A simple interferometric linear position transducer

When an optically rough surface is illuminated by coherent light, the scattered light takes the form of a speckle pattern. If two such patterns interfere, the intensity at any point in the resultant light field is found to depend linearly on the cosine of the relative phase. A simple photodetection circuit allows precise measurement of the position of a target to be made. Two arrangements are described offering out-of-plane and in-plane measurement.