A laser speckle sensor to measure the distribution of static torsion angles of twisted targets

A novel method for measuring the distribution of static torsion angles of twisted targets is presented. The method is based on Fourier transforming the scattered field in the direction perpendicular to the twist axis, while performing an imaging operation in the direction parallel to the axis. The Fourier transform serves to map the angular distribution of the scattered light field at the target into a linear displacement on a two-dimensional array image sensor placed in the Fourier plane. Measuring this displacement facilitates the determination of the angular displacement of the target. A cylindrical lens serves to image the closely spaced lateral positions of the target along the twist axis onto corresponding lines of the two dimensional image sensor. Thus, every single line of the image sensor measures the torsion angle of the corresponding surface position along the twist axis of the target. Experimentally, we measure the distribution of torsion angles in both uniform and non-uniform deformation zones. It is demonstrated both theoretically and experimentally that the measurements are insensitive to object shape and target distance if the image sensor is placed in the Fourier plane. A straightforward procedure to position the image sensor in the Fourier plane is presented. Furthermore, any transverse movement of the target will give rise to partial speckle decorrelation, but it will not affect the angular measurement. The method is insensitive to any wavelength change of the illuminating light source. No special surface treatment is required for surfaces giving rise to fully developed speckle.