Wavefront divergence: A source of error in quantified speckle shearing data

Abstract

Divergent laser illumination is commonly used in current designs of commercial electronic speckle pattern shearing interferometry (ESPSI) or shearography, for qualitative non-destructive testing (NDT) of material defects. The growing demand for quantitative out-of-plane (OOP) and more recently in-plane (IP) ESPSI, is determining the quality of optical system design and analysis software. However, little attention is currently being given to understanding, quantifying and compensating for the numerous error sources. Data describing the measurement inaccuracy due to the divergence of the object illumination wavefront for an OOP interferometer is presented. The errors are measured by comparing divergent object illumination with collimated illumination, with respect to illumination angle, lateral shear and shearing direction. Results indicate that the magnitude of the relative error increases by approximately a power function as the distance from the illumination source decreases.