Sunday, August 9, 2020

schlieren on a stick


schlieren imaging (wikimit and nasa)

Schlieren photography is similar to the shadowgraph (an optical method 
that reveals non-uniformities in transparent media like air, water, or glass)
and relies on the the fact that light rays are bent whenever they encounter changes

shadowgraph (bullet in flight - wiki)

what you need 
*spherical / parabolic / concave mirror (the biggest, lightest,
cheapest you can get) acrylic?
rigid selfie stick (length depends on the mirror focal length)
cellphone (video mode with led powered on)

*spherical mirror will achieve better uniform results

how to adjust
glue the mirror to the far end of the selfie stick
hang the phone on the selfie stick handle
extract the selfie stick to the point which the reflection of the led
seems to be smallest (about twice the focal length of the mirror) 
and that the led reflection covers the phone camera lens
zoom in, adjust the contrast and enjoy your new toy :-)

photo of the wanted (selfie stick) setup


and a video

some more videos


some notes
selfie stick was too flexible to get solid results
need to find a robust selfie stick (carbon fiber?) and make another video
auto exposure of the cellphone app drive me nuts need to find
proper video app with fixed exposure 

further reading:

Raffel, M. & Heineck, James & Schairer, Edward & Leopold, Friedrich & Kindler, Kolja. (2012). Background Oriented Schlieren techniques for helicopter related large scale and flight testing. 38th European Rotorcraft Forum 2012, ERF 2012. 2. 1-9. 


https://epjtechniquesandinstrumentation.springeropen.com/articles/10.1140/epjti/s40485-018-0045-1

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