Schlieren Imaging / wave optics

In summary, the conversation discusses the principles of Schlieren image formation in terms of wave or Fourier optics, rather than ray optics. The technique uses a sharp knife edge in a focal plane to flatten out the transfer function and eliminate oscillations in the power spectrum caused by diffraction from the refractive index inhomogeneities of the sample. This technique is known as shadography, where both positive and negative orders of diffraction interfere and cause oscillations in the power spectrum. The knife edge in Schlieren blocks one of these orders, removing the correlation and oscillations, resulting in a clearer image.
  • #1
Goodver
102
1
Could anyone please explain or advice me where to read about principles of Schlieren image formation NOT in terms of ray optics, but in terms of wave or Fourier optics.

I understand how that works in terms of heuristic ray optics, but would like to get to know how the actual image formation due to the change in spatial index of refraction and presence of the sharp knife edge in a focal plane is explained in terms of wave optics.

Thank you.
 
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  • #2
The knife edge is just there to flatten out the transfer function. Without the knife edge the technique is known as shadography. In shadography, the samples refractive index inhomogeneities can be thought of as diffraction or phase gratings, causing the incident beam to diffract. The diffracted orders from all of these 'gratings' interferes on a screen and the resultant intensity distribution an be Fourier transformed to yield the power spectrum.

In shadowgraphy, both the +tive and -tive orders diffract and when recombining with the incident wave cause oscillations in the power spectrum. The knife edge in schilern blocks on one of those orders of diffracted light removing this correlation that occurs at the detector and removing the oscillations.
 

Related to Schlieren Imaging / wave optics

1. What is Schlieren Imaging?

Schlieren Imaging is a technique used to visualize and study the flow of fluids by detecting small changes in density. It works by using a light source, such as a laser, to illuminate a transparent medium, and then capturing the light that is refracted by changes in density. This allows for the visualization of normally invisible phenomena, such as shock waves and heat transfer.

2. How does Schlieren Imaging relate to wave optics?

Schlieren Imaging relies on the principles of wave optics, specifically the refraction of light, to capture and visualize changes in density. As light passes through a medium with varying densities, it changes direction, and this change can be detected and measured to create an image of the density variations.

3. What are some practical applications of Schlieren Imaging?

Schlieren Imaging has a wide range of applications in various fields such as aerodynamics, heat transfer, and fluid dynamics. It is used in wind tunnel testing to study the flow of air around objects, in combustion studies to observe flame behavior, and in medical imaging to visualize blood flow and tissue density.

4. How is Schlieren Imaging different from other imaging techniques?

Schlieren Imaging is unique in that it can capture and visualize changes in density, rather than just changes in light intensity or color. This allows for the visualization of phenomena that are not normally visible to the naked eye or other imaging techniques. Additionally, Schlieren Imaging does not require any special dyes or contrast agents, making it a non-invasive and versatile imaging method.

5. Are there any limitations to Schlieren Imaging?

While Schlieren Imaging is a powerful tool for visualizing density variations, it does have some limitations. It is most effective when there is a significant difference in density between the medium and its surroundings, and when the density variations are large enough to be detected by the imaging setup. Additionally, Schlieren Imaging is limited to two-dimensional images and cannot capture changes in density in the third dimension.

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