Three dimensional(3D) displacements, which can be translated further into 3D strain, are key parameters for design, manufacturing and quality control. Using different optical setups, phase-shift methods, and algorithms, several different 3D electronic speckle pattern interferometry(ESPI) systems for displacement and strain measurements have been achieved and commercialized. This paper provides a review of the recent developments in ESPI systems for 3D displacement and strain measurement. After an overview of the fundamentals of ESPI theory, temporal phase-shift, and spatial phase-shift techniques, 3D deformation measurements by the temporal phase-shift ESPI system, which is suited well for static measurement, and by the spatial phase-shift ESPI system, which is particularly useful for dynamic measurement, are discussed. For each method, the basic theory, a brief derivation and different optical layouts are presented. The state of art application, potential and limitation of the ESPI systems are shown and demonstrated.

Review of electronic speckle pattern interferometry (ESPI) for three dimensional displacement measurement

Digital shearography has demonstrated great potential in direct strain measurement and, thus, has become an industrial tool for nondestructive testing (NDT), especially for NDT of delaminations and detection of impact damage in composite materials such as carbon fiber reinforced plastics and honeycomb structures. The increasing demand for high measurement sensitivity has led to the need for real-time monitoring of a digital shearographic phase map. Phase maps can be generated by applying a temporal, or spatial, phase shift technique. The temporal phase shift technique is simpler and more reliable for industry applications and, thus, has widely been utilized in practical shearographic inspection systems. This paper presents a review of the temporal phase shift digital shearography method with different algorithms and the possibility for real-time monitoring of phase maps for NDT. Quantitative and real-time monitoring of full-field strain information, using different algorithms, is presented. The potentials and limitations for each algorithm are discussed and demonstrated through examples of shearographic testing.

https://www.spiedigitallibrary.org/journals/optical-engineering/volume-52/issue-10/101902/Real-time-monitoring-of-phase-maps-of-digital-shearography/10.1117/1.OE.52.10.101902.pdf

Real-time monitoring of phase maps of digital shearography

This article is concerned with frequency filtering for electronic speckle pattern interferometry wrapped phase patterns. We propose a robust localized Fourier transform filter which is an extension of the root filtering method (RFM). We improve the RFM from a simple technical process and a filter function in the frequency domain. In our method, the proposed filter function is taken as the power spectrum of the convolution of an image and a Gaussian function to the power α. We test the proposed method on two computer-simulated wrapped phase fringe patterns and one experimentally obtained wrapped phase pattern, and compare our models with the widely used, well-known RFM and windowed Fourier filtering (WFF). The experimental results have demonstrated that our localized Fourier transform filter outperforms the RFM and is comparable to WFF. Our method depends on fewer parameters, as compared with WFF, and can achieve a better balance between the computational complexity and the filtered results.

Localized Fourier transform filter for noise removal in electronic speckle pattern interferometry wrapped phase patterns

Speckle fringe patterns of electronic speckle pattern interferometry (ESPI) are full of high-level speckle noise and are mainly processed by phase-shifting methods that normally require three speckle fringe patterns or more. The author proposes a novel method, the contoured correlation-fringe-pattern (CCFP) method, by which speckle-noise-free fringe patterns can be generated for ESPI. The application of this novel method is extended to the phase-shifting or phase-stepping for ESPI after its improvement. It generates speckle-noise-free phase fringes and remains valid for single phase-step condition, thus eliminates the two main disadvantages of the phase-shifting (stepping) methods of ESPI.

Single-phase-step method with contoured correlation fringe patterns for ESPI.

A comparative study of phase-shifting algorithms in digital speckle pattern interferometry

A set of innovative phase-shifting algorithms developed to facilitate metrology based on electronic speckle pattern interferometry (ESPI) are presented. The theory of a phase-shifting algorithm, called a (5,1) algorithm, that takes five phase-shifted intensity maps before a specimen is deformed and one intensity map after a specimen is deformed is presented first. Because a high-speed camera can be used to record the dynamic image of the specimen, this newly developed algorithm has the potential to retain the phase-shifting capability for ESPI in dynamic measurements. Also shown is an algorithm called a (1,5) algorithm that takes five phase-shifted intensity maps after the specimen is deformed. In addition, a direct-correlation algorithm was integrated with these newly developed (5,1) or (1,5) algorithms to form DC-(5,1) and DC-(1,5) algorithms, which are shown to improve significantly the quality of the phase maps. The theoretical and experimental aspects of these two newly developed techniques, which can extend ESPI to areas such as high-speed dynamic measurements, are examined in detail.

http://ntur.lib.ntu.edu.tw/bitstream/246246/119663/1/13.pdf

Phase-shifting algorithms for electronic speckle pattern interferometry.

An optical wavefront analyzer based on the phase-shearing interferometry technique for measuring a wavefront of a light beam is disclosed. The analyzer includes a single phase shifter, a driving device, a pattern receiving device, and a phase reconstructing device. A method of measuring a wavefront of a light beam is also provided for solving the problems of two-axial focusing and phase inconsistency encountered the prior art by employing phase-shearing and phase-shifting techniques to improve the method of optical wavefront mesurement. A non-iterative, path-independent unwrapping methodology is used to deal with a complicated wavefront.

Optical wavefront analyzer and method using a single wave shifter

Diffractive optical elements (DOE) have been widely used in both scientific and industrial applications. Optical wavefront measurements are one of the most important methodologies in verifying the performance of DOEs. Due to its non-destructive nature, ease of implementation, and relative short operation time, optical interferometry-based systems for wavefront measurements remain popular. The advantages and drawbacks of a non-common path and common path interferometry technique are examined first within this article.

Gym-Bin Yeh

Papers

Phase-shifting algorithms for electronic speckle pattern interferometry.

Optical wavefront analyzer and method using a single wave shifter

New methodology for measuring highly aberrated wavefronts induced by diffractive optical elements