Robust sub-micrometer displacement measurement using dual wavelength speckle correlation

The unique characteristics of speckle correlation techniques including simple setup and fast, non-contact, high resolution measurement capability offer great potential for industrial applications. Robustness is an important requirement for industrial applications, which limits the application of many common techniques such as interferometric or photographic measurements, especially in mechanical workshops. This paper introduces an innovative technique for displacement measurement using speckle photography that is robust to disturbances, imaging errors, and does not require a large number of database patterns for calibration. It uses the relative correlation of the speckle patterns generated by two parallel, overlapping laser beams with an identical spot size and different wavelengths for relative displacement measurement in sub-micrometer order, and requires only one reference pattern that is updated frequently during the measurement process. The method is demonstrated over 200 µm range and is extendable to longer ranges.