Enhancing the robustness of ESPI measurements using digital image correlation
Electronic Speckle Pattern Interferometry (ESPI) provides a sensitive technique for measuring surface deformations. The technique involves comparison of the speckle phase angles within surface images measured before and after material deformation. This phase angle comparison requires that the speckle positions be consistent in all images. A lateral shift between images by just one pixel substantially degrades ESPI measurements, while a shift of two or more pixels typically causes complete
... uses complete speckle decorrelation and compromises the measurement entirely. To prevent such lateral motions, the specimen and the optical system must be rigidly fixed. This requirement typically prevents use of the ESPI method in applications outside laboratories or where it is necessary to remove the specimen from the optical setup between ESPI measurements. Here, Digital Image Correlation (DIC) is used to track speckle motion caused by specimen displacement between ESPI measurements. The measured images can then be mathematically shifted to restore the original speckle locations, thereby recorrelating the ESPI measurements. Examples are presented where ESPI measurements are successfully made with specimen shifts in excess of 60 pixels. A novel ESPI measurement technique where the specimen is removed in between ESPI measurements is also developed and validated.