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Why is a point light source required for Schlieren Photograph?

  1. Apr 5, 2015 #1
    Hi All,

    I have read articles and watched videos demonstrating and explaining the basics of Schlieren photography but one point that I am not at all clear on is why a point light source is needed.

    Can anyone offer a general explanation or point me to a basic illustration that might explain this detail?

    I am thinking it is in part because the goal is to focus the image back onto a single point after the light reflects from a mirror but I get cloudy thinking about how a single point light would show more than just a point of the subject.

    For instance, if I am using a laser to visualize air flow around a soda bottle which is being opened, and the laser is slightly behind the soda bottle, pointed toward the mirror then the beam of light passes through just one small point around the soda bottle on its way to the mirror. Does the mirror itself "widen" the beam of light (if the general idea is right, please correct my terminology).

    I would like to understand why any light source can not be used - that is, what would happen if a flashlight were used instead of a point light source. I am hoping to eventually take some Schlieren photos at home so maybe this will help with the understanding but that will be a while from now and any info you can give me would be appreciated.

    Thanks!
     
  2. jcsd
  3. Apr 7, 2015 #2

    tech99

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    I think the following web site would help you. You need good quality optical components.
    http://www.ian.org/Schlieren/HowTo.html
     
  4. Apr 7, 2015 #3

    sophiecentaur

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    The reason that Schlieren photographs work is that you get diffraction / interference, due to the path differences for the various routes for the light going from source to camera. The light going through a more dense patch will phase lag a bit and interfere with light through normal density air when all the rays get back to the camera. This cancellation (and enhancement) effect will be pretty subtle and it will only show if you have a reasonable amount of coherence for light over the whole field of the object and reflector. If you use a broad source then there is a massive variation in the path differences from different parts of the source, through the system. This means that any cancellation effects on one path will be diluted by uncancelled light on another path and there will be, in effect, many patterns in different places, which will destroy any useful image. A single bright point source will give good illumination and avoid the problems of smearing out of the image.
    The requirement for a coherent source of light is pretty universal when you want to produce any form of interference pattern.
     
  5. Apr 7, 2015 #4

    Andy Resnick

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    Schlieren imaging techniques- and there are many- requires spatially coherent light. Some examples are: knife-edge (Foucault) testing of optical surfaces, phase contrast (Zernike) imaging, and shear interferometry. Because spatial coherence varies inversely with source size, using a point source (or spatially filtering an extended source) provides a sufficiently large coherence area.
     
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