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I'm currently trying to carry out a home-made experiment showing Schlieren effect (or strioscopy effect) without a parabolic mirror (just because I don't have one today, but of course I know I can get a mirror of sufficient quality for a reasonnable price if I really want one). So I use only some magnifying lenses I already have. I explain below the setup, and actually it works. I can get on a cardboard screen an image of a soldering iron and a column of hot air starting from its tip. But this image is very dark. I tried to film it using my camera, but even with manual settings to have a high exposure, I can see something on the LCD screen of the camera before I start to record, but when I turn on the recording everything is becoming black, and the resulting video is completely black (just a little bit noisy but completely black). My son got the same result with a recent smartphone, the better quality of the sensor does not compensate the smallness of its objective lens. I noticed on some youtube videos some people replacing the screen directly by the camera. This way the camera collects more light, that's ok for that, but my problem is that the Schlieren effect totally disappears. I tried with different focus settings on the camera, no way. So I'm wondering why in that case I can see the soldering iron but not the hot air flow coming from its tip. My setup is indeed a bit different from what I could see on other videos. First, in some of them a mirror was used. On other videos using a lens, some people try to place the ccd sensor of the camera at the focus point of the main lens. It implies that there is no dedicated filter on the Fourier plane which is located at the focus point, and for me their effect is much more a phase contrast effect not really comparable with the schlieren effect, which implies to cut a part of the light at the focus point of the main lens. I'm wondering if I missed something regarding the camera setup. It's a bit frustrating to see the effect on a screen but not being able to get a video of it. Any idea is welcome!

Regards, Eric

Setup: I tried different setups, in fact the simplest give the best result. I have first a light source. I used different sources, a small 4.5V filament lamp, a small white LED, a big LED lamp (800lm), another LED lamp from a toy. All of them with an aluminium foil on it with a 1mm hole in it, so that it approximates well a point-like source. I'm currently trying with a laser LED but I need to enlarge the beam sufficiently first. So this source is far away ( about 3m) from a main lens (diameter 8cm, focal length 40cm). At its image focus point I place a knife-edge. It appears that the position of this knife edge is very sensitive for this effect, so I realized a dedicated support for it to get a sufficient precision. About 1m after the knife edge I placed my cardboard screen. The soldering iron is place a few cm before the main lens. When the knife edge is a bit before the focus point its the image on the screen gets black from the opposite side, while when it is after the focus point the blacked part of the image is on the same side. The effect appears when it is precisely at the focus point, the image becomes grey and the hot air flow suddenly appears. The difficulty in tuning this is that the effect is more important when the knife edge cuts a larger part of the spot at the focus point, but of course the brightness of the image also decreases in the same way and the resulting image on the screen is quite dark. On different setups I added a second lens after the knife edge, so that the knife edge is at the object focal point of the second lens to get a parallel beam as an output. I thought that this way it would be better when replacing the screen by a camera, whose focus can be set at infinity. I also tried with additional lenses just after the light source, to act as a condensor and obtain an output beam with much more parallel rays. But this way I could not see any schlieren effect, probably due to alignment problems with these additional lenses.

Schlieren setup

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  • $\begingroup$ Could you be more specific about the samera you are using. Note that smartphones and many personal cameras use very small sensors which are not suitable for low exposure considtions. Smartphones rely on heavy handed processing of images to try and make something pleasing to the eye but not necessarily faithful on detail. $\endgroup$ Jan 13, 2022 at 19:12
  • $\begingroup$ It's a panasonic fz72, it's not a quite recent camera but it is of reasonnable quality. Rgds. $\endgroup$ Jan 13, 2022 at 20:09
  • $\begingroup$ Sounds like you've set too low ISO sensitivity. Normally, when previewing, the camera will maximize gain on its sensor to show you something, to let you frame your video. But when you start recording, the actual settings, including ISO and aperture, take effect. Try doing a still photo at some maximum settings for ISO and smallest f-number for aperture. Limit exposure to something like 33 ms, since your ultimate aim is a video. Do you get anything on the photo? $\endgroup$
    – Ruslan
    Jan 13, 2022 at 21:29
  • $\begingroup$ An FZ-72 has a small senor (a bit bigger than a smartphone, but still small for your needs). Basically any used DSLR would be better for this task. You can shoot comfortably at ISO 800 on a DSLR when an FZ-72 would basically wreck any detail. Borrow a used DSLR and a kit lens (which are good lenses in fact). Don't worry about megapixels, worry about sensor size. Use a timed exposure and shoot RAW if possible - you can do noise reduction in post processing if needed. $\endgroup$ Jan 13, 2022 at 21:49
  • $\begingroup$ Actually I tried to set it up to 6400 ISO with no success (aperture to 2.8 and time exposure to 40ms). I did not try in RAW format, I'll try that. But when I place the camera on the optical axis I have no more light problem, I can set ISO to 100 and aperture to 5.6 with no problem. In that case there is something basic for which I'm not sure. If I have no additionnal lens after the knife edge, should I set the focus also at infinity or at approximately the distance between the camera and the soldering iron? I guess it's the second solution and my problem is in the precision of that focus? $\endgroup$ Jan 13, 2022 at 22:07

1 Answer 1

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So this source is far away ( about 3m) from a main lens (diameter 8cm, focal length 40cm).

Schlieren imaging depends on minute changes in the refractive index of air—which is very close to 1 to begin with. This means that deviation angles are small, and this is why usually large focal length mirrors or lenses are used for this. Your placement of the source far away from the lens reduces the distance to its image, and thus increases the angle of convergence of light rays exiting from the lens. This makes it harder to detect the small deviations. You should instead try to make distance to the image larger by moving the source closer to the lens.

Now, since your focal length is small, you'll get a noticeably magnified image of the source if you try to compensate for the focal length by distance. Thus, you'll also need a smaller source. A diode laser is a very good choice, because its active zone is only several micrometers in size. Just be sure to remove its lens to get a well-diverging beam and thus fill your main lens. Also do pay attention that it should be a diode laser, not a DPSS one, because the latter emits a very thin and quite collimated beam even without lenses. Particularly, avoid the 532 nm laser pointers/modules, because they are DPSS lasers.

Also, instead of a razor, try using a thin strip of material, e.g. a wire. This will let you get deviated light from both sides of this light block instead of one side of the razor.


I've actually tried this setup with the above mentioned changes, using a 520 nm laser module, a lens with 15 cm focal length (7.5 cm diameter), and placing the laser at such a distance that the image was about 1.5 m away from the lens. After light block the camera (Canon 80D with 50 mm lens) was installed. Interestingly, there was so much light that I had to put a polarizing filter after the laser to attenuate it—the opposite problem to yours.

The image was quite small (though larger if I used a 300 mm lens on the camera), but the details can still be seen.

Here's a photo of the main lens, with a candle flame visible below. In this photo the flame was disturbed, so its jet of hot air moved away from the central shaded part. The same result was without the candle at all. The shadow is created by the light block, which was a thin wire, maybe a bit too thin in my setup (so we still see light from the sides), although the main spot of light was indeed blocked.

And here's the case when the flame is undisturbed. The hot air jet is visible in the middle of the shadow.

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  • $\begingroup$ Thanks Ruslan for your interesting comments! So first, I realized my setup with a far away source because in this situation one gets the Fourier transform of the transmittance function on the image focal plane. There may be a similar result for 2 conjugated planes but indeed on most similar setups I could find on the web, the source was either far away, or close to the lens but with a condensor to simulate a source at infinity. I also just tried with a DPSS I had, with somehow the same success on a screen as with a regular led lamp, but with a bit more brightness so I could film it finally! $\endgroup$ Jan 18, 2022 at 7:42
  • $\begingroup$ Also I tried with the raw format but the the pictures are still black. I used UFRaw to see the images, I'll try to see if I can extract an image from the background but I'm pessimistic. And in the case I place the fz72 on the optical axis like you I also have a high luminosity as I said before, and like on your example. As you said it can be a problem with the light rays angle at the image focal point that are too large? I will try again with an additional lens after the light block to send the image at infinity. $\endgroup$ Jan 18, 2022 at 7:52
  • $\begingroup$ Note: with the DPSS I needed 2 additional lenses just after it to enlarge the beam up to 4/5cm in diameter. $\endgroup$ Jan 18, 2022 at 7:54
  • $\begingroup$ The resulting video is there : eric.chopin.pagesperso-orange.fr/P1090830.mp4 (quite dark but at the end when I blow slowly on the soldering iron about 1m away from it, the resulting turbulence is visible on the video). This is obtained by having the camera at about 20cm from the screen. $\endgroup$ Jan 18, 2022 at 8:29
  • $\begingroup$ I could slightly improve the setup to get something visible on the video with the image displayed on a screen. Essentially, following Ruslan idea I modified the direction of the knife edge. Instead of having it orthogonal to the opitical axis so that it cuts half of the image, I placed it in the direction of the optical axis, the cutting edge in the direction of the screen. Moving it forward or backward cuts the beam until the schlieren effect appears. This way the image is a bit less dark: eric.chopin.pagesperso-orange.fr/P1090838.mp4 (best viewed outside a navigator). $\endgroup$ Jan 23, 2022 at 21:29

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