Schlieren effect (strioscopy) visible on a screen but not in a camera

Question

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.

Answer 1

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.

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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.