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Laser speckle imaging for non-destructive analysis

Kalomoiris Filippos

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Summary

When coherent light illuminates a diffuse object, it produces a random interference effect known as a speckle pattern. If there is movement in the object either by itself (blood flow) or by a stimulation (acoustic, thermal, impact) then the speckles fluctuate in intensity. These fluctuations can provide information about the movement. Laser Speckle Imaging is a non-destructive, non-contact, full-field technology that can access these information and build a movement map of the object. This technique has in recent times become a powerful tool for scientific and industrial analysis in many different fields. Its applications range from non-contact surface analysis and archeology to biomedical science. The work that is presented in this thesis is the design of a Laser Speckle Imaging device that will be able to do non-contact analysis in impact and acoustic stimulated surfaces and make spatial and temporal movement mapping with pseudocolors. The implementation employs a sensitive camera, a coherent light source, a light expander, a stimulator and the surface we want to analyze. The light source with the expander illuminates the surface and speckle pattern is produced. This pattern is changed by the stimulator and measurements have been recorded with the camera before and during the stimulation. Measurements can be analyzed in two ways. Either analysing the temporal contrast of some images in the same state or by analysing the spatial contrast of one image. Temporal contrast analysis calculates a speckle contrast image by computing the contrast for each pixel across a sequence of images developed. Spatial contrast analysis implements a square structuring element of pixels to calculate the local speckle contrast. The Laser Speckle Imaging with spatial contrast analysis offers high temporal resolution at the expense of spatial resolution which is offered by the temporal contrast analysis. Our implemantation can do both analyzes in order to achieve both spatial and tempooral resolution. It also quantifies the stimulation's intensity with no inforamtion about the stimulator. These features make our approach suitable in demanding laser speckle imaging applications, such as non-destructive analysis and noninvasive diagnosis.

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