Accurate 3D Measurements Research Articles

Assessment of process chain suitability of the optical 3D measuring system by using influencing factors for measurement uncertainty

Optical 3D measuring systems serve as indispensable tools for the measurement and quality control of complex objects feeding process chains in industrial information integration. However, the accuracy of 3D measurements is influenced by a multitude of parameters, and the associated measurement uncertainties and influential factors remain insufficiently researched. This study investigates the effects of measurement object properties and software on measurement outcomes. Specifically, we examine seven geometries (diameter, distance, roundness, concentricity, flatness, parallelism and verticality) and four influencing factors (surface roughness, coating, polygonization, and interpolation). Our analysis employs variance analysis and compares the results with those obtained through linear regression using machine learning. In conclusion, the analysis of measurement uncertainty for optical 3D measurement systems in the assessment of seven distinct geometric characteristics provides a framework for determination of process chain suitability of the optical 3D measuring system.

Efficient and robust 3D reconstruction: Absolute phase extraction algorithm based on three gray images

The structured light three-dimensional (3D) measurement technology based on optical metrics has gained increasing applications, and there is a growing demand for capturing transient trajectory changes in dynamic scenes. Conventional temporal phase unwrapping (TPU) approaches often require multi-frequency fringe patterns for application. However, the inevitable conflict arises between the number of projected fringe patterns and the measurement speed. This paper proposes a highly accurate and robust 3D measurement method using only 3-step phase-shifting fringe patterns. To precisely reconstruct the 3D shape using the fewest fringe patterns, it is essential to extract as much information as possible from the available patterns. Therefore, we calculate the wrapped phase distribution using 3-step phase-shifting fringe patterns to ensure accuracy. Simultaneously, the trapezoidal phase-shifting method is employed to calculate the intensity ratio distribution. Through the proposed region-based indexing method, the intensity ratio map is transformed into an unwrapped phase map. Additionally, a novel self-correction method is introduced to correct the non-linear errors in the unwrapped phase map, which is then utilized for the calculation of the fringe order required for phase unwrapping. This high-speed measurement method utilizes only 3-step phase-shifting fringe patterns while retaining the advantages of TPU, meeting the requirements of high-speed and full-field measurements. Experimental results on both static and dynamic scenes validate the effectiveness and versatility of the proposed method, demonstrating its flexibility and simplicity in application. In our 3D measurement system, the method achieves 3D reconstruction of the dynamic object's trajectory at a measurement speed of 107 frames per second.

Multi-view fringe projection profilometry for surfaces with intricate structures and high dynamic range

Fringe projection profilometry plays an important role for quality control in production line. However, it is facing challenges in the measurement of objects with intricate structures and high dynamic range that involved in precision manufacturing and semiconductor packaging. In this paper, a multi-view fringe projection profilometry system, which deploys a vertical telecentric projector and four oblique tilt-shift cameras, is presented to address the “blind spots” caused by shadowing, occlusion and local specular reflection. A flexible and accurate system calibration method is proposed, in which the corrected pinhole imaging model is used to calibrate the telecentric projection, and the unified calibration is performed by bundle adjustment. Experimental results show that the 3D repeated measurement error and standard deviation are no more than 10 μm within a measurable volume of 70 × 40 × 20 mm3. Furthermore, a group of experiments prove that the developed system can achieve complete and accurate 3D measurement for high dynamic range surfaces with complex structures.

Parameter selection on a multi-exposure fusion method for measuring surfaces with varying reflectivity in microscope fringe projection profilometry

As industrial and scientific advancements continue, the demand for precise measurement of three-dimensional (3D) shapes and surfaces is steadily increasing. However, accurate 3D measurement of certain surfaces, especially those with varying reflectivities, has always been a challenging issue. Multi-exposure fusion methods have shown stable, high-quality measurement results, but the selection of parameters for these methods has largely been based on experience. To address this issue, this paper has improved the multi-exposure fusion method and introduced a guided approach for parameter selection, significantly enhancing the completeness of measurement results. Additionally, a comparative model is developed to experimentally validate the specific impacts of Gaussian window variance, optimal grayscale range, and attenuation factor variance on the integrity of 3D reconstruction. The experimental results demonstrate that under the guidance of the parameter adjustment method proposed in this paper, the multi-exposure fusion for measuring the 3D topography of high-dynamic surfaces improves the restoration coverage from the original 86% (bright areas) and 50% (dark areas) to over 99%. This provides a selection strategy for parameter adjustment guidance in precise measurements based on the multi-exposure method.

SE-FSCNet: full-scale connection network for single-shot phase demodulation

The accuracy of phase demodulation has significant impact on the accuracy of fringe projection 3D measurement. Currently, researches based on deep learning methods for extracting wrapped phase mostly use U-Net as the subject of network. The connection method between its hierarchies has certain shortcomings in global information transmission, which hinders the improvement of wrapped phase prediction accuracy. We propose a single-shot phase demodulation method for fringe projection based on a novel full-scale connection network SE-FSCNet. The encoder and decoder of the SE-FSCNet have the same number of hierarchies but are not completely symmetrical. At the decoder a full-scale connection method and feature fusion module are designed so that SE-FSCNet has better abilities of feature transmission and utilization compared with U-Net. A channel attention module based on squeeze and excitation is also introduced to assign appropriate weights to features with different scales, which has been proved by the ablation study. The experiments conducted on the test set have demonstrated that the SE-FSCNet can achieve higher precision than the traditional Fourier transform method and the U-Net in phase demodulation.

Accurate 3D Measurement of Complex Texture Objects by Height Compensation Using a Dual-Projector Structure.

Fringe projection profilometry is a widely used technique for 3D measurement due to its high accuracy and speed. However, the accuracy significantly decreases when measuring complex texture objects, especially in the junction of different colors. This paper analyzes the causes of errors resulting from complex textures and proposes a height compensation method to revise the error by employing a dual-projector structure. Moreover, the dual-projector is capable of acquiring a pair of errors with opposite signs, which can be utilized to calculate the accurate 3D information after determining the ratio of this pair of errors. Experiments provide significant improvement in measuring complex texture objects, demonstrating the proposed method's ability.

ANALYSIS OF CALIBRATION ERRORS IN AUTOMOBILE CONTROL TELEVISION SYSTEM

Camera calibration is a critical method for 3D measurement applications based on 2D images. During calibration, internal camera parameters such as principal point and focal length are determined. The principal point is the intersection of the camera's optical axis and the image plane, while the focal length is measured from the lens center to the principal point.This article theoretically investigates calibration errors in two reference planes considering various conditions, such as shifts in the reference plane and changes in focal lengths. Errors in determining the coordinates of reference points in images were used to evaluate these calibration errors.The error in estimating the focal length and principal point is formulated using analytical equations, and the results are compared with simulation outcomes to validate the effectiveness of the proposed formulas. The research reveals that calibration errors arise due to shifts in the reference plane and changes in focal lengths. The obtained results are crucial for enhancing camera calibration methods and improving the accuracy of 3D measurements based on 2D images.This article extends the understanding of camera calibration errors and provides new theoretical approaches for their assessment. The research findings may be beneficial for scientists and practitioners working in the fields of computer vision, image processing, and 3D measurements.Further research in this area may contribute to the development of new calibration methods and approaches that will improve the performance of automotive systems and ensure high accuracy and reliability in the future.

Depth estimation from a single-shot fringe pattern based on DD-Inceptionv2-UNet

The quick and accurate retrieval of an object’s depth from a single-shot fringe pattern in fringe projection profilometry has been a topic of ongoing research. In recent years, with the development of deep learning, a deep learning technique to FPP for single-shot 3D measurement is being used. To improve the accuracy of depth estimation from a single-shot fringe pattern, we propose the depthwise separable Dilation Inceptionv2-UNet (DD-Inceptionv2-UNet) by adjusting the depth and width of the network model simultaneously. And we evaluate the model on both simulated and experimental datasets. The experimental results show that the error between the depth map predicted by the proposed method and the label is smaller, and the depth curve map is closer to the ground truth. And on the simulated dataset, the MAE of the proposed method decreased by 35.22%, compared to UNet. On the experimental dataset, the MAE of the proposed method decreased by 34.62%, compared to UNet. The proposed method is relatively outstanding in both quantitative and qualitative evaluations, effectively improving the accuracy of 3D measurement results from a single-shot fringe pattern.

Experimental Evaluation of Highly Accurate 3D Measurement Using Stereo Camera and Line Laser

This paper proposes a method to improve the accuracy of 3D measurement of a stereo camera by marking a measured object using a line laser. Stereo cameras are commonly used for 3D measurement, but the accuracy of 3D measurement is affected by the amount of texture. Therefore, a new measurement system combining a stereo camera and a line laser is developed. The accuracy of 3D measurement with a stereo camera is improved by using a line laser to mark arbitrary points on the measured object and measuring the marked points, regardless of the amount of texture on the measured object. Because the laser is only used to mark points on the measurement target, calibration is not required with the stereo camera. Experimental evaluation showed that our proposed method can obtain millimeters.

Physics-based learning: Adaptive structured light for active stereo depth estimation

Active stereo systems based on structured light are widely used for 3D vision in various applications, which project specially designed patterns onto object surfaces to encode each position in space for accurate 3D measurements. However, existing approaches use pre-determined patterns that are isolated from the scene properties (object reflectivity, distance, ambient light, inter-reflection), equipment (projector, camera), and reconstruction. Meanwhile, the parameters of the structured light are determined through empirical analysis or several experiments. In this paper, we propose a novel structured light design approach, named Physics-Based Learning Adaptive Structured Light (PBL-ASL), which directly learns the optimal structured light patterns from the scene. To this end, (1) we design a decoder with a non-sinusoidal error suppression module for PBL-ASL, which can accurately estimate the disparity during structured light optimization; (2) we propose a physics-based learning algorithm consisting of a self-supervised objective function and a differentiable imaging model, which computes the disparity error and back-propagates the gradient to the encoding vector to optimize the structured light. Our experiments demonstrate that PBL-ASL can significantly improve the depth estimation accuracy of active stereo systems over several state-of-art methods.

Wide field 3D optical profilometry using a diffraction Lloyd's mirror interferometer.

Interference fringe projection is used as a non-contact optical profilometry method for accurate 3D measurements. In interferometric fringe projection schemes, the maximum measurable size of the test object is limited by the optics of the interferometer. In this work, we report the application of a diffraction Lloyd's mirror interferometer (DLMI) as a wide-field sinusoidal fringe projection system for 3D shape measurement. The DLMI works on diffracted light and therefore generates interference fringes over a large area. This enables measurement of large objects using DLMI as compared to a conventional Lloyd's mirror interferometer. The performance of the proposed system is evaluated in terms of its stability and reproducibility of the results through measurement of the standard deviation in the phase values.

The Accuracy of 3D Measurements in Weightbearing Computed Tomography to Diagnose Lisfranc Instability

Category: Midfoot/Forefoot; Other Introduction/Purpose: Up to one-third of Lisfranc injuries are missed on initial evaluation, resulting in significant morbidity. Prompt diagnosis of Lisfranc injuries is, therefore, critical in optimizing outcomes, and yet there remains a lack of consensus on diagnostic criteria to identify Lisfranc instability using anatomic measurements. Prior studies have underscored the utility of weight-bearing computed tomography (WBCT) in diagnosing Lisfranc instability, which allows for bilateral three-dimensional (3D) evaluation under physiologic load. The aim of this study was to quantify appropriate cutoff values using 3D anatomic measurements of Lisfranc instability under physiologic load and as compared to the uninjured extremity. Methods: A total of 234 adult patients with bilateral WBCT scans of the foot were included: 43 patients with Lisfranc instability & 191 patients without a history of midfoot injuries. Lisfranc instability was confirmed intraoperatively or by clinical evaluation by the surgeon. Patients with prior midfoot surgery, Charcot arthropathy, severe midfoot arthropathy, or significantly displaced fracture of the cuneiforms or 1st, 2nd, or 3rd metatarsal bases were excluded. Lisfranc joint measurements were conducted bilaterally, including C1-M2 diastasis, C1-M2 area and 3D WBCT volume. Patient demographics were collected. Descriptive statistics were calculated for quantitative variables. Percent difference as compared to the contralateral side was calculated for volume, area, and diastasis. Mann Whitney U tests were utilized to determine differences in measurements between patients in the Lisfranc cohort and patients in the uninjured cohort (p-value < 0.05). Diagnostic cutoffs for Lisfranc instability were determined with minimum distance to the corner analysis on ROC curves. Results: The distribution of patients with Lisfranc instability was 58% female with an average age of 38.5±17.6 years. Of the Lisfranc injuries, 36% were purely ligamentous, 18% demonstrated an avulsion injury, and 43% involved a metatarsal base fracture. In the Lisfranc cohort, the median percent difference between injured and uninjured feet was 28.2% (IQR: 20.9%) for volume, 36.3% (IQR: 31.1%) for C1-M2 area, and 40.0% (IQR: 23.3%) for C1-M2 diastasis. Mann-Whitney testing was significant for percent difference between left and right feet for Lisfranc patients versus patients with an uninjured midfoot (p < 0.001). The area under the curve and associated diagnostic cutoffs for Lisfranc instability, were 0.81 and 18% for volume, 0.84 and 20% for area, and 0.91 and 28% for diastasis (Figure 1). Conclusion: Diastasis (1D), area (2D), and volume (3D) measurements are effective measurements to diagnose Lisfranc instability on WBCT. With current diagnostic algorithms, however, 1D measurements afford superior diagnostic sensitivity as compared to 2D and 3D measurements when using WBCT, suggesting Lisfranc instability may be best appreciated in the axial plane. Additional studies are necessary to expand the sample population to assess for corroboration with the current results, especially for subtle Lisfranc instability.

High dynamic reflection surface 3D reconstruction with sharing phase demodulation mechanism and multi-indicators guided phase domain fusion.

Accurate and complete 3D measurement of complex high dynamic range (HDR) surfaces has been challenging for structured light projection technique. The behavior of spraying a layer of diffuse reflection material, which will inevitably incur additional thickness. Existing methods based on additional facilities will increase the cost of hardware system. The algorithms-based methods are cost-effective and nondestructive, but they generally require redundant patterns for image fusion and model training, which fail to be suitable for practicing automated 3D measurement for complex HDR surfaces. In this paper, a HDR surface 3D reconstruction method based on sharing demodulation phase unwrapping mechanism and multi-indicators guided phase fusion strategy is proposed. The division of the exposure interval is optimized via the image entropy to generate an optimal exposure sequence. The combination of temporal-spatial binary (TSB) encoding fringe patterns with time-integration strategy and the variable exposure mode of digital mirror device (DMD)-based projector with a minimum projection exposure time of 233μs enables the proposed approach to broadly adapt complex HDR surfaces. We propose an efficient phase analysis solution called sharing mechanism that wrapped phase sequences from captured different intensity fringe images are unwrapped through sharing the same group of misaligned Gray code (MGC) decoding result. Finally, a phase sequences fusion model guided by multi-indicators, including exposure quality, phase gradient smoothness and pixel effectiveness, is established to obtain an optimum phase map for final 3D reconstruction. Comparative experiments indicate that the proposed method can completely restore the 3D topography of HDR surfaces with the images reduction of at least 65% and the measurement integrity is maintained at over 98% while preserving the measurement accuracy and excluding the outliers.

Tribo-Mechanical Investigation of Glass Fiber Reinforced Polymer Composites under Dry Conditions.

Tribo-mechanical experiments were performed on Glass Fiber Reinforced Polymer (GRFP) composites against different engineering materials, and the tribological behavior of these materials under dry conditions was investigated. The novelty of this study consists of the investigation of the tribomechanical properties of a customized GFRP/epoxy composite, different from those identified in the literature. The investigated material in the work is composed of 270 g/m2 fiberglass twill fabric/epoxy matrix. It was manufactured by the vacuum bag method and autoclave curing procedure. The goal was to define the tribo-mechanical characteristics of a 68.5% weight fraction ratio (wf) of GFRP composites in relation to the different categories of plastic materials, alloyed steel, and technical ceramics. The properties of the material, including ultimate tensile strength, Young's modulus of elasticity, elastic strain, and impact strength of the GFPR, were determined through standard tests. The friction coefficients were obtained using a modified pin-on-disc tribometer using sliding speeds ranging from 0.1 to 0.36 m s-1, load 20 N, and different counter face balls from Polytetrafluoroethylene (PTFE), Polyamide (Torlon), 52,100 Chrome Alloy Steel, 440 Stainless Steel, and Ceramic Al2O3, with 12.7 mm in diameter, in dry conditions. These are commonly used as ball and roller bearings in industry and for a variety of automotive applications. To evaluate the wear mechanisms, the worm surfaces were examined and investigated by a Nano Focus-Optical 3D Microscopy, which uses cutting-edge μsurf technology to provide highly accurate 3D measurements of surfaces. The obtained results constitute an important database for the tribo-mechanical behavior of this engineering GFRP composite material.

Single-Shot Three-Dimensional Measurement by Fringe Analysis Network

Fringe projection profilometry (FPP) has been broadly applied in three-dimensional (3D) measurements, but the existing multi-shot methods, which mostly utilize phase-shifting techniques, are heavily affected by the disturbance of vibration and cannot be used in dynamic scenes. In this work, a single-shot 3D measurement method using a deep neural network named the Fringe Analysis Network (FrANet) is proposed. The FrANet is composed of a phase retrieval subnetwork, phase unwrapping subnetwork, and refinement subnetwork. The combination of multiple subnetworks can help to recover long-range information that is missing for a single U-Net. A two-stage training strategy in which the FrANet network is pre-trained using fringe pattern reprojection and fine-tuned using ground truth phase maps is designed. Such a training strategy lowers the number of ground truth phase maps in the data set, saves time during data collection, and maintains the accuracy of supervised methods in real-world setups. Experimental studies were carried out on a setup FPP system. In the test set, the mean absolute error (MAE) of the refined absolute phase maps was 0.0114 rad, and the root mean square error (RMSE) of the 3D reconstruction results was 0.67 mm. The accuracy of the proposed method in dynamic scenes was evaluated by measuring moving standard spheres. The measurement of the sphere diameter maintained a high accuracy of 84 μm at a speed of 0.759 m/s. Two-stage training only requires 8800 fringe images in data acquisition, while supervised methods require 96,000 fringe images for the same number of iterations. Ablation studies verified the effectiveness of two training stages and three subnetworks. The proposed method achieved accurate single-shot 3D measurements comparable to those obtained using supervised methods and has a high data efficiency. This enables the accurate 3D shape measurement of moving or vibrating objects in industrial manufacturing and allows for further exploration of network architecture and training strategy with few training samples for single-shot 3D measurement.

On-chip size, low-noise fringe pattern projector offering highly accurate 3D measurement.

Fringe pattern projectors are quite useful for highly accurate three-dimensional (3D) measurement when a projector or LED array is used for illumination. We have fabricated a 0.2 mm × 0.2 mm structured light source, which was an on-chip size surface-emitting laser that utilized a holographically modulated two-dimensional (2D) photonic crystal (PC). This will make possible an extremely compact 3D measurement system that will positively impact mobile systems. However, the fringe pattern tends to cause speckle-like noise that leads to severe positional error in 3D measurement. Here we present a simple approach to projecting a low-noise fringe pattern from our surface-emitting lasers by using a one-dimensional (1D) focusing hologram. This method improves the flatness of the fringe pattern by around four times.

Real-time and accurate monocular 3D sensor using the reference plane calibration and an optimized SGM based on opencl acceleration

Speckle projection profilometry (SPP), as a promising structured light projection technique, can achieve global unambiguous 3D measurement by projecting a single random speckle pattern. In addition, the projected speckle pattern is usually etched into the microstructure of highly integrated Vertical-Cavity Surface-Emitting Laser (VCSEL), which makes the hardware system compact enough to be mounted on mobile devices such as robots. However, since the stereo matching algorithm used in SPP involves high computational overhead, it usually runs in real-time on specially customized hardware platforms such as ASIC/FPGA, rather than general-purpose mobile platforms. In this paper, we propose a real-time and accurate 3D measurement method using a monocular 3D sensor based on the infrared speckle projection. Similar to Kinect v1, our sensor mainly consists of an IR dot projector and one IR camera for projecting and capturing speckle images synchronously. Low-cost and high-quality speckle projection is achieved by customizing the projection pattern of VCSEL and using the beam copy function of Diffractive Optical Elements (DOE). Based on the 3D imaging principle of monocular 3D sensors, a reference plane calibration method is proposed to obtain a high-quality reference speckle image for improving the monocular matching accuracy. Then, benefited from the local memory mechanism and multiple operating synchronizations on the OpenCL environment, an optimized semi-global matching (SGM) algorithm using GPU is presented to achieve efficient and accurate depth reconstruction dynamically. Within the measurement range of 0.8m (length) ×0.5m (width) ×1m (depth), the proposed method can achieve real-time and single-shot 3D imaging with an accuracy of 1.277 mm at 75 FPS on GTX 1060 and 15 FPS on ARM Mail G52(mobile platform).

Analysis of the Accuracy Potential of a Stereo High-Speed Camera System in 3D Measurements in Highly Dynamic Experiments.

In the context of setting up a stereo high-speed camera system for accurate 3D measurements in highly dynamic experiments, the potential of a "Fastcam SA-X2" stereo system is evaluated by testing different camera configurations and motion scenarios. A thorough accuracy analysis is performed using spatial rigid-body transformations and relative measurement analyses of photogrammetrically reconstructed surfaces of nondeformable objects. The effects of camera calibration, exposure time, object velocity, and object surface pattern quality on the quality of adjusted 3D coordinates are taken into consideration. While the exposure time does not significantly influence the quality of the static measurements, the results of dynamic experiments demonstrate that not only an insufficient frame rate but also an increased noise level resulting from short exposure times affects 3D coordinate accuracy. Using appropriate configurations to capture dynamic events, the errors in dynamic experiments do not differ significantly from the errors obtained in static measurements. A spatial mapping error of less than 1 μm is obtained through the experiments, with proper testing configurations for an object surface area of 5×20 mm. These findings are relevant for users of high-speed stereo imaging techniques to perform geometric 3D measurements, deformation, and crack analyses.

A Continuous Motion Shape-from-Focus Method for Geometry Measurement during 3D Printing.

In 3D printing, as in other manufacturing processes, there is a push for zero-defect manufacturing, mainly to avoid waste. To evaluate the quality of the printed parts during the printing process, an accurate 3D measurement method is required. By scanning the part during the buildup, potential nonconformities to tolerances can be detected early on and the printing process could be adjusted to avoid scrapping the part. Out of many, shape-from-focus, is an accurate method for recovering 3D shapes from objects. However, the state-of-the-art implementation of the method requires the object to be stationary during a measurement. This does not reconcile with the nature of 3D printing, where continuous motion is required for the manufacturing process. This research presents a novel methodology that allows shape-from-focus to be used in a continuous scanning motion, thus making it possible to apply it to the 3D manufacturing process. By controlling the camera trigger and a tunable lens with synchronous signals, a stack of images can be created while the camera or the object is in motion. These images can be re-aligned and then used to create a 3D depth image. The impact on the quality of the 3D measurement was tested by analytically comparing the quality of a scan using the traditional stationary method and of the proposed method to a known reference. The results demonstrate a 1.22% degradation in the measurement error.

Accuracy Analysis for 3D Model Measurement Based on Digital Close-range Photogrammetry Technique for the Deep Foundation Pit Deformation Monitoring

The deformation of deep foundation pits (DFP) may pose a huge potential threat to the safety of the bracing system and the surrounding environment. A large amount of deformation data facilitates the deformation prediction of DFP and the dynamic adjustment of the bracing scheme. This study proposes and validates a 3D modeling technique based on digital close-range photogrammetry, which can obtain sufficient deformation data and enhance the understanding of deformation patterns. Two typical sets of deformation experiments, namely the vertical deformation test and the horizontal deformation test, were conducted to examine the viability and accuracy of 3D model measurement. Furthermore, the factors affecting the accuracy of the measurement were discussed in detail. The results indicate that the measurement accuracy of 3D models can reach millimeter-level or even sub-millimeter level. The measurement accuracy is affected by the coupling of the pixel resolution of CCD, focal length, photographic distance, and photographic baseline. The increase in the pixel resolution of CCD and focal length improve the measurement accuracy of 3D model measurement, but increasing photographic distance shows the opposite trend. The ratio of photographic baseline to photographic distance (BDR) has a much greater effect on accuracy than other factors. The photographic distance with a resolution of less than 1.0mm and the BDR value of 0.6–1.8 are recommended. Based on the results, it is found that this technique can be adopted as an effective monitoring method in DFP deformation monitoring due to the high precision and labor-saving.