Abstract: | When mutally coherent light beams overlap on an optically rough surface, the scattered light fields or speckle patterns interfere. The intensity at any point depends on the phase difference between the light fields. By matching the sensitive area of a photodetector to speckle size, an electronic signal is obtained which can be used to monitor in-plane rigid body translations of the surface with interferometric precision. Two reliable methods of obtaining directional sensitivity based on polarization properties are described and it is shown that depolarization effects due to rough surfaces, do not cause any difficulty. Intensitivity to motion orthogonal to the plane of the apparatus is demonstrated. |