Schlieren Imaging / wave optics

[Goodver]

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.

 

[Steve Drake]

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.