When a coherent light source illuminates a rough surface, the scattered light waves interfere to form a random granular pattern known as a speckle pattern. By capturing this pattern using a high-resolution camera, and comparing it with a reference pattern (either from a known surface or at a different state of the sample), phase differences can be analyzed to extract precise measurements of surface features. This is known as Digital Speckle Pattern Interferometry (DSPI) or Electronic Speckle Pattern Interferometry (ESPI).
