A review of liquid crystal spatial light modulators:devices and applications

Spatial light modulators,as dynamic flat-panel optical devices,have witnessed rapid development over the past two decades,concomitant with the advancements in micro-and opto-electronic integration technology.In particular,liquid-crystal spatial light modulator(LC-SLM)technologies have been regarded as versatile tools for generating arbitrary optical fields and tailoring all degrees of freedom beyond just phase and amplitude.These devices have gained significant interest in the nascent field of structured light in space and time,facilitated by their ease of use and real-time light manipulation,fueling both fundamental research and practical applications.Here we provide an overview of the key working principles of LC-SLMs and review the significant progress made to date in their deployment for various applications,covering topics as diverse as beam shaping and steering,holography,optical trapping and tweezers,measurement,wavefront coding,optical vortex,and quantum optics.Finally,we conclude with an outlook on the potential opportunities and technical challenges in this rapidly developing field.

Generation of structure light in probe absorption spectrum via microwave-driven Y-type atomic system

Behavior of structure light is investigated by monitoring probe absorption using a microwave-driven Y-type atomic media configuration.The system under consideration is driven by one of the control vortex beams as well as an extra non-vortex control beam to ensure electromagnetically induced transparency.The significant aspect in the generation of structured light is the azimuthal phase-dependent modification for probe absorption.Further intensity distribution for absorption spectra is examined for simultaneously evaluating both the control vortex beams.We also go through the radial distribution of intensity for various orbital angular momentum values.Different modes of structural beams may be distinguished using the suggested approach.Our research gives us a way for rapidly transferring vortex wavefronts from control field to probe absorption profile.This could be useful in quantum information processing.

Seam Tracking and Visual Control for Robotic Arc Welding Based on Structured Light Stereovision

A real-time arc welding robot visual control system based on a local network with a multi-level hierarchy is developed in this paper. It consists of an intelligence and human-machine interface level, a motion planning level, a motion control level and a servo control level. The last three levels form a local real-time open robot controller, which realizes motion planning and motion control of a robot. A camera calibration method based on the relative movement of the end-effector connected to a robot is proposed and a method for tracking weld seam based on the structured light stereovision is provided. Combining the parameters of the cameras and laser plane, three groups of position values in Cartesian space are obtained for each feature point in a stripe projected on the weld seam. The accurate three-dimensional position of the edge points in the weld seam can be calculated from the obtained parameters with an information fusion algorithm. By calculating the weld seam parameter from position and image data, the movement parameters of the robot used for tracking can be determined. A swing welding experiment of type V groove weld is successfully conducted, the results of which show that the system has high resolution seam tracking in real-time, and works stably and efficiently.

Nonlinear optics with structured light

The interest in tailoring light in all its degrees of freedom is steadily gaining traction,driven by the tremendous developments in the toolkit for the creation,control and detection of what is now called structured light.Because the complexity of these optical fields is generally understood in terms of interference,the tools have historically been linear optical elements that create the desired superpositions.For this reason,despite the long and impressive history of nonlinear optics,only recently has the spatial structure of light in nonlinear processes come to the fore.In this review we provide a concise theoretical framework for understanding nonlinear optics in the context of structured light,offering an overview and perspective on the progress made,and the challenges that remain.

Nonlinear effect of the structured light profilometry in the phase-shifting method and error correction

Digital structured light （SL） profilometry is increasingly used in three-dimensional （3D） measurement technology. However, the nonlinearity of the off-the-shelf projectors and cameras seriously reduces the measurement accuracy. In this paper, first, we review the nonlinear effects of the projector-camera system in the phase-shifting structured light depth measurement method. We show that high order harmonic wave components lead to phase error in the phase-shifting method. Then a practical method based on frequency domain filtering is proposed for nonlinear error reduction. By using this method, the nonlinear calibration of the SL system is not required. Moreover, both the nonlinear effects of the projector and the camera can be effectively reduced. The simulations and experiments have verified our nonlinear correction method.

On-line detection system of weld-based rapid prototyping process based on structured light

In order to realize the closed-loop control for rapid prototyping process based on gas metal arc welding, the geometric parameters of weld beads should be detected. In this study, a vision sensor system consisting of a linear laser projector and a CCD camera was designed to collect images of weld beads. Then, an image processing approach which combines with a Gaassian filter and an improved gravity method was used to extract the centerline of a light stripe based on VC ＋＋. Feature points of the centerline were identical directly by means of an image fusion algorithm. Experimental results show that image fusion is an effective approach to measure the width and height of the weld bead with high accuracy. This method can identify beads effectively in multi-pass welding and avoid designing different modes to suit all kinds of shapes.

3D surface reconstruction based on binocular vision using structured light

A 3D surface reconstruction method using a binocular stereo vision technology and a coded structured light,which combines a gray code with phase-shift has been studied.The accuracy of the 3 D surface reconstruction mainly depends on the decoding of gray code views and phase-shift views.In order to find the boundary accurately,gray code patterns and their inverses are projected onto a human eye plaster model.The period dislocation between the gray code views and the phase-shift views in the course of decoding has been analyzed and a new method has been proposed to solve it.The splicing method is based on feature points.The result of the 3D surface reconstruction shows the accuracy and reliability of our method.

Construction for M-arrays and application in structured light

M-arrays are random arrays in which an appropriate sub-window appears only once in the whole array. Coded structured light based on M-arrays is one=shot technique to rapidly acquire 3D information of unknown surfaces by projecting suitable patterns onto a measuring surface. This paper presents a method to construct large size M-arrays based on the piece growing algorithm in which an array is constructed by many pieces through splicing each other. Reconstructing 3D shapes by utilizing the designed pattern based on constructed M-arrays for two objects are given.

SEGMENTATION ALGORITHM BASED ON EDGE-SEARCHING FOR MULTI-LINEAR STRUCTURED LIGHT IMAGES

Aiming at the problem that the existence of disturbances on the edges of light-stripe makes the segmentation of the light-stripes images difficult, a new segmentation algorithm based on edge-searching is presented. It firstly calculates every edge pixel＇s horizontal coordinate grads to produce the corresponding grads-edge, then uses a designed length-variable l D template to scan the light-stripes＇ grads-edges. The template is able to find the disturbances with different width utilizing the distributing character of the edge disturbances. The found disturbances are eliminated finally. The algorithm not only can smoothly segment the light-stripes images, but also eliminate most disturbances on the light-stripes＇ edges without damaging the light-stripes images＇ 3D information. A practical example of using the proposed algorithm is given in the end. It is proved that the efficiency of the algorithm has been improved obviously by comparison.

Three-Dimensional Measurement Using Structured Light Based on Deep Learning

Three-dimensional(3D)reconstruction using structured light projection has the characteristics of non-contact,high precision,easy operation,and strong real-time performance.However,for actual measurement,projection modulated images are disturbed by electronic noise or other interference,which reduces the precision of the measurement system.To solve this problem,a 3D measurement algorithm of structured light based on deep learning is proposed.The end-to-end multi-convolution neural network model is designed to separately extract the coarse-and fine-layer features of a 3D image.The point-cloud model is obtained by nonlinear regression.The weighting coefficient loss function is introduced to the multi-convolution neural network,and the point-cloud data are continuously optimized to obtain the 3D reconstruction model.To verify the effectiveness of the method,image datasets of different 3D gypsum models were collected,trained,and tested using the above method.Experimental results show that the algorithm effectively eliminates external light environmental interference,avoids the influence of object shape,and achieves higher stability and precision.The proposed method is proved to be effective for regular objects.

Statistical Analysis of Ship Impact Forces on Light Wharf Structures

In the present study,the formula calculating ship impact forces on light wharf structures is presented when the elastic deformation of the hull and the pier structures as well as the nonlinear deformation of the fender are taken into account. The ship impact forces are statistically analyzed with the Monte-Carlo method according to the known probability distribution types of random variables.Based on the simulated results, the distribution of ship impact forces which is characterized by bimodal distribution can be expressed as the combining probability density function of beta distribution and normal distribution. The corresponding parameters of the probability density function can be estimated with the maximum likelihood method. The results show that ship impact forces on light wharf structures follow the distribution of type I extreme value.The mean coefficient and variation coefficient are 1.11 and 0.008 respectively during 50 years of design reference period.

MULTIPARAMETER MEASUREMENT FOR RACEWAY GROOVE OF BEARING BASED ON 3D RECONSTRUCTION WITH DIGITAL STRUCTURED LIGHT

A fast 3D reconstruction method based on structured light to measure various parameters of the raceway groove is presented. Digital parallel grating stripes distributed with sine density are projected onto the raceway groove by a DLP projector, and distorting of stripes is happened on the raceway. Simultaneously, aided by three-step phase-shifting approach, three images covered by different stripes are obtained by a high-resolution CCD camera at the same location, thus a more accuracy local topography can be obtained. And then the bearing is rotated on a high precision computer controlled rotational stage. Three images are also obtained as the former step at next planned location triggered by the motor. After one cycle, all images information is combined through the mosaics. As a result, the 3D information of raceway groove can be gained. Not only geometric properties but also surface flaws can be extracted by software. A preliminary hardware system has been built, with which some geometric parameters have been extracted from reconstructed local topography.

Application of color structured light pattern to measurement of large out-of-plane deformation

Measurement of out-of-plane deformation is significant to understanding of the deflection mechanisms of the plate and tube structures.In this study,a new surface contouring technique with color structured light is applied to measure the out-of-plane deformation of structures with one-shot projection.Through color fringe recognizing,decoding and triangulation processing for the captured images corresponding to each deformation state,the feasibility of the method is testified by the measurement of elastic deflections of a flexible square plate,showing good agreement with those from the calibrated displacement driver.The plastic deformation of two alloy aluminum rectangular tubes is measured to show the technique application to surface topographic evaluation of the buckling structures with large displacements.

Filtering algorithm of line structured light for long-distance obstacle detection

Since unmanned ground vehicles often encounter concave and convex obstacles in wild ground, a filtering algorithm using line structured light to detect these long distance obstacles is proposed. For the line structured light image, a ranked-order based adaptively extremum median （RAEM） filter algorithm on salt and pepper noise is presented. In the algorithm, firstly effective points and noise points in a filtering window are differentiated; then the gray values of noise points are replaced by the medium of gray values of the effective pixels, with the efficient points＇ gray values unchanged; in the end this algorithm is proved to be efficient by experiments. Experimental resuits demonstrate that the image blur, resulting into proposed algorithm can remove noise points effectively and minimize the protecting the edge information as much as possible.

Enhancing light absorption for organic solar cells using front ITO nanograting and back ultrathin Al layer

To address the discrepancy between carrier collection and light absorption of organic solar cells caused by the limited carrier mobility and optical absorption coefficient for the normally employed organic photoactive layers,a light management structure composed of a front indium tin oxide(ITO)nanograting and ultrathin Al layer inserted in between the photoactive layer and the electron transport layer(ETL)is introduced.Owing to the antireflection and light scattering induced by the ITO nanograting and the suppression of light absorption in the ETL by the inserted Al layer,the light absorption of the photoactive layer is significantly enhanced in a spectral range from 400 nm to 650 nm that also covers the main energy region of solar irradiation for the normally employed active materials such as the P3HT:PC_(61)BM blend.The simulation results indicate that comparing with the control device with a planar configuration of ITO/PEDOT:PSS/P3HT:PC_(61)BM(80-nm thick)/ZnO/Al,the short-circuit current density and power conversion efficiency of the optimized light management structure can be improved by 32.86%and 34.46%.Moreover,good omnidirectional light management is observed for the proposed device structure.Owing to the fact that the light management structure possesses the simple structure and excellent performance,the exploration of such a structure can be believed to be significant in fabricating the thin film-based optoelectronic devices.

Lateral Performance for Wood-Frame Shear Walls–A Critical Review

Wood is a green material in line with the sustainable development strategy.From the excellent performance of engineering wood products,modern wood structures represented by light wood structures have gained more development opportunities.As an indispensable part of light wood structure systems,the wood-frame shear wall plays a vital role in the bearing capacity and earthquake resistance of light wood structure systems.This paper is focused on a review of the lateral performance of wood-frame shear walls and classifies the influencing factors in relevant experimental research into three categories,including internal factors such as shear wall structure,external factors such as test scheme,and other factors of material production and test process.Finally,the research prospects in this field were introduced based on the summary of the research status.This work can be a reference for further research on the lateral performance of wood-frame shear walls.

Spatial light modulation for femtosecond laser manufacturing:Current developments and challenges

Since the invention of lasers,spatial-light-modulated laser processing has become a powerful tool for various applications.It enables multidimensional and dynamic modulation of the laser beam,which significantly improves the processing efficiency,accuracy,and flexibility,and presents wider prospects over traditional mechanical technologies for machining three-dimensional,hard,brittle,or transparent materials.In this review,we introduce:(1)The role of spatial light modulation technology in the development of femtosecond laser manufacturing;(2)the structured light generated by spatial light modulation and its generation methods;and(3)representative applications of spatial-light-modulated femtosecond laser manufacturing,including aberration correction,parallel processing,focal field engineering,and polarization control.Finally,we summarize the present challenges in the field and possible future research.

A 3D Measurement Method Based on Coded Image

The binocular stereo vision system is often used to reconstruct 3D point clouds of an object.However,it is challenging to find effective matching points in two object images with similar color or less texture.This will lead to mismatching by using the stereo matching algorithm to calculate the disparity map.In this context,the object can’t be reconstructed precisely.As a countermeasure,this study proposes to combine the Gray code fringe projection with the binocular camera as well as to generate denser point clouds by projecting an active light source to increase the texture of the object,which greatly reduces the reconstruction error caused by the lack of texture.Due to the limitation of the camera viewing angle,a one-perspective binocular camera can only reconstruct the 2.5D model of an object.To obtain the 3D model of an object,point clouds obtained from multiple-view images are processed by coarse registration using the coarse SAC-IA algorithm and fine registration using the ICP algorithm,which is followed by voxel filtering fusion of the point cloud.To improve the reconstruction quality,a polarizer is mounted in front of the cameras to filter out the redundant reflected light.Eventually,the 3D model and the dimension of a vase are obtained after calibration.

Development of an In-Situ Laser Machining System Using a Three-Dimensional Galvanometer Scanner

In this study, a three-dimensional (3D) in-situ laser machining system integrating laser measurement and machining was built using a 3D galvanometer scanner equipped with a side-axis industrial camera. A line structured light measurement model based on a galvanometer scanner was proposed to obtain the 3D information of the workpiece. A height calibration method was proposed to further ensure measurement accuracy, so as to achieve accurate laser focusing. In-situ machining software was developed to realize time-saving and labor-saving 3D laser processing. The feasibility and practicability of this in-situ laser machining system were verified using specific cases. In comparison with the conventional line structured light measurement method, the proposed methods do not require light plane calibration, and do not need additional motion axes for 3D reconstruction;thus they provide technical and cost advantages. The insitu laser machining system realizes a simple operation process by integrating measurement and machining,which greatly reduces labor and time costs.

3D reconstruction of worn parts for flexible remanufacture based on robotic arc welding

3D reconstruction of worn parts is the foundation for remanufacturing system based on robotic arc welding, because it can provide 3D geometric information for robot task plan. In this investigation, a novel 3D reconstruction system based on linear structured light vision sensing is developed. This system hardware consists of a MTC368-CB CCD camera, a MLH-645 laser projector and a DH-CG300 image grabbing card. This system software is developed to control the image data capture. In order to reconstruct the 3D geometric information from the captured image, a two steps rapid calibration algorithm is proposed. The 3D reconstruction experiment shows a satisfactory result.