Sijin Wu

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

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.

Real-time monitoring of phase maps of digital shearography

Abstract. 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.

Spatiotemporal three-dimensional phase unwrapping in digital speckle pattern interferometry

We propose a hybrid spatiotemporal three-dimensional phase unwrapping algorithm for use in digital speckle pattern interferometry (DSPI). The feature of the proposed algorithm is the integration of one-dimensional temporal and two-dimensional spatial phase unwrapping algorithms. By demodulating the phase on a single reference point or multiple reference points using temporal phase unwrapping and on each separated phase map region using spatial phase unwrapping, the DSPI with the spatiotemporal three-dimensional phase unwrapping algorithm can realize the measurement of dynamic absolute displacements and the determination of abrupt phase changes which are usually caused by object discontinuities. We demonstrate that the presented algorithm can overcome the drawbacks of the traditional spatial and temporal phase unwrapping algorithms.

Stroboscopic digital shearographic system for vibration analysis of large-area object

Digital shearography has become a competitive tool for vibration analysis due to its capability of full-field, high resolution, and high-speed measurements. Many stroboscopic shearographic systems have been developed in recent years. However, they are only suitable for vibration analysis of a small area. This paper presents a shearographic system that was developed to carry out the vibration analysis of a large-area object whose area is 1 m2 or even bigger. The system can use both time-average and stroboscopic illumination methods to analyze the forced vibration distribution. The setup of the system is introduced and experimental results are presented.

Synchronous measurement of three-dimensional deformations by multicamera digital speckle patterns interferometry

Abstract. We present a spatial phase-shift digital speckle patterns interferometry (SPS-DSPI) system with the capability of measuring three-dimensional (3-D) deformations under a dynamic loading condition simultaneously using multiple cameras. The Fourier transform method is utilized to calculate the phase difference. The consistency of different cameras is achieved using digital image correlation (DIC) technology. Calibration and calculation programs are compiled to make sure each subset on the measuring surface is uniform. SPS-DSPI and DIC techniques are combined to provide a direct measurement of the 3-D deformation of the entire surface area simultaneously. The theory, application result, and validation experiment are presented.

Investigation of hidden diffuse surfaces using phase-shifting endoscopic digital speckle pattern interferometry

Digital speckle pattern interferometry (DSPI) is an important optical tool which is widely used in many sophisticated applications. However, a traditional DSPI system can only be used to investigate the outer surfaces which can be easily observed. Therefore, an endoscopic DSPI was proposed to detect the internal and hidden surfaces. It has a rigid or flexible endoscopic tube to allow a hidden surface being clearly imaged. A fiber-optics-based setup makes the proposed DSPI system compact and robust. The temporal phase-shifting technique is used to help precise extraction of phase distributions from speckle patterns.

Precise Detection of Wrist Pulse Using Digital Speckle Pattern Interferometry

Pulse diagnosis is one of the four diagnostic methods of traditional Chinese medicine. However it suffers from the lack of objective and efficient detection method. We propose a noncontact optical method to detect human wrist pulse, aiming at the precise determination of the temporal and spatial distributions of pulse. The method uses the spatial-carrier digital speckle pattern interferometry (DSPI) to measure the micro/nanoscale skin displacement dynamically. Significant improvements in DSPI measurement have been made to allow the DSPI to detect the comprehensive information of the arterial pulsation at locations of Cun, Guan, and Chi. The experimental results prove that the spatiotemporal distributions of pulse can be obtained by the proposed method. The obtained data can be further used to describe most of the pulse parameters such as rate, rhythm, depth, length, width, and contour.

Spatial phase-shift dual-beam speckle interferometry

The spatial phase-shift technique has been successfully applied to an out-of-plane speckle interferometry system. Its application to a pure in-plane sensitive system has not been reported yet. This paper presents a novel optical configuration that enables the application of the spatial phase-shift technique to pure in-plane sensitive dual-beam speckle interferometry. The new spatial phase-shift dual-beam speckle interferometry (SPS-DBSP) uses a dual-beam in-plane electronic speckle pattern interferometry configuration with individual aperture shears, avoiding the interference in the object plane by the use of a low-coherence source, and different optical paths. The measured object is illuminated by two incoherent beams that are generated by a delay line, which is larger than the coherence length of the laser. The two beams reflected from the object surface interfere with each other at the CCD plane because of different optical paths. A spatial phase shift is introduced by the angle between the two apertures when they are mapped to the same optical axis. The phase of the in-plane deformation can directly be extracted from the speckle patterns by the Fourier transform method. The capability of SPS-DBSI is demonstrated by theoretical discussion as well as experiments.

Spatial carrier color digital speckle pattern interferometry for absolute three-dimensional deformation measurement

Abstract. It is difficult to measure absolute three-dimensional deformation using traditional digital speckle pattern interferometry (DSPI) when the boundary condition of an object being tested is not exactly given. In practical applications, the boundary condition cannot always be specifically provided, limiting the use of DSPI in real-world applications. To tackle this problem, a DSPI system that is integrated by the spatial carrier method and a color camera has been established. Four phase maps are obtained simultaneously by spatial carrier color-digital speckle pattern interferometry using four speckle interferometers with different illumination directions. One out-of-plane and two in-plane absolute deformations can be acquired simultaneously without knowing the boundary conditions using the absolute deformation extraction algorithm based on four phase maps. Finally, the system is proved by experimental results through measurement of the deformation of a flat aluminum plate with a groove.

Dynamic measurement of first-order spatial derivatives of deformations by digital shearography

This paper presents a simple spatial phase shift shearography based on the Michelson interferometer. A novel digital shearography set-up with a large angle of view, which is based on a 4f system is demonstrated. In the system, the Michelson interferometer is used as a shearing device to generate a shearing distance by tilting a small angle in one of the two mirrors. In fact spherical wave fronts become plane after going through the first lens in 4f system. Tilting the mirror in the Michelson interferometer also generates spatial carrier frequency. Sinusoid fitting method is applied to evaluate the phase. This system can generate a phase map of shearography by using only a single image. The effects of shearing angle, the choice of algorithm and comparison result are discussed in detail. The theory and the application are presented.