The Window Fourier Ridge (WFR) is one of the useful methods in local frequency extraction from a fringe pattern. But the background of the pattern and the inherent noises existed in the pattern will disturb the extracted result, which need to be removed before operation. To solve this problem, we modify the WFR method by spectrum normalization of the fringe pattern in its frequency domain. Then, the accuracy of the extracted local fringe frequency is enhanced greatly due to the inhibition of the disturbance from its background. The principle of the normalized spectrum method is detailed by spectrum analysis of the fringe pattern. The conducted simulations and experiment results show that the local fringe-frequency estimated from the fringe pattern with background by the proposed method is as good as that from the fringe pattern without background by traditional WFR method.
In this work, we propose a new calibration technique for locating the projector in the structured light measurement system. Using Brox optical flow, the estimation of three coordinate components of the projector only requires two images captured before and after the motion of the calibration plate. The calibration principle presented on geometry depicts the relation between the position of the projector and that of the camera, to the optical flow caused by the movement of the calibration plate. The accuracy of the proposed method is verified by numerical simulation. The measurement system is calibrated by using the proposed calibration method and by Falcao’s method simultaneously for comparison. After calibration, eight-step phase shift technique of a recognized high precision is performed to reconstruct the 3-D shape. The results show that the height distribution of the specimens is in good agreement with its true values, and the maximum absolute error is less than 0.1 mm.
The dark spots in the speckle pattern are the phase singularities, which contain rich information regarding the phase of the speckle field and determine the distribution of the speckle field. The generation of optical vortices using speckle is a simple, effective, and easy to implement method. we designed an experimental scheme that can generate a optical vortex to explain the properties of phase singularities in a speckle field. Through numerical simulation, we obtained the distribution of the real part zero line and the imaginary part zero line in the speckle field, and drew the singular skeleton of the speckle field, indicating that there are phase singularities in the speckle field.The experimental results are in agreement with those obtained from numerical simulations, thus verifying that the generation of optical vortices can be produced using the scattering method. The experimental scheme is simple and easy to operate.
Phase extraction and applications of the speckle vortex field with spiral wavefront and stable core structures of phase singularities has become an important research content in the field of optical measurement. Phase singularities in the speckle field constitute singular skeleton. We use an experimental system to measure the deformation displacement based on the singular skeleton. In the experiment, a liquid crystal spatial light modulator (LC–SLM) is used to generate speckle field, and the output speckle image is captured by the CCD camera. By processing the speckle pattern, the singular skeleton of the speckle field is depicted. Based on the singular skeleton, the deformation translational displacement of the test sample is obtained. The experimental results show that high-resolution optical measurement methods can be developed based on the singular skeleton of the speckle field.
Linear gratings and circular gratings are most commonly used in the applications of Talbot effect, due to their simple patterns and ease of analysis of Moiré fringes. In the present paper, we demonstrate a scheme to measure the displacement of a deformed object which based on the Talbot effect. We designed a hologram of a two-dimensional square-hole grating. For the test, we used a computer-controlled liquid crystal spatial light modulator (LC–SLM) and the computer-generated Talbot array hologram. The Talbot array hologram is displayed on the LC–SLM screen. The beam diffracted by the grating is transmitted to the surface of an object. By adjusting the distance between the LC-SLM and the object, a clear image is formed on the surface of the object. A speckled pattern of the object surface is captured by a CCD camera. Two speckle patterns before and after the deformation is recorded. The information about the displacement is codified in the intensity pattern. The displacement can be obtained by digital image correlation (DIC). The DIC obtains the displacement components by comparing the gray intensity changes between the digital images of an object surface before and after the deformation. This method is simple and easy, and can be used as an alternative method to measure in-plane and out-of-plane displacements. Theoretical and experimental results are presented.
Speckle fields contain many random dark and bright spots. In traditional speckle interference measurement technique, the information of the bright areas is fully utilized, and the speckle vortices that exit in the dark areas are often overlooked. The generation of speckle vortices by using phase-only liquid crystal spatial light modulator (LC-SLM) combined with the double Archimedes’ spiral micro-holes array is experimentally and theoretically studied. In the experiment, the gray image of double Archimedes’ spiral micro-holes array is displayed on the screen of LC-SLM, and the output optical field is captured by a CCD camera. The numerical simulations and experimental results show that speckle vortices can be generated by using this method.
In ESPI experiment, object beam and reference beam are always planar light. The plane light can be replaced by vortex beam. Vortex beams can be generated by a reflective liquid crystal spatial light modulator (LC-SLM) which added in the optical path. The generated vortex beam can be used as object light or reference light in out-of-plane displacement measurement. The out-of-plane displacement is simulated and analyzed before and after the object deformation. By phase shifting method and unwrapping, the distribution of phase difference is obtained. The simulation results demonstrate the efficacy of the proposed method for the out-of-plane displacement measurements.
A method for three-dimensional (3-D) deformation measurement is presented by combining Digital Speckle Correlation Method (DSCM) with Electronic Speckle Pattern Interferometry (ESPI). The combination is completed based on a typical ESPI system, in which the reference light is controlled to turn on or shut down. The in-plane displacement components are obtained by using DSCM when the reference light is shut. A phase shifting ESPI is formed when the reference light is used, which can be used for the measurement of the out-plane displacement component. A typical three-point-bending experiment is completed. Experiment results show that the three displacement components can be obtained by the combination effectively.
A simple setup for 3-D deformation measurement is offered. In the scheme a novel Cube Beam-Splitter, called Non- Cube Beam-Splitter (NCBS), is used for 3-D phase-shift Electronic Speckle Pattern Interferometry (ESPI). By using the NCBS lights from a tested object and lights from a reference surface, the reference and the object light can be combined and then interfere each other on a CCD camera when a laser beam illuminate the test object and the reference surface simultaneously. When three laser beams illuminate the test object at different incident angles respectively before and after deformation, three interference fringe patterns are formed. Then three phase maps corresponding to three lasers can be calculated by using phase-shift, by which three displacement components are completed. The principle of the method is presented and proved by a typical three-point bending experiment. Experimental results are offered.
Speckle plays an important role in the optical field. Optical vortices which exist in random speckle fields usually contain useful phase information. The distribution of speckle field is determined by these optical vortices. In order to study speckle vortices quantitatively, we established a micro-holes array model based on the law of Archimedes’ spiral arrangement. Speckle vortices can be generated by the random diffuse reflection points (spiral micro-holes). In the experiments, the gray image of Archimedes’ spiral micro-holes are displayed on the screen of liquid crystal spatial light modulator (LC-SLM), and the output optical field is captured by a CCD camera. The numerical simulations and experimental results show that the model can be used to generate speckle vortices.
A technique for deformation measurement by carrier is presented, which is based on large image-shearing shearography. A reference object is fixed on the side of a test object. They are all illuminated by one expanded laser beam. When a large image-shearing Wollaston crystal is used in front of a charge-coupled device (CCD) camera, one image of the object is superposed by one image of the reference surface. The carrier can be introduced by tilting the reference surface at a small angle. When Fourier transform is used to demodulate the modulated fringe pattern, the phase of deformation can be calculated and the deformation can be measured accurately. The principle of spatial carrier frequency modulation in large-shearing electronic speckle pattern interferometry (ESPI) is discussed. A typical experiment using a centrally loaded clamped circular plate is completed. Some experimental results are presented. The experimental results prove that the method can modulate a speckle pattern very well and the displacement fields can be measured effectively.
A method for three-dimensional (3D) displacement measurement by separating out-of-plane displacement from in-plane displacement is presented. A reference beam is added to a dual-beam symmetric illumination electronic speckle pattern interferometry (ESPI) system and shared by the two illuminations. The test object is illuminated by the two object beams, respectively. Two phase maps, which include out-of-plane and in-plane displacement, can be obtained by phase-shifting techniques. In order to decrease electronic noises in the phase maps, one of the phase maps is calculated by the reversed phase-shifting method presented. By using inverse phase distribution, out-of-plane displacement can be easily separated from in-plane displacement by subtraction and can greatly decrease electronic noises. The principle of the method is presented and proved by a typical three-point bending experiment. Experimental results are offered.
A phase-shifting technique for full-field measurement of the strain of an object in electronic speckle-shearing interferometry is presented. The object under study is placed on a rotating platform and illuminated by an expanded laser beam. Its images are recorded at short intervals by a shearing CCD camera and stored directly into a computer. By subtracting the series of images from the first one, fringe patterns that represent the object surface strain can be obtained and shown on a computer monitor if the object is loaded. When the object is given a small angle of rotation, an additional phase will be introduced into the fringe patterns. Theoretical analysis shows that this additional phase is linear. Therefore, the electronic speckle shearing phase-shifting technique can be realized by rotating the test object. The small angle can be obtained exactly by a rotating platform controlled by a computer. The theory of the method and system as well as some experimental results is presented.
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