Computer-aided electronic speckle pattern interferometry (ESPI). Deformation analysis by fringe manipulation

When performing deformation measurements by interferometric techniques, one component of displacement is measured without direction. In this paper we present a simple method for obtaining both the direction and magnitude of displacement by phase-stepping the reference beam in an electronic speckle pattern interferometry (ESPI) camera. The interferograms are digitized in real time and analysis is performed by a personal computer (PC). The algorithm for obtaining the displacement data only requires four primary interferograms: two for the reference state of the object and two for the deformed state. By suitable averaging over neighbouring pixels, displacement slopes of up to 15λ across the TV target can be measured with an accuracy of ∼λ/15, where λ is the wavelength of the laser.