Speckle Photography

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The speckle photography is an optical method to measure displacements perpendicular to the direction of observation (so-called. Plane in shifts).

The object to be examined (e.g. a component) is illuminated with light of sufficient coherence length and imaged with a lens on a photo plate, see top picture of the figure. An image of the object overlaid with speckles is created in the image plane (photo plate) . The size of the speckle is determined by the aperture.

Speckle Photography

The object is mechanically or thermally stressed. Corresponding to the in-plane component of the object displacement, the speckle pattern also shifts in the image plane. The photo plate is exposed in every load condition. After developing the double-exposed photo plate, two speckle patterns of the object that have been shifted according to the in-plane displacement are stored there. This photo plate is then illuminated point by point with an unexpanded laser beam , as shown in the lower part of the figure. A stripe pattern is created by the diffraction of the laser beam on the speckles. In analogy to the diffraction at the double slit , this diffraction pattern is called “Young's striped pattern”.

With a known imaging scale, the amount of in-plane displacement can be determined by measuring the stripe spacing. The direction of the shift results from the direction of Young's Striefen.

As an alternative evaluation method, the double-exposed photographic plate can also be Fourier-transformed in an optical filter structure; the shift is then obtained by analyzing the spatial frequency spectrum.

This classic speckle photography with photographic recording of the speckle pattern is hardly used today. Instead, the speckle patterns are recorded directly with an electronic camera. As in the classic variant, the Young's stripe pattern can in principle be calculated from the speckle patterns using numerical methods. However, it is more practical to directly correlate the electronically recorded speckle patterns in each load state and thus calculate the shift

Individual evidence

  1. KJ Gasvik: Optical Metrology . John Wiley & Sons, 1987, ISBN 0-471-91246-8 , chapter 6.2
  2. ^ W. Brünings, H. Scheithauer: Measuring ranges and measuring errors in speckle photography . Laser and Optoelectronics 21/5, 1989 pp. 62 - 69
  3. D. Chen, F. Chiang: Digital processing of young's fringes in speckle photography : Opt. Eng. vol. 29, No 11, 1990, pp. 1413-1420
  4. U. Schnars: Digital recording and mathematical reconstruction of holograms in interferometry . VDI Progress Reports, Series 8, No. 378, 1994, Chapter 6.5.2, download: https://sites.google.com/view/ulfswebsite/#h.p_kwZY0w8EEySE
  5. ^ D. Holstein: Spatially resolved characterization of mechanical properties of laser beam welded connections . Strahltechnik, Volume 15, BIAS-Verlag, 2001, ISBN 3-933762-04-9
  6. P. Synnergren, M. Sjödahl: Mechanical testing using digital speckle photography . Proc. SPIE, vol. 4101B, 2000, pp. 520-531