastname@example.org if you have any questions of leave a comment. I should have more progress on this to write about next week.