As speckle interferometry various optical measurement methods are referred to for measuring small changes in distance that are based on the analysis of speckle patterns. Speckles occur when a surface is illuminated with light of sufficient coherence length . The word interferometry in the title of this article refers both to the interference of the waves scattered from neighboring points during the generation of speckle patterns and to the coherent superposition of the speckle pattern with a reference wave. However, not all of the methods described here use a reference wave. Depending on the method, changes in distance of a few nanometers up to a few tens of micrometers can be recorded.
Application in materials research
The Electronic speckle pattern interferometry (ESPI) is a method to visualize small deformations of test objects. It is used in materials research, among other things, to investigate vibration , thermal stress, tensile stress or shear stress . The basis of the method is the speckle effect of laser light, which is scattered on a rough surface of the test object and superimposed with a reference wave. During a controlled loading of the object, successive images are recorded without changing the relative position of the camera, object and laser. The load leads to small deformations of the object and causes the speckle points to move on the images. The deformation can now be quantitatively reconstructed from a computer-aided analysis of the speckle interferograms.
Another variant of speckle interferometry is shearography . In contrast to the ESPI method, with which the surface displacement can be measured, the stripe pattern in Shearography represents the deformation gradient. Shearography is more robust in terms of annoying vibrations and is also used under industrial conditions.
The speckle photography is an optical method to measure displacements perpendicular to the direction of observation (so-called. Plane in shifts). An overlay with a reference wave (as in the ESPI method and shearography) does not take place. The in-plane change in distance is determined directly from the correlation of the speckle pattern recorded before and after the shift.
Application in astronomy
In astronomy, speckle interferometry is a method of eliminating the effects of atmospheric disturbances on astronomical recordings. Air turbulence in the atmosphere is the reason why the light from a star reaches the telescope on paths of slightly different lengths. Therefore, in the focal plane of the telescope instead of a produced diffraction theoretically sharp image by interference a pattern of individual spots (the so-called speckles ). The resolving power of a telescope is therefore limited to the mean diameter of this speckle pattern (see also Seeing ). The fine structure of the pattern changes completely in less than a second. This air turbulence also creates the well-known flickering (sparkling) of the stars.
For speckle interferometry, many short-term exposures are taken instead of a single long exposure. The individual exposure times are between a few milliseconds and a maximum of one second. By non-linear averaging of the individual images, it is possible to compensate for the deteriorating influence of the earth's atmosphere and to obtain a sharp image of the object (for example the star) based on diffraction theory. This technique was first used in the 1970s. Their use is limited to lighter objects due to the necessarily short exposure times.
Important methods are also:
- Knox-Thomson method (1973)
- Speckle masking interferometry (G. Weigelt, 1977 and AW Lohmann, 1983)
- Multichannel / multiframe blind image deconvolution
- Oskar von der Lühe : Interferometry in Astronomy , tutorial of the Faculty of Physics, Albert Ludwig University of Freiburg
- Swapan K. Saha: Speckle interferometry , Indian Institute of Astrophysics , arxiv : astro-ph / 9910506v1
- David L. Fried : Optical Resolution Through a Randomly Inhomogeneous Medium for Very Long and Very Short Exposures , 1966, bibcode : 1966OSAJ ... 56.1372F
- Antoine Labeyrie : Attainment of Diffraction Limited Resolution in Large Telescopes by Fourier Analyzing Speckle Patterns in Star Images , 1970, bibcode : 1970A & A ..... 6 ... 85L
- Stresstech GmbH: Borehole device and ESPI for residual stress determination (technical application of Electronic Speckle Pattern Interferometry (ESPI) in material testing for residual stress analysis with the borehole method)