Enhancing the measurement range of laser speckle systems

Abstract

Laser speckle-based systems provide sensitive measurements of displacement and stain. However, the typical saturation of correlation functions restricts the application of such techniques regarding their dynamic range. Therefore, this paper proposes using the extended zero-mean normalized cross-correlation (EZNCC) algorithm to automatically update the reference speckle image according to a programmable threshold, allowing for exceeding the decorrelation limit. Experiments investigated the effect of rigid body displacements and thermal strain at the light scattering surface (a ceramic container). The EZNCC yielded ~30 μm resolution over a continuous 1500 μm range for displacement measurements, and ~0.2°C resolution between 21.5°C and 61.5°C for the temperature analysis. As this method requires minor updates to the traditional correlation algorithms, the ENZCC does not require modifications to the optical setups, improving the dynamic range and ensuring the high sensitivity of speckle correlation systems in a feasible, straightforward approach.