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In a recent published work, we developed the technique to enhance the reliability of absolute phase maps by using the fringes of three spatial frequencies. However, it is time-consuming to capture the fringe images of three spatial frequencies with single channel in time sequence. To increase the efficiency of our proposed three-frequency technology, in this paper we propose a method to capture the fringe images of three spatial frequencies with multi-color channels and 3CCD camera. The projected spatial frequencies can be selected to guarantee the correctness of recovered fringe orders and avoid the chromatic aberration effect on frequency distortion. The cross talk among color channels can be eliminated effectively and the measured object can be reconstructed with high accuracy. The effectiveness of this method is verified by experimental results. [ABSTRACT FROM AUTHOR]

A new temporal approach is presented for the recovery of the absolute phase maps from their wrapped versions based on the use of fringe patterns of three different spatial frequencies. In contrast to the two-frequency method recently published, the method proposed is characterized by better anti-error capability as measured by phase error tolerance bound. A general rule for the selection of the three frequencies is presented, and its relationship to the phase error tolerance bound is derived. Theoretical analysis and experimental results are also presented to validate the effectiveness of the proposed three frequency technique. [ABSTRACT FROM AUTHOR]

Abstract: This paper presents a multiple-step triangular-pattern spatial shift estimation approach for 3D object profilometry measurement. The proposed technique is based on the intensity ratio maps, but, compared to existing intensity ratio approaches, it employs spatial shift estimation to work out the 3D shape. The spatial shift is determined by making use of the inverse function of the intensity ratio map obtained from the reference plane, which is obtained by the least-square polynomial fitting technique. In contrast to existing intensity ratio approaches, the proposed method does not suffer from the influence of the nonlinear distortion associated with digital projection. Also, compared to the existing phase shifting method with phase error correction, the proposed technique is simple and thus is more suitable for fast measurement. The performance of the proposed technique has been tested and the advantages have been demonstrated by experiments. [Copyright &y& Elsevier]

We introduce an experimental methodology designed to detect the position-vectors of omnidirectional targets located in a circular environment. This is achieved using multiple channels of probe chaotic waves generated by an asymmetric coupling semiconductor lasers network (ACSLN). The probe waves' key features of time–space uncorrelation and broad bandwidth are integral to this process. These features enable us to accurately measure position-vectors for a multitude of omnidirectional targets. This is accomplished by correlating the multiple channels of the probe waves with their respective reference waves. Our experimental outcomes indicate impressively low relative errors in the position-vector detection of omnidirectional targets, not amounting to more than 0.4%. By fine-tuning the coupling strength, we were able to achieve ranging-resolutions for the omnidirectional targets as high as 4.63 mm. While a single target within a distance of no more than 4.3 m has minimal impact on the resolution of ranging measurements, it significantly influenced both the relative error and the peak sidelobe level. Our successful detection of omnidirectional targets' position-vectors with the ACSLN opens up fascinating prospects for potential applications; notably in the fields of driverless cars and object tracking systems that require omnidirectional vision. These results may indeed revolutionize future developments in these areas. • We have developed an experimental configuration that enables precise ranging for omnidirectional multi-targets using the multiple channels of the probe chaotic wave from the laser network. • Our experimental outcomes validate that the detections for the position-vectors of the omnidirectional targets carry extremely low relative errors, not exceeding 0.4%. The ranging resolution to multi-targets can achieve an impressive highpoint of 4.7 mm. • The distance to arbitrary target minimally impacts the ranging resolution, but markedly impacts both the relative-error and peak sidelobe-level. [ABSTRACT FROM AUTHOR]

• A serial single-light-source multi-parameter laser self-mixing interferometry (SMI) sensor is proposed for measuring multiple elastic moduli. • Two specimens made of brass and aluminum 6061 were evaluated to validate the feasibility of the designed sensor. It provides a valuable tool for accurately assessing the mechanical properties of materials, facilitating both fundamental research and engineering applications. • This work provides a promising direction for developing a serial multi-object measurement solution based on SMI using a single laser source. The mechanical properties of materials hold significant importance in fundamental research, material, mechanical and civil engineering. These properties are often characterized by some elastic parameters, e.g., Young's modulus, shear modulus and Poisson's ratio. Self-mixing interferometry (SMI) is a unique non-destructive sensing technology that requires minimal components. In this work, we have developed a time-multiplexed fiber-coupled laser SMI sensor in conjunction with impulse excitation technique to simultaneously measure Young modulus, shear modulus and Poisson's ratio of materials. Two specimens made of brass and aluminum 6061 were assessed to validate the feasibility of the designed sensor. The measured values of Young's modulus (101.2 GPa for brass and 69.5 GPa for aluminum 6061), shear modulus (36.3 GPa for brass and 29.5 GPa for aluminum 6061), and Poisson's ratio (0.39 for brass and 0.34 for aluminum 6061) aligned with values reported in existing literature and industry standards. The standard deviations for these three measured parameters were 0.4 GPa, 0.05 GPa, and 0.06 for brass and 0.12 GPa, 0.02 GPa, and 0.003 for aluminum 6061, respectively. The proposed sensor offers several advantages, including a simple structure, high measurement precision, and ease of operation, which contributes to a precise tool for assessing material mechanical properties, benefiting both fundamental research and practical engineering applications. It also provides a promising direction for the development of a serial multi-object measurement solution based on SMI using a single laser source. [ABSTRACT FROM AUTHOR]

Abstract: Phase Shift Profilometry (PSP) method is widely used in 3D shape measurement. The advantage of the PSP method is that it is less sensitive to the surface reflectivity variations and the object can be measured point by point. But if only a single wavelength λ 1 is used, the step height cannot be larger than λ 1/4. In order to measure step height object, Gray Code (GC) and Multi-Wavelength Phase Shift Profilometry (MWPSP) method are used. But the GC method cannot solve the problem of the objects with a variety of colors under different illumination. The shortcoming of MWPSP method is that its speed is too slow because a lot of images with multi-wavelengths should be captured by the system. In order to solve the problem of measurement speed and measurement range, a new phase unwrapping algorithm based on Three Wavelength Phase Shift Profilometry (TWPSP) is proposed in this paper. A series of fringe images only with three wavelengths λ 1, λ 2 and λ 3, are captured by the 3D measurement system. Three equivalent wavelengths λ 12, λ 23and λ 123 based on the original three wavelengths are calculated to perform the phase unwrapping algorithm. Compared with the GC method, the proposed method can measure the objects with different colors under different illuminations. Compared with the MWPSP method, the proposed method is faster and high precision. Experiments demonstrate that the proposed method is robust, high precision, low noise and suit to measure the object with variety colors under different illumination. It can be used to measure arbitrary step height and multiple objects together. [Copyright &y& Elsevier]

Existing compressed ultrafast photography (CUP) mainly acquires the light intensity and loses the phase information that characterizes the spatial features. In this paper, we propose a compressed ultrafast holographic imaging system to achieve phase measurement in ultrafast dynamic samples. In the proposed imaging system, the Mach-Zehnder interference structure is incorporated into compressed ultrafast photography. Based on this imaging system, a reconstruction method combining the plug-and-play generalized alternating projection (PnP-GAP) framework and the depth filtering of prior FastDVDNet is proposed to recover the hologram sequence in ultrafast temporal resolution. FastDVDNet employs the correlation between frames to enhance denoising performance and maintain temporal continuity, thus obtaining better results for ultrafast hologram sequence reconstruction. The simulation results indicate that the proposed method outperforms other reconstruction methods in terms of average peak signal-to-noise ratio (PSNR) and structural similarity (SSIM). • A compressed ultrafast off-axis holographic imaging system was developed to achieve phase measurement in ultrafast dynamic samples. • Based on this imaging system, a reconstruction method combining the plug-and-play generalized alternating projection (PnP-GAP) framework and the deep filtering of prior FastDVDNet was proposed. • The performance of different reconstruction algorithms for reconstructing dynamic holograms was analyzed. [ABSTRACT FROM AUTHOR]

• A mathematical sensing model of a DOFI system working in P1 state was established. • Factors that determine the sensitivity enhancement were analyzed,and the operation region of a DOFI system in P1 state was investigated, based on which, design criteria of designing a DOFI-P1 sensing system were summarized. • Preliminary experiments have been performed to verify the high measurement sensitivity of a DOFI-P1 system as compared with the OFI system. • The results of this work provide helpful guidance for the construction of high-sensitivity DOFI sensing and measurement systems. Laser optical feedback interferometry (OFI) is a unique one-channel interferometric measurement technology with the advantages of compact structure, low implementation cost and easy alignment. However, it has a critical challenge that the visibility of interference fringe is often low and further induces low measurement sensitivity, especially in occasions when the optical feedback strength is feeble, e.g., biomedical and fluid detection. Recently, it has been demonstrated that period-one (P1) dynamics can enhance the measurement sensitivity by introducing extra optical feedback to form a dual optical feedback interferometry (DOFI) system. However, the details of the enhancement mechanism and how to design such a system are still required to unveil. In this paper, comprehensive discussions of designing a DOFI system with high measurement sensitivity enhanced by P1 dynamics are presented. First, based on the modified Lang-Kobayashi equations, a mathematical sensing model of a DOFI system working in P1 state was established. Then, simulations were carried out to verify the validity of the proposed model. After that, factors that determine the sensitivity enhancement were analyzed, based on which, theoretical analysis of achieving optimal sensitivity enhancement was performed. Next, we discussed the operation region of a DOFI system in P1 state and investigated the influence of the system parameters on the operation region. At last, experiments were conducted to verify the theoretical analyses. The results of this work provide helpful guidance for the construction of high-sensitivity DOFI sensing and measurement systems. [ABSTRACT FROM AUTHOR]

Multi-agent reinforcement learning (MARL) has been successfully applied in many fields. In MARL, the policy evaluation problem is one of crucial problems. In order to solve this problem, distributed Temporal-Difference (TD) learning algorithm is one of the most popular methods in a cooperative manner. Despite its empirical success, however, the theory of the adaptive variant of distributed TD learning still remain limited. To fill this gap, we propose an adaptive distributed temporal-difference algorithm (referred to as MS - ADTD) under Markovian sampling over time-varying networks. Furthermore, we rigorously analyze the convergence of MS - ADTD , the theoretical results show that the local estimation can converge linearly to the optimal neighborhood. Meanwhile, the theoretical results are verified by simulation experiments. • This paper proposes a distributed adaptive TD learning algorithm under Markovian sampling. • It theoretically analyzes the non-asymptotic convergence performance of the proposed algorithm. • It verifies the performance of the proposed algorithm by experiments. [ABSTRACT FROM AUTHOR]

This work presents a new technique for 3D shape measurement of moving object in translational motion, which finds applications in online inspection, quality control, etc. A low-complexity 1D fast Fourier transform (FFT)-based spatial matching approach is devised to obtain accurate object displacement estimates, and it is combined with single shot fringe pattern prolometry (FPP) techniques to achieve high measurement performance with multiple captured images through coherent combining. The proposed technique overcomes some limitations of existing ones. Specifically, the placement of marks on object surface and synchronization between projector and camera are not needed, the velocity of the moving object is not required to be constant, and there is no restriction on the movement trajectory. Both simulation and experimental results demonstrate the effectiveness of the proposed technique. [ABSTRACT FROM AUTHOR]

It is a challenging issue to get satisfied results in terms of 3D imaging for shiny surface with fringe projection profilometry (FPP), as the wide variation of surface reflectance for shiny surface will lead to bad exposure, which requires the high dynamic range imaging (HDRI) technique. HDRI with monochromatic illumination and single-shot raw data of the color camera is presented in this paper. From the single-shot raw data, 4 monochrome sub-images corresponding to R, G, G and B channels can be separated respectively. After the attenuation ratios between R&G, G&B channels are calibrated, an image with higher dynamic range can be synthesized with the 4 sub-images, which can help to avoid the impact of bad exposure and improve the accuracy of phase calculation. Experiments demonstrate the validity of proposed method for shiny surface. [ABSTRACT FROM AUTHOR]

• A method to reconstruct the HDR moving object. • Address the HDR issue without projecting multiple-level intensity. • The feature of the HDR moving object is analyzed. High quality 3D reconstruction of HDR object can be achieved by projecting extra fringe patterns with different intensity. However, it requires the object to be kept static. The object motion not only introduces phase shift among the fringe patterns, but also changes the position of the overexposure pixels. This paper proposes a new method to reconstruct the moving HDR object by utilizing the features of motion. For the specific point on the object, overexposure will not happen among all the fringe patterns as the overexposure position is changed with the motion. The unsaturation fringe patterns are selected from the captured fringe patterns and the correct phase information is retrieved based on unequal phase shift algorithm. The effectiveness of the proposed algorithm is verified by the experiments. [ABSTRACT FROM AUTHOR]

Objectives The characterization of the physical properties of dental resin composites is fraught with difficulties relating to significant intra and inter test parameter variabilities and is relatively time consuming and expensive. The main aim of this study was to evaluate whether optical fiber Bragg grating (FBG) sensing system may become a viable tool to study dental material characteristics. Of particular focus was the potential for the system to demonstrate a multi parameter all-in-one feature. Methods A miniature FBG was embedded in six different dental resin composites and employed as a sensor to evaluate linear polymerization shrinkage, thermal expansion and water sorption. Six commercially available dental composites with different filler types and volume are evaluated. The tests are repeated with three sets of samples. The curing characteristics and residual strain gradient exhibited by the cured dental composites were also observed and commented. Results Among the studied samples, SDR shows lowest polymerization shrinkage, while Beautifil FO3 shows the highest. The results also show clear distinction between particle filler type and fiber reinforcement based composites in their polymerization shrinkage properties. The agreement of the results with existing literatures show that FBG based system provides accurate results. Polymerization shrinkage rate of the samples are also obtained. Thermal expansion of the composites are measured using the FBG sensing method for the first time and is correlated with resin type, volume, filler type and glass transition temperature. The water sorption characteristics of the dental composite are also successfully measured using the FBG sensing method. The high level of repeatability and the low standard deviations shown in the results indicate good reliability with the use of FBG sensors. Significance This study demonstrates how optical fiber technology can provide simple and reliable methods of measuring the critical physical properties of dental composites. In addition due to the embedding and preservation of the sensor within the samples multiple parameters can be tested for with the same sample. These features are expected to greatly assist material science researchers in dentistry as well as other biomedical fields. Of some interest the phenomenon of stress relaxation of dental composite at higher temperature was observed. [ABSTRACT FROM AUTHOR]

Skeleton extraction is of great significance for image recognition processes. In this paper, a novel skeleton extraction method is proposed based on the anisotropic diffusion of the gradient vector field. Firstly, the gradient vector is calculated for the original image. Then, the gradient vector field is updated by an anisotropic partial differential equation to strengthen the source point or sink point of the image energy. At last, according to the divergence property of the final gradient vector field, the skeleton of the initial image is obtained. The experimental results demonstrate that the proposed method is robust for noise and can give the exact skeleton for images. [ABSTRACT FROM AUTHOR]

• In this work, we proposed to use a Microwave Photonic (MWP) sensing signal to achieve high displacement sensing resolution. The MWP sensing signal is generated by a laser diode (LD) with dual-cavity. In this configuration, the LD serves as both an MWP source and a sensor. The MWP sensing signals are directly generated by laser dynamics without need any external modulator. The information about the measurand is carried by both optical and microwave components of the MWP sensing signals. Hence, rich information about the measurand can be retrieved from the generated MWP sensing signals to greatly improve sensing performance. This work has the following contributions and significances. • We push an optical feedback interferometry (OFI) to operate at a laser state called 'period one' by using a dual-cavity configuration. In such configuration, one cavity is linked with measurand target, the other is used to control the LD dynamic states. Under the optimal system operation condition, a modulated laser intensity in microwave range (we call it as MWP sensing signal) can be generated and it is sensitive to a target movement external the LD and thus is used for displacement sensing. • It is found that the high frequency component contained in an MWP sensing signal can be used to subdivide OFI fringe signals. This is a novel method for subdivision of interference fringe and it is able to achieve super high sensing resolution (in a nano scale in theory) compared to the existing methods. Laser diodes (LDs) experiencing external optical feedback (OF) are known to demonstrate complex nonlinear dynamics and found various applications. Typical sensing configuration using an LD with OF is called optical feedback interferometry (OFI) that can produce an interferometric-like fringe signal (called OFI fringe) with a fringe resolution as half laser wavelength for displacement measurement. This work uses a dual-cavity OFI and operates the LD at Period-One (P1) state. By optimal configuration for such system, a microwave photonic (MWP) signal can be generated in oscillation form at a very high frequency with its amplitude modulated by an OFI fringe signal. The high frequency component contained in the MWP signal is used for subdivision of an OFI fringe to greatly improve sensing resolution. Both simulation and experiment are conducted to confirm the proposed approach for OFI fringe subdivision. [ABSTRACT FROM AUTHOR]

We conduct a systematic study of thin film solar cells consisting of a GaAs needle profile (NP) grating structure as a light-trapping layer. The influence of geometric parameters on the optical absorption of the NP grating is investigated using rigorous coupled wave analysis and the finite element method. This type of structure can lead to broadband optical absorption enhancement throughout the wavelength range that we studied. Our simulation results reveal that the absorption efficiency of NP grating can be improved significantly compared with its rectangular grating counterpart. The proposed structure is expected to illuminate the design and fabrication of high-efficiency solar cells. [ABSTRACT FROM AUTHOR]

• In this paper, we exploit deep learning to reduce phase error caused by the nonlinear response and random noise. • We design a three-to-three deep learning framework, called Tree-Net, to construct six step phase-shifting patterns from three-step. • The training process of Tree-Net is complicated and time-consuming, but once the training is completed, Tree-Net can rapidly and simultaneously generate the desired three fringes. • Tree-Net has excellent performance when coping with different PSP systems after only one training, which provides great potential for the high-efficiency batch application of phase shift profilometry. • Experimental results demonstrate that the phase error can be reduced by about 90% in three-step PSP, which verified the effectiveness, universality, and robustness of the proposed method. The nonlinear effect in phase shifting profilometry (PSP) is an essential source of phase error in 3D measurement. In this paper, we propose a universal phase error compensation method with a three-to-three deep learning framework (Tree-Net). Perfectly meeting the phase error compensation requirements, Tree-Net can construct six-step phase-shifting patterns from three-step. As a result, this compact network of fringe-to-fringe transformation has excellent performance when coping with different PSP systems after only one training. Experimental results demonstrate that the phase error can be reduced by about 90% in three-step PSP, which verified the effectiveness, universality, and robustness of the proposed method. [ABSTRACT FROM AUTHOR]

• A comprehensive review of the literatures related to the reduction of motion-induced errors in phase shifting profilometry is conducted. • Existing approaches are categorized into object tracking-based approaches, Fourier-assisted approaches, motion prediction approaches and other approaches, with principles elaborated. • Guidance on the selection of appropriate methods based on application and suggestions on future directions are provided. 3D shape measurement using phase shifting profilometry (PSP) has been extensively studied by the researchers in the past few years. With the recent binary defocused method, the measurement speed of these systems have been increased significantly. Yet, measurement errors are inevitable while imaging dynamic scenes. This is mainly because PSP methods require the object to be static for a short period of time. Researchers have developed various methods to alleviate the motion-induced errors for PSP systems. This paper reviews the various state-of-the-art motion-induced error reduction methods. The various research methods are categorized into different groups and the principles behind each method was explained. We conducted experiments to compare the potential of different algorithms in compensating the motion-induced errors. A comparative discussion is also provided to serve as a reference for selection of methods under different application scenarios. Finally, further discussions are also provided to point out opportunities for future research in this field. [ABSTRACT FROM AUTHOR]

• A two-step phase-shifting algorithm dedicated to fringe projection profilometry (FPP) is proposed. • Considering the physical process of FPP, the captured fringe image is formulated with two independent variables. • The phase error due to ambient light is analyzed via a line-circle model. • The wrapped phase is refined based on the estimation of the actual fringe contrast. Improving the time efficiency of fringe projection profilometry (FPP) is an attractive problem. For FPP using phase-shifting, it is desired to improve the efficiency by reducing the step number for phase retrieval. This paper proposes a two-step phase-shifting algorithm dedicated to FPP. Considering the physical process of FPP, the captured fringe image is formulated with two variables, i.e. surface reflectance and phase value. And a phase shift is introduced to get the two equations, which lead to the close-form solution for phase calculation. Then the phase error due to ambient light is analyzed via a line-circle model, and an algorithm of refining the phase calculation is proposed based on the estimation of the actual fringe contrast. The validity of the proposed approach is demonstrated with experiments. [ABSTRACT FROM AUTHOR]

Automatic detection of micro-aneurysm in color retinal image is important for early screening and diagnosis of diabetic retinopathy. In this paper, a new method is proposed for micro-aneurysm detection based on circular bilateral Gabor filtering. Firstly, a circular bilateral Gabor filter is developed to extract micro-aneurysm candidates. Secondly, false positives are reduced by eliminating small vessels through a process involving local gradient analysis. The proposed method is tested on the retinal images from the Retinopathy Online Challenge database and Tianjin Medical University Metabolic Diseases Hospital. Evaluation results at both image and lesion level demonstrate the efficacy of the proposed method in detecting micro-aneurysm accurately. [ABSTRACT FROM AUTHOR]