This week Otto Jr. and I did a bit more experimentation with the Homemade Schlieren Photography that we were doing last week (details in last weeks post HERE). The setup that we are using is essentially the same using the 5 inch 3 dioptre lens and the LED light source. HERE is the setup diagram of the basic setup. Last weeks results were pretty good all things considered but there were several things that we wanted to improve and try. Here is a picture that we did of a candle burning and is similar to last weeks pics HERE and HERE.
|Schlieren Image of Candle Burning|
One of the things that I tried to 'fix' was the chromatic aberration that can be seen around the lens in the above image. Rainbows are really cool but in this case I think it's a bit of a distraction. Most Schlieren setups use a mirror as a focusing element which doesn't have this type of disadvantage (and has other big advantages) but I don't have a mirror... The obvious solution was to switch to a monochromatic light source that wouldn't have all the colors of the rainbow and would also be lacking the pot of gold you sometimes find at the end. This was especially true in this case because it didn't work! Later what I found out was I need to find the optical Circle Of Least Confusion!!! (That is my new favorite technical term)
Click below to read more about the second Schlieren setup complete with videos!
Last week we were using a super bright white 'jumbo' LED to illuminate the lens so this time I tried a blue one. Specifically a Radio Shack 276-0006 Ultra High Brightness which in non marketing speak is a 8000mcd 466nm LED. I did the same thing as last time and cut / polished off the rounded bullet shaped point lens and covered it in aluminum tape with a small hole poked in it. This makes a decent single point light source. As I mentioned above it didn't really work all that well. Although it illuminated the lens really nice with an even color and shade of blue the Schlieren effect was very difficult to see. There was almost no contrast between the more dense and less dense gas areas. I'm not exactly sure why but Otto Jr. and I suspect that the single frequency of light when refracting doesn't cause as much of an interference in the light and therefore doesn't make as a pronounced contrast. There is limited info on the web about Schlieren but usually they say to use a white light source, lasers don't work too well and a mirror works best to avoid chromatic aberration. We had to try though...
The next option was to optically correct for the aberration with more lenses in the optical path. This is the usual way to get around chromatic aberration in cameras and other optical systems but it requires that you have 'achromatic lens pairs' and a thorough understanding of optics. I have neither so I dug around my box-o-lenses that has all the lenses I have taken out of broken cameras, copy machines, toasters and vibrating appliances and found some that seemed to work on the bench. I mounted several to a liner slide so I could position them to focus properly but whenever I corrected for the aberration the Schlieren effect was gone.
|Schlieren with Chromatic Aberration Correction|
The problem was setting the focus to get rid of the aberration meant that I was focusing through the 5" lens to a point behind it and not focusing on and object in front of the lens. I'm sure with the proper optics I could get rid of the rainbow and have the correct focus but not with what I have laying around the house. In the above picture you can see three of the lenses on the linear slide, a polarization filter clamped to a dial indicator stand (more about that below), the camera tripod in the lower left and the 5" lens in the upper right (behind the candle).
While digging around in the box-o-lenses I found a few light polarizing filters that I had from a quantum mechanics experiment like THIS one that I did years ago. I found that adding a polarizing filter (as shown in the pic above, lower left) in the optical path with the filter set at about a 45 degree angle to the optical axis - the filter itself at 45 degrees - not the polarizing angle - the Schlieren effect was more pronounced. In part because the filter is physically at a angle to the light some of the light is reflected away by the glass filter material and the reduced light level made it clearer. Also while holding the filter at an angle and rotating it (rotating the plane of polarization) there was a point where the effect was much clearer. I have to think about why this is but when light passes from one density to another it not only refracts but it is polarized. I think that the polarizing filter is messin' with the Schlieren and interacting with the Schlieren stop ("knife edge Schlieren Stop" or "point Schlieren stop" - see below)
Butane Lighter video with Point Schlieren Stop
As I mentioned using a parabolic or spherical mirror would solve the chromatic aberration problem and it would increase the Schlieren effect because the light would be passing through the air twice instead of once. I decided to give up on the chromatic aberration problem for now and try to get better resolution from the lens setup because I don't have a mirror.
Up until now I have been using a razor blade as a Schlieren stop and thought that trying something different might give better resolution. HERE is a neat picture of a Schlieren setup at Harvard using a mirror and a razor. One of the problems with using a straight knife edge as a Schlieren stop is it is not sensitive to Schlieren refraction that is happening parallel to the blade edge. What happens is one side of the 'Schlieren' tends to be dark and the other lighter. I read that a better effect can be seen with using a right angle edge like the corner of the razor blade. I tried that and it gave better results but then I thought why not a sharper angle? I tried using the point of a pin in the optical path thinking that might reveal better details because instead of a straight edge or two edges at right angles the pin point would essentially have "all angles" as it comes to a sharp point. Check out the above video of the Butane Lighter using the pin (you can see it as a blue pyramid) and then have a look at the below video done with the knife edge.
Candle video with Knife Edge Schlieren Stop
Look closely and you can see that using the Point Stop highlights the Schlieren effect more evenly than using the Knife Edge Schlieren stop. Eventually I would like to try a colored filter like THIS setup but I think I should fix the chromatic aberration problem first by getting a mirror. The other thing that I would like to try is cutting back some of the light with an iris based on the effect that the polarizer had. I have an iris from an old video camera that I can mount to something and try closing it down while filming. I might even try using it as a Schliren stop just to see what happens. Email me at email@example.com if you have any suggestions for improving this or any errors in what I have described or if you just want to make your own and have questions.
One last video...
Another Butane Lighter Schlieren Video