Compensation Based Displacement Measurement Using Objective Laser Speckles

Objective laser speckles can be observed as intensity pattern from free space propagation of coherent light that is diffracted from a technically rough surface. Utilizing this pattern allows for very localized non-contacting measurement of surface displacement as well as strain, and carries a high potential for miniaturization. Existing objective speckle techniques suffer from decorrelation effects, dramatically decreasing the performance of such measurement techniques. In this paper we present a compensation based displacement measurement scheme, where we keep the measured speckle pattern displacement close to zero by actively following the measuring objects movement in a feedback operation. This converts the image sensor into a zero-detector, what considerably improves the speckle pattern correlation over a significantly extended measurement range.