"Shadowgraphy" with UV light - an experiment to detect fluctuations in the optical density of otherwise homogeneous, transparent panes of glass

[Kai]

In yesterday's article, MING dealt with the topic of "streaks" in optical filters and asked about their detection/visualization. 

LINK

ULF referred to a corresponding article by SCHOTT.
Shadowgraphy is presented there as a simple and rapid method for visualizing striae (variations in the refractive index in an otherwise homogeneous medium).

 

Today I looked to see whether shadowgrapy (without having to make optically demanding setups) could also be suitable for us to assess the quality of our UV filters.

The result is encouraging, I think.

 

A PTFE plate has proven to be a suitable screen. However, it should be fresh and finely ground to ensure a homogeneous background. My plate was too rough and shows too many structures. You can of course take a reference image and subtract it from the measurement image. But that doesn't make things easier.

Setup:
3 W LED 365 nm as light source, 4 m distance to the projection surface, PTFE plate.
Canon 6D-FS with EF 2.8 90mm TS-E and ZWB2 filter. Camera-screen distance 2 m.

Distance of the examined filters to the screen also 2 m.
Everything reasonably coaxial (camera slightly offset, compensation by shift lens).

 

The resolution can be improved by reducing the relative size of the light source (i.e. either a greater distance from the screen or a lens hood in front of the lamp). Of course, this also reduces the amount of light. White balance and focus on the screen. Aperture 2.8, 1/8s, 3200ASA.

 

 

To check the sensitivity, I recorded butane gas escaping from a lighter and a butane gas flame.

 

 

 

 

I examined three filters as objects: ZWB2, QB21 and a laboratory cobalt glass (mounted as a round filter).
The two optical filters show no streaks or other optical defects, the laboratory glass clearly shows them.

 

 

 

 

 

 

 

 

 

Finally, a picture of float glass (glass of a picture frame), which visually appeared completely homogeneous and absolutely flat.
Light source: VIS LED. Diemensions according to the UV stup, but with a 1mm pinhole. Rotation of the glass plate.
Result: The optical density of this float glass is very inhomogeneous! 

 

 

 

 

The method can certainly be fine-tuned and the "Bokeh striation method" of MING seems to be a lot more sensitive.
But the shaowgraphy is quickly set up and surprisingly good for a first qualitative evaluation!
Maybe someone will do comparative tests on it...

 

 

[Andy Perrin]

This is a form of Schlieren photography, which I explored back here via a different method. It can indeed be very sensitive to tiny changes in refractive index:

https://www.ultravioletphotography.com/content/index.php?/topic/4513-flow-visualization-by-background-oriented-schlieren-imaging/#comment-44516

 

There was another example here:

https://www.ultravioletphotography.com/content/index.php?/topic/4539-hot-iron-with-bos/#comment-44772

 

Bernard did a version closer to yours using image subtraction:

https://www.ultravioletphotography.com/content/index.php?/topic/4529-flow-visualisation-for-dummies/#comment-44638

 

[KhanhDam]

that's really cool thanks for sharing!  a photo of the set up would be nice to see.

[Nate]

Cool test, I got to try this

[Kai]

 @ Andy:
It's exciting that you've already shown experiments on BOS technology here. Nice results!
In fact, the BOS technique (depending on how it is carried out) is almost as sensitive as the Toeppler technique with large mirrors.

 

Attached are two BOS videos in which I visualize the warm air over my hand and butane gas from a lighter. The live image is on the left and the difference to the original image is on the right. I took the pictures in 2019 with a simple USB camera. The resolution is very low. Anyway have fun ;)
 

 

 

The sensitivity of the BOS technique is orders of magnitude better than that of shadowgraphy. However, the shadowgraphy shown here by me only requires a flashlight, a screen, the camera and the Schlieren object. And you can see the result without image processing.
It is probably the simplest method of making streaks visible.

 

There are a number of techniques that work with lenses, mirrors, structured backgrounds or retroreflective materials. Optical density fluctuations in transparent media are always made visible as an image. A nice overview of such methods and their applications can be found in the book (376 pages) by Gary Settles (Penn State University, Gas Dynamics Laboratory), for example as a book:
Settles, (Gary S.), Schlieren and shadowgraph techniques: visualizing phenomena in transparent media
ISBN 978-3-642-56640-0 (eBook)

PS:
If you want to see something really good using Schlieren-technique to visualize the supersonic effect of a whip:
https://youtu.be/AnaASTBn_K4

 

==========================================================================================
 @ KhanhDam

The setup of shadowgraphy is very simple:

Take an LED torch and illuminate a homogeneous white surface (e.g. room wall).
Distance - if possible - 4 m.
Take a lighter and stand in the middle between the lamp and the wall so that the shadow of your hand and the lighter is clearly visible.
Turn on the lighter and look at the wall. Finished :)

If you vary your distance, you can change the size and sharpness of the image.
Now you can of course also photograph or film the whole thing.

Have fun!!!
 

[Andy Perrin]

The image processing for BOS is nearly as simple and requires the same setup as shadowography so aside from the advantage of needing no processing I think either is pretty good. The differencing technique for BOS does not give high quality results though, much better to download a PIV (Particle Imaging Velocimetry) program for free and use that. 
 

I love your hand video!

 

ETA: I tried the BOS on my 8mm filter (the one that was blurry) with somewhat interesting results:

It looks like either my filter is not flat, or it isn't MOUNTED flat. I cannot see it by eye, so it must be subtle. I wonder if this is why the image was blurry? Perhaps the glass is fine but the mounting was done badly. Since all UVP members' filters probably were cut from the same sheet, and only mine seemed to have problems, I'm leaning towards "user error" — badly done mounting — now as the explanation...

 

(Ignore the light leak and the hotspot - none of that affects BOS, so I didn't work very hard to make the image nice.)

[Ming]

Fantastic demonstrations, Kai.

 

Andy, thanks a lot for the links to prior discussion.

 

[Andy Perrin]

I removed the filter from the ring, and redid the test. This time I noticed something else. Even without the effects of the filter being at a slant, the filter surface is not truly flat. It has a dip in it that is not visible to the eye but shows up nicely on the BOS (along with all the surface scratches, which are surely my fault, argh!):

 

I did the BOS at higher resolution this time:

 

It looks almost like something "dinged" my filter!

[Doug A]

Very useful tests and examples @Kaiand @Andy Perrin.  The tests provide methods to gauge filter construction and precision. Andy, even though the 8mm filter has a dip, do you believe remounting it will improve sharpness?

Thanks for sharing,

Doug A

[Andy Perrin]

Doug, I am not sure. The slight tilt of the filter wasn’t even visible when I examined it by eye, and that BOS method is so sensitive it can see air movements. I can try another test but the dip seems more likely to be the problem, because how many other filters do I own that must be slightly crooked? I didn’t test any others but I would bet most are a tiny bit off. On the other hand, a dip in the filter could act like a lens and cause blurring.