A method for measurement of continuous displacements and deformations is presented. The method may be used in e.g. speckle interferometry, moire, structured light and other optical measurement methods based on evaluation of phase changes. The initial random phase of the interference pattern is either known or evaluated before displacement take place using e.g. phase shifting. The changing phase thereafter is achieved only from one image at a time by a least square algorithm. The technique can be used for measuring shape deformations such as transients and other dynamic events, heat expansion as well as other phenomena where it is difficult to accomplish phase shifting during deformation.
An improved method for measurement of continuous displacement and deformations with digital speckle pattern interferometry is presented. The initial random phase of the speckle pattern is evaluated by having many phase-shifting steps before the deformation. By this way the accuracy of the initial phase estimation can be increased and the handling of the image noise is improved. This makes it also possible to use the phase stepped speckle patterns as references for comparison with the speckle patterns of the deformed object, thereby increasing the reliability and accuracy of the phase estimations of the deformed patterns. The technique can be used for measuring deformations such as transients and other dynamic events, heat expansion as well as other phenomena where it is difficult to accomplish phase shifting during deformation.
The heat expansion of the solder joint on a PCB board has been investigated using speckle interferometry. Set-ups for measuring in-plane and out-of-plane displacements have been separately used. The component was stepwise heated by increasing the working current. During the heating the temperature was measured by a thermocouple. For each step, the heating was halted until thermal equilibrium occurred, then four phase-shifted interferograms were recorded for each temperature Tn. In this way the deformation between two temperatures Tn+1 and Tn were evaluated. The deformation was measured in different directions and the joint solder's strains were calculated.
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