Follow Up: Chromatic Dispersive Light Schlieren

For the last couple of weeks I have been tinkering with the Chromatic Dispersive Light Schlieren that I built. If you haven't read the other couple of posts about this setup check them out HERE and HERE. To summarize the only difference between this setup and the other ones that I have built is the light source. As the title suggests this uses a Chromatic Dispersive Light source for the Schlieren illumination or CDLS as I like to call it. To make this I placed a prism and a collimating lens right after the LED light source to break the white light from the white diode into all the colors of the rainbow (at least as many as a white LED can produce anyway!). Check out the first link above to see what I mean.

  

The reason I decided to try this is getting a Schlieren stop in just the right place, at the focus of the mirror, with a camera that has a zoom lens is difficult and sometimes impossible. My hope was the Chromatic Dispersive Light Schlieren would eliminate the need for a Schlieren stop and so far it seems to work.

 

 

Above is a short video showing the results that i got with a candle. The reason I am using a candle to test this is because it's a easy thing to photograph because of all the wonderful hot air it makes ;-)

First Test: Chromatic Dispersive Schlieren Light Source

From the title of this post I bet you can guess what it's going to be about. Last week I wrote about a Chromatic Dispersive Light Source that I was building for my Schlieren setup. I managed to get time to work on it this week and test it a bit. But before I get to that I'd like to say that I'm going to abbreviate the Chromatic Dispersive Schlieren Light Source from now on as CDLS. That either stands for Chromatic Dispersive Light Source or Chromatic Dispersive Light Schlieren. Why give it an acronym? Why have the S mean either Schlieren or Source? Everything sounds better and more important if it has an acronym associated with it!! Better still if it's a little ambigious and mysterious. At some point if I mess around with this more and make it better I'll have to come up with a retronym for this version. OK I'm kidding...

Enough of that, HERE is last weeks posts about what I am trying to build. As I mentioned I ground down the bullet lens end of a white LED and polished the surface flat. Then I wrapped it in aluminum foil tape, poked a pin hole in the foil and mounted it in front of a prism. You can see a picture of the assembly HERE. One thing that I did add that is not in the picture is right after the prism I mounted a plate with a 0.1" hole in it to block and light that is bouncing off the prism. I also built a small cardboard cover with the whole thing to help block any light from coming off it.

  

The first attempt at using this didn't work too well because the 'rainbow' coming off the beam and out the hole was very narrow and didn't illuminate the mirror very well. I tried putting a lens after the 0.1" hole and that works better but is still not perfect. Below is a picture of what I got.

Burning Candle With CDLS

  

See how much better that caption on the picture looks with CDLS instead of Chromatic Dispersive Light Schlieren? What can be seen in the picture is a nice rainbow of colors caused by the refracting light from the hot air rising off the candle. Before you even ask "why are you taking pictures of candles?" Click below to read more and for the answer to that question.

Chromatic Dispersive Schlieren Light Source

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.

Another Long Week and Little Time to Play

I had huge plans for this last week related to the experiments that I want to do with high voltage and my Schlieren photography setup. Unfortunately for various reasons I didn't have time to do much. One reason for my lack of "experiment time" is I have not won the lottery and am unable to have all the 'normal' things taken care of by an army of personal assistants. I did add "build robotic servant" to my list of things to do but that is for another day.

What I am writing about this week is a continuation of lasts weeks post. You can re-red that post HERE just because it was so interesting! Just in case you didn't read it and don't want to let me explain a bit what I am doing. I have a high voltage power supply that I am connecting to two electrodes. The high voltage wire is connected to one electrode made from a stainless steel sewing needle. The other electrode is a small 'banana'  electrode that is movable. It's movable so I can raise it up and change the gap between the two electrodes. This whole setup is sitting in front of the parabolic mirror that I use to make beautiful and interesting Schlieren videos ;-) Below is a picture of the setup.

High Voltage Electrode Schlieren Setup

Do you notice something strange about the above picture? If you know me well you won't think anything is strange because you know that I can't spell! I noticed the error after I made the picture and I'm too lazy busy to fix it. The picture is pretty self explanatory if you read the explanation I wrote above. What isn't so obvious is why I'm doing this. Continue reading to find out and don't miss the exciting and entertaining video!

High Voltage Electrode Schlieren Images

This week was another busy week but I managed to get some time to experiment a bit. I decided to look a little more closely at the airflow around a high voltage electrode with the Schlieren setup. To do that I put together a quick test rig that allows me to raise and lower an electrode in front of the parabolic mirror. I'm working on editing a video of what I saw going on but in the meantime I have a couple of pictures. The first picture below is of the test setup.

 

High Voltage Electrode Schlieren Setup

The black cylinder near the bottom of the mirror is a plastic tube mounted to a small piece of wood. In the center of that is a smaller plastic cylinder that has a rounded and blunt electrode attached to it. There is a wood 'lever' going off to the left side of the photo that I can use to raise and lower the electrode by sliding it in the larger tube. The red wire at the top is connected to a sewing needle that is mounted right above the movable electrode that I just described. The red wire of course is connected to the power supply. What does all this mean?? I don't know but I'm going to try and figure it out and explain it to you. Continue reading to find out!