Schlieren imaging (or photography) with a small mirror

<p><a href="http://www.answers.com/topic/schlieren-photography">Schlieren photography</a> has traditionally required elaborate setups involving large parabolic mirrors.</p>

<p>This fellow on YouTube demonstrates how he achieves an impressive approximation with a common flat mirror; something everyone can experiment with:<br>

<a href="

- 6:17 run time</p>

<p>I haven't put in a lot of thought as to the exact nature of what's going on with his setup to enable detection of minute differences in air density, maybe you'd like to offer an explanation. </p>

 

<p>It's not Schlieren. I built Schlieren system for scientific study of flames at Bell Labs so I know what's involved. The described technique is a Shadowgraph and the long focal length "pinhole mirror" provides the required collimated beam of light (or at least close enough to it). It's an old technique dating back to the 16th century (possibly even earlier).</p>

<p>The techniques are related but different. Schlieren is significantly more complicated (especially when dual axis and color coded) but can proved more useful (and quantitative) information as well as higher sensitivivity to refractive index changes.</p>

<p>Not sure if either technique has much application to pictorial photography, though at one time Bell Labs sold a giant poster of one of my dual axis color Schlieren images of gas flow around a cylinder suspended in a torch flame. I didn't get any of the proceeds :-(</p>

<p>Thanks, Bob. An old technique that's new to me. I'll look it up. </p>

<p>Hi Michael,</p>

<p>I used this setup for watching an eclipse one year. I used a mechanic's mirror clamped to our back yard fence about 100' away from the house. The mechanic's mirror had an articulating head that is useful for aligning the reflection. The diameter of the mirror was maybe 2". I reflected the sunlight through a window in the house to an opposite wall.</p>

<p>I used the full power of the reflection to place a spot on the curtain on that window at the desired location, and then I dropped a paper sleeve over the mirror's head with a small punch hole in it to give me a smaller aperture (hence sharper image). Then I opened the curtain slightly to let the reflected light hit the wall. It was a really fussy setup, but it worked well, so my young kids could watch the eclipse safely from indoors.</p>

<p>A few years later, I made a better solar observatory out of a mirror and two camera lenses, projecting about a 4' diameter image of the sun on a projector screen at the end of the hallway in my children's elementary school. Classrooms would take turns sitting in front of my screen and sketching the eclipse. That was a blast!</p>

<p>With regard to your "how's it work" question, the changes in air density or composition result in different indices of refraction, and that enables small pockets or columns of gasses (whether heated, cooled, or just of a different density or composition) to behave as lenses that change the direction of the well collimated light. I never thought to play around with my solar eclipse setup in this manner!</p>

<p>Sarah -- very clever. I've got to try that.</p>

<p>Bob is correct: This is not true Schlieren -- it's just a shadowgraph. Done using the sun, it's also far from being new or innovative. If I'm not mistaken, there's a story about Robert Hooke (of microscopy fame) first doing this in the 1600's.</p>

<p>T</p>

<p>PS - No, I wasn't there. ;-)</p>

<p>PPS - I used a Schlieren setup in the mid-to-late 1970's to understand mixing of the reagents and shock fronts in very high power supersonic gas dynamic lasers, energy deposition in electric discharge gas lasers, etc. </p>

 

<p>Sarah, thanks for the feedback. </p>

<p>Funny how the mind wonders. I watched the video and began wondering about the drift problem and imagining a 1" mirror attached to my GoTo scope mount, then came the problem about how to polar-align during the day to achieve enough accuracy to at least observe 100' away for a few minutes without manual correction. Then I got distracted by another thread. :-) </p>

<p>My solar observation has been limited to Baader film in front of an 80mm/F5 scope but never liked the idea of looking into something that can potentially burn my eyes. Some day I'll break down and spring for that little $700 <a href="http://store.meade.com/home-page-featured/best-selling-products/coronado-personal-solar-telescope-pst.html">Coronado solar scope</a>. </p>

<p>Tom, Bob, you guys have a lot of experience first hand on this sort of advanced stuff few of us ever get to play with, and I'm sure many here will be interested in your experiences if you can dumb it down just enough, without worms, so we can all learn from it. Not just about Schlieren setups either. How about it? :-) </p>

<p>http://en.wikipedia.org/wiki/Synthetic_schlieren<br>

for another kind of effort to emulate the effect</p>

<p>You should be able to do the same thing with an expanded laser beam since all you need is a collimated beam of light to make shadowgraph images. Actually you don't even need a colimated beam as long as you have a point (or very small) light source. Not sure if a laser pointer would have enough intensity to make viewing easy though. A pinhole (even a large one) would work just as well as a mirror. In fact the mirror just acts as a virtual pinhole.</p>

<p>Bob, thinking out loud:</p>

<ol>

<li>If I use a green laser pointer and shine it through a ball lens, would that approximate a beam expander? </li>

<li>If so, then maybe I can use a drop of pure water suspended from the tip of a syringe needle as a ball lens? </li>

<li>If that approximates a collimated light source, then the back side can work as a shadowgram in a darkened room? </li>

</ol>

<p>I'd try it but my green laser went kaput, but a replacement might go on the list if the scheme has a chance of working. </p>

<p>Usually the way to expand and collimate a laser would be to use a microscope objective to focus the beam, followed by a convex lens placed one focal length away from the focus point. </p>

<p>You can also use a telescope in reverse (shine the laser into the eyepiece). In fact most beam expanders (for small expansions) use a reverse Galilean telescope configuration.</p>

<p>A single lens will expand the beam, but won't collimate it into a parallel beam.</p>

<p>You could also use a bright LED. It's not quite a point source, so you can't get a perfectly collimated beam, but it's probably close enough for this purpose. Since the LED probably already projects a somewhat focused beam, you can probably get away with just using a single convex lens with the LED at its focus. The collimated beam will then be the same diameter as the focusing lens.</p>