Sagnac Interferometer Based Digital Shearography for Simultaneous Dual Sensitivity Measurement

Abstract

This paper presents a single camera digital shearography system with dual sensitivity to enable simultaneous measurement of minor and large shearograms in a single field measurement. Digital shearography has many benefits, such as its non-contact measurement, high sensitivity and resistance to turbulence. It has been successfully used for testing of minor deformation gradients in nondestructive testing (NDT). Most digital shearography systems can only work in a single measurement field. As a result, minor shearograms may be overlooked, resulting in incomplete detection in a full-field measurement. The application must allow for multiple measurements in different fields of view or overloading to reveal small defects. The former increases costs, while the latter can result in huge surface deformation that may mask important information. A novel dual sensitivity shearographic system that can simultaneously identify minor and large shearograms in a single field measurement has been developed. The setup is based on the modified Sagnac Interferometer, which uses a polarized Michelson interferometer as a substitute for one of its three mirrors to create additional shearing channels. Two pairs of orthogonally polarized waves carry the large and small field of view information individually without cross-interference on the target screen. The carrier frequency spatial phase shift method is adopted to separate the signal on the frequency domain. The system's performance for dynamic imaging of an object with defects was experimentally investigated. Theory derivation and description of the setup are shown in detail in this paper.