Measurement of curvature distribution using digital speckle three-shearing aperture interferometry

Digital speckle three-shearing-aperture interferometry used for the measurement of curvature distribution fields of a deformation object is proposed for the first time. In this method, pure curvature distribution fringes without containing slope distribution fringes can be obtained by using digital speckle three-shearing-aperture interferometry. Two specklegrams, one before and the other after object deformation, are recorded by a CCD camera and stored in a computer. A 2-D fast Fourier transform (FFT) and a 2-D inverse FFT (IFFT) are performed on the two specklegrams. The subtraction of the transformed specklegrams will produce curvature distribution fringes, and the speckle noises in curvature distribution fringes can be removed after these fringes pass through low-pass filtering. Results from theory and experiments are in good agreement.     

Speckle shearography using a multiband light source

Applications for optical metrology usually use lasers as light sources, because of the excellent spatial and temporal coherence of the emitted light. By comparison, the demands of speckle shearography concerning the coherence of the light source are low. This enables certain white-light sources to be an option.

In this paper, the feasibility of low coherence shearography is shown. For this purpose an experimental setup is designed, composed of a mercury arc lamp, a spatial filter and a Michelson interferometer. With respect to speckle shearography, important characteristics of the light source are described and the mercury arc lamp is shown to be suitable. Finally, some experimental investigations of an object under load are presented.     

用数字滤波消除散斑条纹图噪声

本文讨论了散斑摄影条纹图的噪声来源及模型,设计合适的数字滤波器.采取正交实验手段,全面分析了噪声、条纹对比度和密度对条纹方向及间距测量精度的影响.     

Phase retrieval of speckle fringe pattern with carriers using 2D wavelet transform

A novel two-dimensional (2D) Gabor continuous wavelet transform (CWT) for phase retrieval and fringe filtering of speckle fringe patterns with spatial carriers is proposed. Theoretical analysis of 2D Gabor CWT is presented and results are compared with advanced fan 2D CWT using both the computer simulated and experimental speckle fringe patterns. It is shown that noise reduction by 2D Gabor CWT demonstrates better results than that of the advanced fan 2D CWT. Two-dimensional Gabor CWT is also compared with 2D Fourier transform and results show that 2D Gabor CWT algorithm has better noise immunity.     

Development of Matlab filtering techniques in digital speckle pattern interferometry

In interferometric fringe pattern analysis, specular and speckle fringe patterns are the two main divisions. While specular fringes are characterized by quality fringes, speckle (that obtains due to the diffuse scattering of the coherent radiation from an optically rough surface) fringe patterns are characterized by noisy fringes. This paper concentrates on this aspect and the Matlab based filtering methods to improve the quality of speckle fringe patterns by developing the appropriate software. Further, the newly developed software “Macurv” will be presented which can give the second order derivative (curvature) fringe information. A software with several functions is written using Matlab. The objective of the software is to provide a more effective way for the post-processing of speckle interferometric fringes. The algorithm and functions of the developed software “Macurv” will be explained.     

Measurement of vibration with Young's fringe modulated speckle patterns in a photorefractive correlator

A technique has been proposed to analyze small in-plane component of vibrations by measuring the fringe shift, and analysing the correlation output of captured vibrating Young's fringes with a photorefractive correlator. Fringes are formed due to interference between the reconstructed speckled beam recorded for stationary object beam, and vibrated speckled beam transmitting through a BaTiO₃ crystal after introducing a small shift in the diffuser position. The correlation peak intensity is affected due to mismatch of vibrations between two piezo-mirrors in the object beam. High correlation output gives an indication of the frequency matching of known reference and unknown vibrations, thus giving information on the frequency of vibration. Vibration amplitudes of the order of a wavelength of light have been measured by counting the number of fringes passing past a line in the observation plane.     

Determination of strain and stress distribution on shearwalls by using the speckle photography technique

Speckle photography (SP) is a powerful tool that is adequate to determine small displacements in micrometer range. This information shows other characteristics of structure deformation under loads and can be determined as stress and strain distribution. In this paper we present the results of the application of the SP technique used to study the behaviour of discontinuities in a shearwall model. These structural elements are very important to the stability of buildings. The displacement whole field around the discontinuities and loading points was determined using the pointwise method. This allows us to determine stress distribution at the point of interest by means of the suitable equations. We also present the stress distribution obtained through the finite element method in order to compare the results obtained by means of these two techniques. Good correspondence was found between the displacements determined by both techniques (r=0.982) and also between the stress values we obtained.     

Measurement of tilt of a diffuse object by double-exposure speckle photography using speckle fanning in a photorefractive BaTiO3 crystal

A technique has been proposed for the measurement of tilt of a diffuse object using speckle fanning in a photorefractive BaTiO₃ crystal. Free space geometry has been used for recording in which the axis of rotation of the object lies away in three-dimensional space. The method is based on recording of two exposures, one before- and another after the tilt, thus producing speckle correlation fringes due to overlapping of two speckle fans. We consider a generalized case of tilt, generating curved fringes with center shifted in the transverse direction. The axis of rotation lies perpendicular to the optical axis in three-dimensional space having the transverse and axial components of the shift in the center of rotation with respect to the diffuser plane. Fourier transform of the curved fringes produces correlation output. Measure of separation between the correlation peaks in transverse and axial directions provides information about the tilt angle, the direction of tilt, and the distance of rotation axis from the optical axis. Experimental results have been presented to validate the theoretical predictions.     

Fourier fringe processing using a regressive Fourier-transform technique

Since the introduction of a fourier fringe algorithm by Takeda, it has been possible to determine the phase of a particular light source impinging on an object from one sole image. This has led to applications in many whole field optical measurement techniques such as ESPI, holography, profilometry and so on. However, the basic processing technique, in case of the 2D-Fourier transform, is subject to a major drawback. Because this technique supposes periodicity in a fringe image, the so-called leakage effects occur. This gives rise to non-negligible errors, which can be resolved by using a regressive Fourier transformation technique. In the method introduced in this article, the fringe signal is represented by a model using sines and cosines where the frequency is not fixed (which is the case for classical FFT-techniques). The coefficients of those sines and cosines together with the frequency components are then estimated locally by means of a frequency domain system identification technique. This allows the fringe pattern to be unwrapped without any distortion. This method will be applied in particular to Fourier-transform profilometry (determines object geometry using shifts of projected fringes) although it can be used in any of the techniques mentioned above. Moreover, it will be shown that the proposed method can deal with other distortions that occur in practice such as over-modulation and varying fringe visibility. The proposed technique will be validated on both simulations and on a profile measurement of a pipe section.     

Phase retrieval in two-wavelength DSSI using a combined filtering method

Phase retrieval in two-wavelength DSSI (digital speckle shearing interferometry) using a combined filtering method is proposed for small deformation derivative measurement. A simultaneous two-wavelength illumination and 3-CCD camera are employed in the experimental setup. The proposed method, which uses a two-wavelength technique does not require the conventional spatial phase unwrapping and has the advantage of good noise suppression for phase retrieval. Experimental results obtained show that using the proposed combined filtering method sensitivity similar to the single wavelength technique can be achieved.