Quasi single-frame measurements with phase-stepped ESPI

Abstract

Phase-stepped ESPI measurement techniques typically require the acquisition of sets of speckle images before and after specimen deformation. This process limits the use of the technique to quasi-static specimens. A novel technique is proposed here where speckle images are measured in a continuous phase-stepped sequence and where interpolation of nearby images is used to infer all four phase-stepped images corresponding to each measured image. This process effectively creates a single-frame ESPI measurement technique that is suitable for observing moving objects. Small phase changes measured by a high-speed camera are observed incrementally between successive images, with cumulative phase changes determined by summation. This approach avoids the need for phase unwrapping, and so retains an absolute zero phase datum. The sequence of small phase changes also preserves good correlation between successive measurements, thus enabling successful cumulative phase measurements over large specimen displacements, even into the mm range. The phase visibility at each pixel is available within every measured image, thereby enabling the use of pixel repair strategies if desired.