Speckle imaging
The speckle interferometry, a method for measuring small changes in distance. Flared coherent laser light produced on a surface of speckles. By the interference pattern changes when tension in the reflective surface or lateral movements of a rough surface the distance to a fraction of the wavelength ( typical movements: 5 nm to 50 nm) change.
In astronomy, speckle interferometry is a procedure for image enhancement, which eliminates the in ground-based telescopes caused by the turbulent atmosphere deterioration.
Application in materials research
Speckle interferometry can make small deformations of test objects visible. It is used in materials science, among others, for the investigation of vibration, thermal stress, tension or shear stress. The basis of the method is the speckle effect of the laser light is scattered by a rough surface of the test object. During a controlled load on the object of the relative position of the camera, objects and laser now are successively added the following images without modification. The stress leads to small deformations of the object and leaves wander the speckle points on the images. From a computer-assisted analysis of the speckle movements now, the deformation can be reconstructed quantitatively.
Application in astronomy
In astronomy, the speckle interferometry is a method to eliminate the effect of atmospheric turbulence on astronomical images. Air turbulence in the atmosphere are the reason that the light from a star reaches to slightly different long paths the telescope. Therefore produced in the focal plane of the telescope instead of a diffraction image by theoretically sharp interference pattern of a single spot ( the so-called speckles ). The resolving power of a telescope is therefore limited to the average diameter of this speckle pattern ( see also seeing ). The fine structure of the pattern changes completely in less than a second. This air turbulence produce the well-known flickering ( sparkle ) of the stars.
For the speckle interferometry many short-term exposures are taken instead of a long-exposure frame. The individual exposure time is between a few milliseconds to a maximum of one second. By non-linear means for each image to compensate for the influence of the Earth's atmosphere and undeteriorated to obtain a diffraction theoretically sharp image of the object (for example of the star ) is possible. This technique was used in the 1970's for the first time. Their application is limited by the necessarily short exposure times to brighter objects.
Important methods are:
- Knox -Thomson method ( 1973)
- Speckle - Masking Interferometry (G. Weigelt, 1977 and AW Lohmann, 1983)
- Multi Channel / Multi Frame Blind Deconvolution Image