Absolute phase retrieval is an important part of 3D measurement. The conventional phase-unwrapping algorithm requires the projection of extra patterns to retrieve the absolute phase, which reduces the measurement speed. Based on the theory of phase domain modulation, the phase is pre-modulated in the form of a specific code embedded into the phase shift method to reduce the number of projected images for improving the speed. To improve the number of codewords and for the accurate determination of the fringe order, the coding strategy of segmented quantization is adopted at the time of coding. When decoding, the original wrapped phase is obtained by solving the wrapped phase map based on the wrapped phase map with the corresponding fringe order for absolute phase retrieval. The experimental results reveal that this method requires only one set of phase-shift modes to realize absolute phase retrieval. This feature improves the measurement speed and simultaneously maintains an accuracy that is consistent with that of the phase-coding method.
In traditional calibration methods for line-structured light vision sensors, the use of two-dimensional planar targets usually requires multiple placements of the target in the measurement space to capture more images to improve the final calibration accuracy, which complicates the calibration process. We propose a new calibration method for line structured light vision sensors (LSLVS) based on freely placeable circular targets. The circular target only needs to be placed freely twice, and the camera captures the images of the circular target and the optical strip. Based on the geometrical relationship of the circle under the camera projection and the nature of the vanishing line, the target plane is first reconstructed, and then multiple three-dimensional points on the target plane are obtained according to the perspective projection relationship, and finally the reconstruction of the optical plane is completed. Compared with the traditional calibration method, this method can be applied to measurements in a limited space, and it does not need sophisticated auxiliary equipment to control the target motion during the whole calibration process, which makes the calibration process more simplified. The final experimental results show that the method can reach an accuracy of 0.0523 mm when only two images are used and can be effectively applied to the line structured light measurement system.
KEYWORDS: 3D metrology, Phase unwrapping, Cameras, 3D modeling, Optical engineering, Optical spheres, Point clouds, Fringe analysis, 3D projection, Image segmentation
In the existing binocular fringe projection methods, the continuous phase is commonly employed for performing 3D scene reconstruction. However, this method has phase ambiguity issues that require the addition of patterns or the utilization of embedded signals to be solved. However, this approach not only leads to the introduction of cumbersome steps but also fails to accurately reconstruct objects without obvious features. To effectively overcome these challenges, an active stereo 3D measurement method was proposed that eliminated the need for phase unwrapping. More specifically, the proposed method first applied binary processing to the wrapped phase map, followed by extracting phase order lines, and assigning symbolic labels to the phase intervals between them, accomplishing thus rough matching. Finally, based on the region block matching, the phase values were used to refine the disparity map. The proposed method was evaluated using a standard sphere with a diameter of 60 mm, and the root-mean-square error of the proposed method is 0.053 mm.
A photonic crystal fiber (PCF) consisting entirely of circular air holes based on hexagonal cladding and cross-shaped core structure is proposed. The transmission properties of the proposed PCF are simulated calculation by using TOPAS as the background material, the finite element method as the calculation method, and the circular perfectly matched layer (PML) as the boundary condition. The results show that very low transmission loss, including effective material loss (EML) with 1.04 × 10-3 cm-1 , the confinement loss with 2.3 × 10-6 dB/cm, and the bending loss (when the bending radius is 1 cm) with 1.23 × 10-17 cm-1 can be got. When the proposed PCF under the optimal condition, extremely large effective area about 9.69 × 1016 μm2 and flat dispersion about 0.46 ± 0.04 ps/THz/cm can be obtained, and the proposed PCF is in the single mode. Large effective area and ultra-low loss make the proposed PCF hold great future in low loss terahertz systems. Additionally, the proposed PCF with simple structure can be drawn by many methods such as the extrusion method.
Three-dimensional (3D) measurement of complex surfaces is of great importance in industrial inspection and production. To solve the problem of high accuracy measurement on complex surfaces, a phase unwrapping method based on multi-frequency binary is proposed, which can directly calculate fringe order by using wrapped phase. First, the four-step phase shift fringe patterns whose period changes exponentially are projected onto the measured object (The base is 2, the index is n, n=1, 2, 3...). Then, the four-step phase shift algorithm is used to obtain the wrapped phase of each period. Binary fringe is generated by binarization of the wrapped phase of each period according to the threshold value of 0. The fringe order K is calculated by binary coding and decoding algorithm, and then the wrapped phase in the highest period is unwrapped by K which can obtain the continuous phase of the object. The simulation and experiment verify the validity of the proposed method, and it has high robustness for complex surface and isolated object. The proposed method is simple in calculation, flexible in control and easy to operate. It provides a simple and effective method for improving the accuracy and robustness of the fringe projection profilometry system.
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