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Defocused speckle imaging for remote surface motion measurements

University of British Columbia

Defocused Speckle Imaging (DSI) is an optical method where a laser source illuminates a rough object surface, and a defocused camera records the scattered interference speckle pattern that characterizes the surface. The speckle pattern appears to move if the object displaces or rotates. Speckle motion tracking thus enables non-contact surface motion measurements. The observed speckle motion magnitude increases with distance, which makes DSI particularly attractive for remote measurements. As the camera focal plane position controls the effective sampling distance, measurement sensitivity can be tuned by simple camera defocus adjustment. However, despite its great potential, DSI has not been previously utilized for measurements at large distances. This is because the observed speckle motions are influenced by both surface displacements and rotations, and because the measurement sensitivity depends on geometric parameters that are challenging to extract in field conditions. This thesis first presents a geometric Speckle Hemisphere Model to allow easy visualization of the speckle phenomenon. The thesis next proposes an optimum approach to separate linear and rotational speckle motion components using a simple combination of two cameras focused at different distances. Finally, the thesis presents a measurement self-calibration principle by combining multi-wavelength laser illumination with speckle pattern diffraction analysis to determine geometric distance and angle parameters directly from the captured speckle patterns. A set of experimental measurements validates the Speckle Hemisphere Model and illustrates the general sensitivity characteristics of DSI; at low sampling distances, measurement is mostly sensitive to in-plane displacements, whereas large sampling distances have much higher relative tilt sensitivity. Multiaxial motion experiments performed at 4–16 meters demonstrate the method’s suitability for large distances. The self-calibration principle validation shows capability to determine sampling distances and oblique surface angles up to 45˚ at high accuracy (1.7% and 0.7˚). The final study presents self-calibrated surface motion measurements performed at a 30.7-meter distance, with surface angles of 2.5–7.4˚. The dual-camera configuration can effectively determine the sampling distances (6.4%) and the surface angles (0.2˚). The speckle motions resulting from microscopic in-plane displacements (400μm) and very fine tilt motions (0.003˚) are tracked robustly at high accuracy (6.0%).

Text

2021-02-12

Attribution-NonCommercial-NoDerivatives 4.0 International

http://hdl.handle.net/2429/77287

Mechanical Engineering

University of British Columbia

UBCV

http://creativecommons.org/licenses/by-nc-nd/4.0/