In order to avoid frequency aliasing,improve the spatial resolution of the phase map in Fourier transform Profilometry(FTP),an approach based on the digital time-multiplexing technique is proposed to remove the background component from the deformed fringe pattern. Firstly, a sinusoidal fringe pattern is projected onto the tested object by digital-light-processing( DLP) projector, the fringe pattern modulated by the object’s surface is captured by a CCD camera.Secondly, apply Fourier transform to the captured fringe pattern to obtain the spectrum. Thirdly, rotate the specimen’s fringe pattern 90-deg to obtain the rotated fringe pattern , then obtain the new spectrum corresponding to the rotated fringe pattern.Fourthly,the new spectrum is subtracted from the original spectrum ,clip the negative going values in the resultant spectrum by digital manipulation.Fifthly, filter out the first-order spectrum from the resultant spectrum by the band filter,apply inverse Fourier transform to the selected spectrum to obtain complex fields,then retrieve the phase, unwrap the wrapped phase map by the phase unwrapping algorithm.Finally, the simulation and experimental evaluations are conducted to prove the validity and performance of the proposed method. The results are analyzed and compared with those of the conventional method.The effectiveness and superiority of the proposed method have been demonstrated and verified.
Digital holography is a powerful tool for non-contact quantitative phase imaging. Off-axis configuration remains a popular choice among the digital holography systems due to its ability to separate the dc and cross-terms in the recorded hologram in Fourier spectral space.However, compensating the off-axis tilt of the reference wave is one of the open challenges in the off-axis digital holography.Deng et al. proposed an off-axis tilt compensation method based on hologram rotation [DENG et.al. Opt. Let., 2017]. The off-axis tilt is removed by subtracting the phase of the digital reference hologram obtained by rotating the original specimen’s hologram from the retrieved phase corresponding to the original hologram. Nonetheless, Deng’s method is extremely time consuming due to the computation of Fourier transform,inverse Fourier transform and phase unwrapping for many times. In this paper, we propose a simple algorithm to compensate the off-axis tilt . Firstly, apply Fourier transform to the original off-axis hologram, filter out the first-order spectrum by band filter, then determine directly the spectrum of digital reference hologram from the spectrum of the original hologram, then filter out the first-order spectrum from the spectrum of digital reference hologram, apply inverse Fourier transform to the two first-order spectra to obtain two complex fields, then retrieve directly the phase difference from the two complex fields using the direct phase difference algorithm, then unwrap the wrapped phase map by the phase unwrapping algorithm. Finally, simulations and experiments are conducted to prove the validity of the proposed method. The results are analyzed and compared with those of Deng’s method, demonstrating that our method not only can speed up by more than 50% the calculation time, but also can improve measurement accuracy.
Nondestructive optical techniques have been widely used to satisfy engineering applicat ion. 2D digital image correlation (DIC) method has superiority of convenient manipulation and high accuracy in measuring planar displacement. Moreover, fringe pattern profilometry (FPP) method has become a highly developed technique to measure surface profile. Combined with 2D-DIC method, FPP method can be applied to measure three-dimensional displacements conveniently. As a semi-custom integrated circuit, Field-Programmable Gate Array (FPGA) has been popular for its powerful programming performance on controlling experimental instrument. Furthermore, Labview, an efficient graphical programming language which excels in instrument communication, can be used to program FPGA. In this paper, a corrected dynamic FPP method combined with 2D-DIC method has been presented and achieved by Labview programming to measure dynamic deformation. An experimental system including a projector and a camera is used to project fringe patterns and acquire images alternately at a high speed. By the referred method a series of spatial-discrete displacement data in equal intervals of time are obtained. Then a four-dimensional interpolation is adopted to get full-field and continuous-time displacement data. Thus, the planar and out-plane displacements can be simultaneously measured. Experiments were performed and verified the feasibility of proposed method.
The digital gradient sensing (DGS) method is a full-field optical technique to measure the in-plane stress gradients of transparent materials. Elasto-optic constant is an important material characteristic for the DGS method and is usually not given. On the basis of introducing the working principle of DGS method, a calibration method of elasto-optic constant based on the least-squares method is proposed, and the accuracy of iterative convergence is improved by introducing liner error. A theoretical model of a semi-infinite plane under concentrated normal load on boundary is chosen to verify the feasibility of the proposed method. The full-field experimental data of polymethyl methacrylate specimen is obtained from the digital image correlation method. The experimental results show that the least-squares calibration method can determine the elasto-optic constant of transparent materials effectively.
According to the characteristic of the optical field arrangement, parallel illumination and parallel receiving in shadow moire method, that the reference grating and the shadow moire can be shifted equal distance at the same time, are proposed in this paper. First, the calculation formula for determining exact phase-shifting values and its experimental techniques were derived. Second, an exact simulation technique in shadow moire topography was constructed on the basis of the conceptions of geometrical interference mode for forming the virtual shadow moire fringes and the calculation formula of exact phase-shifting values. Third, to verify the validity of the calculation principle and simulation technique, the experimental results of the two typical models were offered, one was the cylinder and the other was sphere. These results indicate that the method proposed in this paper can improve the precision of three dimensional shape reconstructions and promote the development of shadow moire topography.
In this article, a metallographic microscopy, an atomic force microscopy and a field emitting scanning electronic microscopy was used to investigate the surface and the cross-sectional morphology of porous silicon films, respectively. Simple micro-structure and micro-mechanical models are established to explain the origin mechanism of residual stresses in the porous silicon. Experimental results reveal that the residual stresses have close relation with the micro-structure of the porous silicon and consist of the lattice mismatch stress, capillary stress, oxidation stress, Van der Walls force and so on. Combining micro-Raman spectroscopy with x-ray diffraction measurements, we get the total residual stress of 900MPa, and its components of the lattice mismatch stress is about of 815.8MPa, the capillary stress of 13.2MPa and the oxidation stress of 71MPa for a chemical etched porous silicon sample with a certain porosity. It can be seen that the lattice mismatch between the porous layer and the Si substrate is a major source (about 91%) for the total residual stress of the porous silicon.