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 tor design, manufacturing and quality control. Using different optical setups, phase-shift methods, and algorithms, several different 3D electronic speckle pattern interferometry（ESPl） 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 ESP! 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.

Measurement of the surface morphology of plasma facing components on the EAST tokamak by a laser speckle interferometry approach

The laser speckle interferometry approach provides the possibility of an in situ optical noncontacted measurement for the surface morphology of plasma facing components(PFCs),and the reconstruction image of the PFC surface morphology is computed by a numerical model based on a phase unwrapping algorithm.A remote speckle interferometry measurement at a distance of three meters for real divertor tiles retired from EAST was carried out in the laboratory to simulate a real detection condition on EAST.The preliminary surface morphology of the divertor tiles was well reproduced by the reconstructed geometric image.The feasibility and reliability of this approach for the real-time measurement of PFCs have been demonstrated.

Measurement of in-plane deformations of microsystems by digital holography and speckle interferometry(Invited Paper)

The reliability of microsystems is an important issue and for their quality inspection, it is necessary to know the displacements or deformations due to the applied mechanical, thermal, or electrostatic loads. We show how interferometrical techniques like digital holography and speckle interferometry can be used for the measurement of in plane deformations of microsystems with nanometric accuracy and we give a description of the measurement uncertainties.

Measuring Vibration of Body with Speckle Interferometry

Speckle interferometry is an efficient method to analyze a vibration. In certain conditions, this technique has some outstanding advantage, and need not strict shock--proof condition, compared with the holographic method for measuring vibration. Therefore,it is suitable to analyze a vibration with a large amplitude.Real-time interferometry is a rapid and simple method for measuring vibration of a body, gives speckle pattern containing amplitude distribution of body-surface. By means of time-averaged method, the speckle pattern is recorded in Fourier transform plane, or vibration lines are seen directly with eyes, so as to analyze efficiently amplitude, phase, and model of a vibration. This paper deduces the intensity distribution function with real-time method, and gives experimental demonstration of vibration body-the vibration lines with different frequencies.

Diagnosis of the surface morphology of laser-ablated materials using the weightedDCT approach in laser speckle interferometry for application to plasma facing materials

To implement on-line, real-time monitoring for the surface morphology of Plasma-Facing Materials(PFMs) in tokamak, we developed a Laser Speckle Interferometry measurement approach. A laser ablation method was used to simulate the erosion process during Plasma-Wall Interactions in a tokamak. In the present investigation, we evaluated the results of laser ablation morphology changes on the surface of Mo material reconstructed by four different approaches(Flood-fill, Quality-guided, Discrete Cosine Transform(DCT) and Weighted-DCT). The morphology results measured by the weighted-DCT approach are very close to the measurement results from confocal microscopy with an average error rate within 7%. It is verified that the weighted-DCT algorithm has high accuracy and can efficiently reduce the influence of noise pollution coming from laser ablation, which is used as a proxy for erosion from plasma wall interaction. Additionally, the CPU computer time has been shortened. This is of great significance for the real-time monitoring of PFMs’ morphology in the Experimental Advanced Superconducting Tokamak(EAST) in the future.

New speckle pattern interferometry for precise in situ deformation measurements

A new electronic speckle pattern interferometry method is proposed to realize in situ deformation measurements.The feature of the method is the combination of a high-speed camera and multiple laser Doppler vibrometers(LDVs)for synchronous measurements.The high-speed camera is used to record and select effective interferograms,while the LDVs are used to measure the rigid body displacement caused by vibrations.A series of effective interferograms with known shifted phase values are obtained to calculate the deformation phase.The experimental results show that the method performs well in measuring static and dynamic deformations with high accuracy in vibrating environments.

Speckle reduction by selective spatial-domain mask in digital holography

An improved method of using a selective spatial-domain mask to reduce speckle noise in digital holography is proposed.The sub-holograms are obtained from the original hologram filtered by the binary masks including a shifting aperture for being reconstructed. Normally, the speckle patterns of these sub-reconstructed images are different. The speckle intensity of the final reconstructed image is suppressed by averaging the favorable sub-reconstructed images which are selected based on the most optimal pixel intensity sub-range in the sub-holograms. Compared with the conventional spatial-domain mask method, the proposed method not only reduces the speckle noise more effectively with fewer sub-reconstructed images,but also reduces the redundant information used in the reconstruction process.

Possibility to break through limitation of measurement range in dual-wavelength digital holography

By using the beat frequency technique,the dual-wavelength digital holography(DWDH)can greatly increase the measurement range of the system.However,the beat frequency technique has a limitation in measurement range.The measurement range is not larger than a synthetic wavelength.Here,to break through this limitation,we propose a novel DWDH method based on the constrained underdetermined equations,which consists of three parts:(i)prove that the constrained underdetermined equation has a unique integer solution,(ii)design an algorithm to search for the unique integer solution,(iii)introduce a third wavelength into the DWDH system,and design a corresponding algorithm to enhance the anti-noise performance of DWDH.As far as we know,it is the first time that we have discovered that the problem of DWDH can belong in a problem of contained underdetermined equations,and it is also the first time that we have given the mathematical proof for breaking through the limitation of the measurement range.A series of results is shown to test the theory and the corresponding algorithms.More importantly,since the principle of proposed DWDH is based on basic mathematical principles,it can be further extended to various fields,such as dual-wavelength microwave imaging and dual-wavelength coherent diffraction imaging.

Phase-shifting digital holography in image reconstruction

A phase-shifting digital holography scheme developed to investigate internal defects in artworks is described. Phase-shifting is utilized to obtain a clear reconstructed object wave from a rough surface texture. A reverse-transform algorithm is employed to reconstruct the object wave on its original position of unknown distance or the imaging position from the object wave information on the holographic plane. To get the clearest reconstruction the exact registration of the unknown distance is determined by applying the intensity sum as the auto-focusing function, The spatial resolution of the reconstruction image is also investigated for a variety of affecting factors. Laboratory results of reconstruction images under deformation are presented.

Holography and speckle in phase-shifting digital holography(Invited Paper)

The correlation properties of the optical field diffusely reflected from a rough surface under coherent illumination are analyzed numerically. The cross-correlations of the complex amplitudes and the intensities before and after translation and/or tilt of the surface are calculated at an arbitrary observation plane in three-dimensional （3D） space. The results provide us with the 3D distributions of phase changes and speckle displacement that lead to the distributions measured by holographic interferometry and speckle relationships and physical interpretation of them are and signal-to-noise ratio of the displacement to be correlation technique. Comparisons with analytical also discussed.

Measurement of out-of-plane deformation of curved objects with digital speckle pattern interferometry

Digital speckle pattern interferometry （DSPI） is a high-precision deformation t technique for planar objects. However, for curved objects, the three-dimensional （3D） shape information is needed in order to obtain correct deformation measurement in DSPI. Thus, combined shape and deformation measurement techniques of DSPI have been proposed. However, the current techniques are either complex in setup or complicated in operation. Furthermore, the operations of some techniques are too slow for real-time measurement. In this work, we propose a DSPI technique for both 3D shape and out-of-plane deformation measurement. Compared with current techniques, the proposed technique is simple in both setup and operation and is capable of fast deformation measurement. Theoretical analysis and experiments are performed. For a cylinder surface with an arch height of 9 mm, the error of out-of-plane deformation measurement is less than 0.15 μm. The effectiveness of the proposed scheme is verified.

Simultaneous 2D in-plane deformation measurement using electronic speckle pattern interferometry with double phase modulations

Electronic speckle pattern interferometry（ESPI） and digital speckle pattern interferometry are wellestablished non-contact measurement methods. They have been widely used to carry out precise deformation mapping. However, the simultaneous two-dimensional（2D） or three-dimensional（3D） deformation measurements using ESPI with phase shifting usually involve complicated and slow equipment. In this Letter, we solve these issues by proposing a modified ESPI system based on double phase modulations with only one laser and one camera. In-plane normal and shear strains are obtained with good quality. This system can also be developed to measure 3D deformation, and it has the potential to carry out faster measurements with a highspeed camera.

LASER SPECKLE MOVEMENT CAUSED BY SURFACE DEFORMATION AND LIGHT WAVELENGTH CHANGE

When a surface is illuminated by laser,the field of diffusing reflective light can be described by Fresnel-Kirchhoff integration in the region of Fresnel.If the correlation,before and after the change of intensity field,is analysed by statistical method,three governing equations for variations of intensity field and speckle movement formulas can be obtained.In these equations,the surface motion,deformation and wavelength change are all considered to cause the speckle movement.

A sequence pulse counting method for shape measurement in dual-beam speckle interferometry

A temporal approach to fast shape measurement is presented.In principle,the rotational object method is used in combination with the sequence pulse counting method (SPCM) to determine the height of the object through calculating the related phase.Two specimens are tested to demonstrate the validity of the approach.One is an object covered by a Chinese character (tea) with a height variety of 0.3 mm,and the other is an object surface with a relatively large fluctuation of 3.5 mm.The experimental results are compared with mechanical measurements.An axis shifting method is also proposed to determine shapes with relatively large fluctuations.Effects of such parameters on the height measurement as incident angle of the dual light beams,tilting angle of the object,and azimuth angle of the measured point are discussed as well.

Measuring the Object Deformation by Real Time Holographic Interferometry with Automatically Calculating Hologram

Used Ar + laser as a light source,BSO(Bi 12 SiO 20 ) crystal as a hologram recording material,CCD camera as a detector,and a holographic setup controlled by a computer,we have realized real time holographic interferometry in terms of 3-interfe-rogram method.The deformation of a plate is measured under a pressure.

POLARIZED SPECKLE-PHOTOELASTICITY FOR 3-D STRESS ANALYSIS

Combining speckle interferometry with photoelasticity,the absolute retardation fringes in 3-D model are obtained by interference of the polarized rays reflected from the two surfaces of a sandwiched plate inside the model.This method can avoid the troubles introduced by Poisson's ratio μ=0.5 and stress-optical constants ratio A/B=-2 of the photoelastic material in a stress-frozenstate,and yield the re- sults which provide necessary supplementary information for the completely-experimental solution of 3-D stress state.A comparison of experimental results with theoretical ones shoves the feasibility of the method proposed in the present paper.

Homomorphic partial differential equation filtering method for electronic speckle pattern interferometry fringes based on fringe density

Noise reduction is one of the most exciting problems in electronic speckle pattern interferometry. We present a homomorphic partial differential equation filtering method for interferometry fringe patterns. The diffusion speed of the equation is determined based on the fringe density. We test the new method on the computer-simulated fringe pattern and experimentally obtain the fringe pattern, and evaluate its filtering performance. The qualitative and quantitative analysis shows that this technique can filter off the additive and multiplicative noise of the fringe patterns effectively, and avoid blurring high-density fringe. It is more capable of improving the quality of fringe patterns than the classical filtering methods.

Variational denoising method for electronic speckle pattern interferometry

Traditional speckle fringe patterns by electronic speckle pattern interferometry （ESPI） are inherently noisy and of limited visibility, so denoising is the key problem in ESPI. We present the variational denoising method for ESPI. This method transforms the image denosing to minimizing an appropriate penalized energy function and solving a partial differential equation. We test the proposed method on computer- simulated and experimental speckle correlation fringes, respectively. The results show that this technique is capable of significantly improving the quality of fringe patterns. It works well as a pre-processing for the fringe patterns by ESPI.

Speckle noise reduction in digital holography by use of multiple polarization holograms

A novel speckle reduction technique for digital holography is proposed. Multiple off-axis holograms are recorded using a circularly polarized illumination beam and a rotating linearly polarized reference beam. The speckle noise in the reconstructed images is suppressed by averaging these fields. We demonstrate the effectiveness of this technique experimentally and conduct additional statistical evaluation.

Speckle noise reduction in digital holography with spatial light modulator and nonlocal means algorithm

An integrated method based on optical and digital image processing is presented to suppress speckle in digital holography. A spatial light modulator is adopted to introduce random phases to the illuminating beam. Multiple holograms are reconstructed and superimposed, and the intensity is averaged to smooth the noise. The adaptive algorithm based on the nonlocal means is designed to further suppress the speckle. The presented method is compared with other methods reduction is improved, and the proposed method is effective The experimental results show that speckle and feasible.