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

Divergent laser illumination is commonly used in current designs of commerc ial electronic speckle pattern shearing interferometry (ESPSI) or shearogra phy, for qualitative non-destructive testing (NDT) of material defects. The growing demand for quantitative out-of-plane (OOP) and more recently in-pl ane (IP) ESPSI, is determining the quality of optical system design and ana lysis software. However, little attention is currently being given to under standing, 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 a re measured by comparing divergent object illumination with collimated illu mination, with respect to illumination angle, lateral shear and shearing di rection. Results indicate that the magnitude of the relative error increase s by approximately a power function as the distance from the illumination s ource decreases.