High quality three-dimensional (3D) shape measurement using intensity-optimized dithering technique

In past decades, there has been an upsurge in the development of three-dimensional (3D) shape measurement and its applications. Over the years, there are a variety of technologies developed including laser scanning, stereo vision, and structured light. Among these technologies, the structured-light technique has the advantages of fast computation speeds and high measurement resolution. Therefore, it has been extensively studied in this field of research. Nowadays, with the rapid development of digital devices, different kinds of patterns can be easily generated by a video projector. As a result, digital fringe projection (DFP), a variation of the structured light method, has had many applications owing to its speed and accuracy. Typically, for a DFP system, ideal sinusoidal fringe pattern projection is required for high accuracy 3D information retrieval. Since traditional DFP projects 8-bit sinusoidal fringe patterns, it suffers from some major limitations such as the speed limit (e.g., 120 Hz), the requirement for nonlinear gamma calibration, and the rigid synchronization requirement between the projector and the camera. To overcome these limitations, the binary defocusing technology was developed which projects 1-bit square binary pattern and generates ideal sinusoidal pattern through projector defocusing. In the past few years, the binary defocusing technique has shown great potential for many applications owing to its speed breakthroughs, nonlinear gamma calibration free and no rigid synchronization requirement between the camera and the projector. However, a typical square binary pattern suffers from some major limitations: (1) high-order harmonics, introduced by a square wave, which affect the accuracy of measurement, cannot be completely eliminated by projector defocusing; (2) a reduced measurement volume since the projector needs to be properly defocused to generate the desired high-quality sinusoidal patterns; and (3) difficulty achieving high-quality measurements with wider square binary patterns. The binary dithering technique, originally developed for printing technology, is found to have great potential for overcoming these aforementioned limitations of the square binary method. However, the binary dithering technique, which simply applies a matrix operation to the whole image, still has great room for improvement especially when the fringe patterns are not sufficiently defocused. Although there have been past efforts made to improve the performance of dithering techniques for 3D shape measurement, those approaches are either computationally expensive or fail to improve the quality with different amounts of defocusing. In this research, we aim at further improving the binary dithering technique by optimizing the dithered patterns in intensity domain. We have developed both global and local optimization frameworks for improving dithered patterns. Our simulation and experimental results have demonstrated that: the global optimization framework improves the Bayer-order dithering technique by approximately 25% overall and up to 50% for narrower fringe patterns (e.g. fringe period of T = 18 pixels); the local optimization framework can improve the performance of a more advanced error-diffusion dithering technique by 20% overall and up to 40% for narrower fringe patterns (e.g. T = 18 pixels). Moreover, since the local algorithm involves optimizing a small image block and building up the desired-size patterns using symmetry and periodicity, it is much faster in terms of optimization time than the global algorithm

High-speed 3D shape measurement with fiber interference

This paper presents a miniaturized fringe projection system that only uses two fibers to potentially achieve superfast (e.g., MHz to GHz) 3D shape measurement speeds. The proposed method uses two optical fibers that carry the same wavelength of laser light with polarization and phase information properly modulated to generate high-quality sinusoidal fringe patterns through interference. The high-speed phase shifting is achieved by employing a high-speed Lithium Niobate (LN) electrooptic phase modulator. Since only two optical fibers are used to generate sinusoidal patterns, the system has a great potential of miniaturization for applications where the sensor size is critical (e.g., 3D endoscopy). Principle of the proposed techniques will be introduced, and preliminary experimental results will be presented in this paper to prove the success of the proposed metho

Improve dithering technique for 3D shape measurement: phase vs intensity optimization

This paper presents a thorough comparison between a phase-based and an intensity-based optimization method for 3D shape measurement with the binary dithering techniques. Since for a 3D shape measurement system utilizing digital fringe projection techniques, the phase quality ultimately determines the measurement quality, and thus these two methods are compared in phase domain. Both simulation and experiments find that the phase-based optimization method can generate high-quality phase under given conditions. However, this method is sensitive to the amount of blurring (or defocusing). On contrast, the intensity-based optimization method can consistently generate high-quality phase with various amounts of defocusing. Both experiments and simulations will be presented to compare these two optimization method

Comparison between LCOS projector and DLP projector in generating digital sinusoidal fringe patterns

Digital-light-processing (DLP) and liquid-crystal-on-silicon (LCOS) are two digital projection technologies which enjoy great popularity. This paper will demonstrate the performance of the two technologies in generating digital sinusoidal fringe patterns in the two following ways: focused-sinusoidal-patterns (FSP) method and defocused-binary-patterns (DBP) method. Experiment shows that for the FSP method, LCOS projector is a better choice since nonlinear gamma effect is less significant and there is no need for precise synchronization; While for the DBP method, DLP projector has the advantage over LCOS projector since the generated images have higher contrast ratio

Flexible real-time natural 2D color and 3D shape measurement

The majority of existing real-time 3D shape measurement systems only generate non-nature texture (i.e., having illumination other than ambient lights) that induces shadow related issues. This paper presents a method that can simultaneously capture natural 2D color texture and 3D shape in real time. Specifically, we use an infrared fringe projection system to acquire 3D shapes, and a secondary color camera to simultaneously capture 2D color images of the object. Finally, we develop a flexible and simple calibration technique to determine the mapping between the 2D color image and the 3D geometry. Experimental results demonstrate the success of the proposed technique

Measurement Studies Utilizing Similarity Evaluation between 3D Surface Topography Measurements

In the realm of quality assurance, the significance of statistical measurement studies cannot be overstated, particularly when it comes to quantifying the diverse sources of variation in measurement processes. However, the complexity intensifies when addressing 3D topography data. This research introduces an intuitive similarity-based framework tailored for conducting measurement studies on 3D topography data, aiming to precisely quantify distinct sources of variation through the astute application of similarity evaluation techniques. In the proposed framework, we investigate the mean and variance of the similarity between 3D surface topography measurements to reveal the uniformity of the surface topography measurements and statistical reproducibility of the similarity evaluation procedure, respectively. The efficacy of our framework is vividly demonstrated through its application to measurements derived from additive-fabricated specimens. We considered four metal specimens with 20 segmented windows in total. The topography measurements were obtained by three operators using two scanning systems. We find that the repeatability variation of the topography measurements and the reproducibility variation in the measurements induced by operators are relatively smaller compared with the variation in the measurements induced by optical scanners. We also notice that the variation in the surface geometry of different surfaces is much larger in magnitude compared with the repeatability variation in the topography measurements. Our findings are consistent with the physical intuition and previous research. The ensuing experimental studies yield compelling evidence, affirming that our devised methods are adept at providing profound insights into the multifaceted sources of variation inherent in processes utilizing 3D surface topography data. This innovative framework not only showcases its applicability but also underlines its potential to significantly contribute to the field of quality assurance. By offering a systematic approach to measuring and comprehending variation in 3D topography data, it stands poised to become an indispensable tool in diverse quality assurance contexts.This article is published as Liu, Lijie, Beiwen Li, Hantang Qin, and Qing Li. "Measurement Studies Utilizing Similarity Evaluation between 3D Surface Topography Measurements." Mathematics 12, no. 5 (2024): 669. doi: https://doi.org/10.3390/math12050669. Copyright: © 2024 by the authors. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https://creativecommons.org/licenses/by/4.0/)

Similarity evaluation of topography measurement results by different optical metrology technologies for additive manufactured parts

The surface topographic measurements can be used by the additive manufacturing (AM) industry for in-situ quality inspection. However, disagreements may arise when we use different technologies to measure the topography of the same sample surface due to noise, sampling or optical properties of the sample surface, which may cause miscommunications or confusions between manufacturers. Thus, proposing methods for rating the similarities to match surface topographic data measured by various optical techniques is of crucial importance. This research investigates similarity evaluation methods for three-dimensional point-cloud topography data acquired by different technologies. Two different optical techniques (focus variation microscopy and structured light scanning) are used as testbeds. We propose two similarity evaluation methods for three-dimensional point-cloud data based on image distance method and Pearson’s correlation coefficient. The experimental results demonstrate that the proposed methods are effective and informative in determining whether the measured data are collected from the same sample, even though the measuring systems have different working principles and resolutions. This research facilitates our understanding of the discrepancies between different measuring systems, and meanwhile benefits a cyber-manufacturing system where unified inspection methods are unavailable among different manufacturers sharing the metrology data in cyber space

Insight into the Interaction of Metal Ions with TroA from Streptococcus suis

The scavenging ability of sufficient divalent metal ions is pivotal for pathogenic bacteria to survive in the host. ATP-binding cassette (ABC)-type metal transporters provide a considerable amount of different transition metals for bacterial growth. TroA is a substrate binding protein for uptake of multiple metal ions. However, the function and structure of the TroA homologue from the epidemic Streptococcus suis isolates (SsTroA) have not been characterized.Here we determined the crystal structure of SsTroA from a highly pathogenic streptococcal toxic shock syndrome (STSS)-causing Streptococcus suis in complex with zinc. Inductively coupled plasma mass spectrometry (ICP-MS) analysis revealed that apo-SsTroA binds Zn(2+) and Mn(2+). Both metals bind to SsTroA with nanomolar affinity and stabilize the protein against thermal unfolding. Zn(2+) and Mn(2+) induce distinct conformational changes in SsTroA compared with the apo form as confirmed by both circular dichroism (CD) and nuclear magnetic resonance (NMR) spectra. NMR data also revealed that Zn(2+)/Mn(2+) bind to SsTroA in either the same site or an adjacent region. Finally, we found that the folding of the metal-bound protein is more compact than the corresponding apoprotein.Our findings reveal a mechanism for uptake of metal ions in S. suis and this mechanism provides a reasonable explanation as to how SsTroA operates in metal transport

Adaptive channel selection in IEEE 802.15.4 TSCH networks

Additional files 6: Table S5. Four conjugative transposon gene clusters in the Chryseobacterium indologenes J31 genome

Superfast 3D Shape Measurement with Application to Flapping Wing Mechanics Analysis

The goal of measurement is to allow a person to perceive the three-dimensional (3D) world around us, to know about a substance, and to obtain or produce new knowledge. However, performing accurate measurements of dynamically deformable objects has always been a challenging task, which in fact has huge potential to applications in areas of manufacturing, robotics, non-destructive evaluations, etc. Over a decade of efforts, scientists have made significant progresses along this direction. In particular, the binary defocusing method, which performs fringe analysis upon the camera captured distorted 1-bit binary patterns projected by an out-of-focus projector, has reached unprecedented superfast measurement speeds (e.g. kHz) with high spatial resolutions. Despite of the speed breakthrough, there are still a number of challenges associated with such technology: (1) requiring an out-of-focus projector brings about difficulty in achieving high measurement accuracy; (2) motion induced artifacts and errors are still present if measuring a scene with fast moving objects; (3) it is difficult to perform subsequent analysis (e.g. deformation, mechanics, etc.) solely by interpreting those uncorrelated frames of acquired dynamic 3D data. The first challenge is mainly caused by the difficulty of performing an accurate calibration for a camera-projector system with an out-of-focus projector. To deal with this problem, we have theoretically proved and experimentally validated that a camera pixel can be virtually mapped to a projector pixel in phase domain even if the projector is substantially out-of-focus. Based on this foundation, we developed novel calibration approaches that can successfully achieve high accuracy under different scales: in a macro-scale measurement range [e.g. 150 mm(H) X 250 mm(W) X 200 mm(D)], we achieved an accuracy up to 73 µm; in a medium-scale measurement range [e.g. 10 mm(H) X 8 mm(W) X 5 mm(D)] , we achieved an accuracy up to 10 µm. The second challenge is quite common in dynamic measurements if the sampling rate is not high enough to keep up with the object motion. Although employing hardware with higher measurement speeds is always a potential solution, it is more desirable to innovate software algorithms to reduce the hardware cost. We developed two different software approaches to deal with the problems associated with object motion: (1) a single-shot absolute 3D recovery method to increase the sampling rate; (2) a motion induced error reduction framework. The first approach successfully overcame the difficulty of absolute 3D recovery for existing single-shot fringe analysis methods by taking advantage of the geometric constraints of a camera-projector system. The second approach successfully alleviated motion induced errors and artifacts by hybridizing the merits of two commonly used fringe analysis techniques: Fourier transform and phase shifting. Addressing both aforementioned challenges has enabled us to perform simultaneous superfast and high-accuracy 3D shape measurements with reduced motion induced errors or artifacts. Under such platform, we are seeking to introduce the technologies to a different field and explore an application. Finally, a particular topic presented in this dissertation is our research on 3D strain analysis of robotic flapping wings. Measuring dense 3D strain map of flapping wings could potentially produce new knowledge for the design of bio-inspired flapping wing robots. Such topic, however, is not well documented so far owing to the lack of an appropriate technological platform to measure dense 3D strain maps of the wings. In this dissertation research, we first measured the dynamic 3D geometries of a robotic bird with rapid flapping wings (e.g. 25 cycles/second) using a superfast image acquisition rate of 5,000 Hz. Then, we developed a novel geometry-based dense 3D strain measurement framework based on geodesic computation and Kirchhoff-Love shell theory. Such an innovation could potentially benefit bio-inspired robotics designers by introducing a new method of geometric and mechanical analysis, which could be used for better design of robotic flapping wings in the future. In summary, this dissertation research substantially advances the research of 3D shape measurement by achieving simultaneous superfast and high-accuracy measurements. Meanwhile, it demonstrates the potential of such technology by developing geometry-based 3D data analytics tools and exploring an application. Contributions of this research could potentially benefit a variety of different fields in both academic and industrial practices, where both speed and accuracy are major concerns and where subsequent mechanics analysis are necessary