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For fringe projection profilometry (FPP), the nonlinear response of the camera and projector in a 3-D measurement system causes the captured fringe to be nonsinusoidal, which results in periodic phase errors. This article reports a theoretical analysis of the phase error caused by the nonsinusoidal fringe and proposes a triple $N$ -step phase shift (PS) algorithm. Experimental results demonstrate that the proposed algorithm outperforms the existing double $N$ -step PS algorithm in terms of reducing the phase error caused by the nonsinusoidal captured fringe. [ABSTRACT FROM AUTHOR]

A predistortion-based 3-D phase mapping method with linear epipolar constraints for fringe projection system calibration and measurement is reported. Hybrid system calibration methods have been developed recently to provide accurate measurement, high speed, and flexibility for fringe projection profilometry (FPP). However, these methods still have significant limitations. With projector distortion, for example, rigorous post-undistortion often results in low efficiency in computation. Conversely, simple, approximated undistortion is only effective for low levels of distortion. As a result, projector undistortion is usually not prioritized in practice. Aiming to achieve high-accuracy 3-D reconstruction with FPP, an easily implementable projector undistortion method is proposed in this article. Theoretical analyses and simulations confirm that the proposed method is rigorous. Experiments validate that the pixelwise root-mean-square reconstruction error is 0.042 mm, a 24% reduction from that of a simple post-undistortion method. The proposed method is also efficient computationally. Finally, a 3-D phase mapping lookup table is built. The results show that the proposed method is effective for achieving high accuracy and speed in 3-D surface measurement. [ABSTRACT FROM AUTHOR]

This article presents a composite (COM) method to obtain the high-resolution pulse shape discrimination (PSD) for the neutron and gamma-ray pulses generated from scintillation detectors. The method, which is based on a selective combination of the digital charge integration (DCI) with the reference pulse method, aims to reduce the mixed radiation events in the low-energy range. An EJ-301 liquid scintillation detector together with a fast sampling analog-to-digital converter (ADC) is used to measure and digitize the pulses induced from the radioactive decays of 60Co and 252Cf, which are then analyzed by our COM method. The proposed method is evaluated using the figure of merit (FoM) and separation quality function $F(u)$ , and the results are compared with three known methods, namely the DCI, standard event fit (SEF), and artificial neural network (ANN) methods. We show that the average values of FoM and $F(u)$ obtained within the COM method are about ten times higher than those obtained within the DCI and SEF in the whole energy range from 50 to 1000 keV electron equivalent (keVee). In particular, by using the COM method, the percentage of gamma events being confused as neutrons ranges from 0.32% to 8.80% when the energy is reduced from 400 to 50 keVee. This finding, which is significantly lower than those obtained by using the DCI and SEF, indicates that the proposed COM method should be considered as a leading method for producing a neutron/gamma PSD counter system with high resolution. [ABSTRACT FROM AUTHOR]

Precision measurement and reconstruction of detailed surfaces topography is a challenging task for non-diffuse complex parts. Although coordinate measurement machines (CMM) with the touch-trigger probe are widely used in current industry, the measurement efficiency limits their application in the measurement of complex surfaces. This article proposes a multisensor data fusion strategy by integrating the technical merits of CMM and photometric stereo (PS) to achieve multiscale reconstruction of a complex surface with high efficiency. Considering the complementary measurement characteristics of the two approaches, the sparse points from CMM are used to provide global shape information, and the high-resolution surface normal map from PS is used to provide local detailed structure. A multistage neural network is then proposed to fuse these two kinds of modality information such that the global features from the sparse points and the local features from the surface normal map are fused in a coarse-to-fine multistage process so as to make the training process more stable and the reconstruction more accurate. To enhance the generality of the fusion neural network, a synthetic training data set is also designed to include a large variety of multiscale features enriched surfaces. Experiments are conducted to verify the effectiveness of the proposed multisensor fusion strategy in accurate reconstruction of complex surfaces with high efficiency. [ABSTRACT FROM AUTHOR]

Shape measurement plays an important role in the condition monitoring and operation control of inflatable rubber dams. This article presents a method to measure the cross-sectional shape of a rubber dam using an array of inertial measurement units (IMUs) placed on the circumference of the dam. Accelerometer and gyroscope measurements are combined using an adaptive complementary filter to determine the tangent angles of the dam circumference. The adaptive complementary filter adjusts the weights of the accelerometer and gyroscope measurements dynamically in order to reduce the uncertainty in orientation estimation due to external acceleration under dynamic conditions. A natural cubic spline that interpolates the measured tangent angles at discrete locations is used to represent the tangent angles along the dam circumference as a continuous function of the arc length. Finally, the cross-sectional shape is reconstructed by integrating the continuous tangent angle function along the circumference of the dam. Experimental assessment of the measurement system was performed on a purpose-built test rig using a digital camera as a reference measuring device. Results under a typical static condition show that the measured and reference shapes agree well with each other, with a similarity index being 3.74%, a mismatch distance of the last IMU node being 12.3 mm, and a relative error of height measurement being −2.44%. Under dynamic conditions, the measurement results deteriorate due to external acceleration, but considerable improvement is achieved in comparison with an accelerometer-only approach. In addition, the elimination of faulty nodes from shape reconstruction has negligible influence on the results, suggesting that the measurement system enjoys a high degree of fault tolerance. [ABSTRACT FROM AUTHOR]

Phase-coding methods have been widely used for three-dimensional (3-D) shape measurement, which use sinusoidal phase-shifting patterns to recover wrapped phase and the stair phase-coding patterns to determine the fringe order. However, due to the system nonlinearity, random noises, and image blurring, the total number of codewords that can be used is limited. In order to increase the number of codewords, this article presents a two-digit phase-coding (TDPC) strategy, in which each fringe of the sinusoidal phase-shifting patterns corresponds to a two-digit codeword encoded in the phase-coding patterns. In this way, more codewords can be generated without increasing additional patterns. Experiments of the cup and sculptures have been carried out, and the results confirm that the TDPC method can recover the 3-D shape of objects more robustly. Meanwhile, the precision experiment of a standard ball proves the high accuracy of the proposed method. [ABSTRACT FROM AUTHOR]

Amplitude-modulated continuous-wave (AMCW) laser scanner with focusing optics is expected to realize high-precision 3-D measurement, which requires accuracy of mm or less. Since such a modulation scheme employs periodical intensity modulation, the longitudinal resolution and the unambiguous range are in a tradeoff. Another problematic situation is the case that the target object is large so that the scanning range exceeds the unambiguous range. The acquired 3-D point clouds contain phase jumps at the boundary of the unambiguous range imposed by the modulation frequency. In this article, we propose multiple solutions to cope with these problems. First of all, our system utilizes dual-frequency modulation to overcome the tradeoff between the resolution and the unambiguous range. With appropriate electronics for demodulation, the hardware imposed a longitudinal resolution of 19.2 ± 38 $\mu \text{m}$ and the unambiguous range of 48 ± 614.3 $\mu \text{m}$ were realized. The unambiguous range can be shifted within 5-m range using a mechanical focusing optics. However, such an attractive laser scanner still suffers from ranging ambiguity. The noise on the laser light modulated by the lower modulation frequency results in phase deviation, which can incur ranging errors at the integer times of the half cycle of the higher modulation frequency. We have coped with such ranging ambiguity by synthesizing ranging errors with the data in the correct range. Especially, defocused 3-D point clouds contaminated by severe ranging errors were analyzed and restored. The measurement range can thus be elongated by >20 times of the depth-of-focus using such data processing without manipulating the mechanical focusing optics. Next, we developed an algorithm as a remedy to prevent phase jumps in the unambiguous range. With exploitation of the relationship between the intensity and spatial information, the phase unwrapping was performed to recover the spatial continuity. Thereafter, the unambiguous range can be elongated to be >48 cm. With all the abovementioned configuration and data processing, we have overcome the ranging ambiguity inherent in the AMCW laser scanner. [ABSTRACT FROM AUTHOR]

This article presents a new practical calibration method of a stripe laser imaging system for the 3-D shape measurement of an object. The proposed calibration method has an integrated mathematical approach for the camera imaging parameters and the stripe laser light plane. A simple calibration block with a checkerboard pattern attached is used that has different inclination angle at each face. Because the same scene with the laser light ON and OFF is used for the integrated calibration of the camera and the laser light plane, the calibration process is fast and efficient and the resultant calibration parameters are consistent. The experimental results demonstrated the performance of the proposed calibration method. [ABSTRACT FROM AUTHOR]

A laser scanner is a powerful tool for 3-D geometry measurement. The amplitude-modulated continuous-wave (CW) laser scanner, which benefits from high accuracy measurement operating in the intermediate range, is suitable for high-precision 3-D industrial inspection. However, due to the limited dynamic range, laser scanners suffer from data loss and deterioration. This poses challenges for 3-D modeling, conversion of 3-D point cloud data to 3-D solid data, and so on. We chose the operation wavelength in the 1530-nm region. The dynamic range of an amplitude-modulated CW laser scanner was enhanced by the implementation of a combination of a low-noise and high-gain optical preamplifier and p-i-n photodiodes, a pellicle beamsplitter, and automatic gain control using a polarization-independent high-speed variable optical attenuator based on electrooptic ceramics in the receiver. In these manners, receiver sensitivity of −72 dBm was achieved and receiver dynamic range of around 37 dB was realized. We expect that our results contribute to high-definition industrial inspection for Industry 4.0. [ABSTRACT FROM AUTHOR]

Fringe projection profilometry (FPP) is widely used in the 3-D noncontact measurement. In FPP, one of the most important steps is obtaining the absolute phase through fringe orders and wrapped phase. The existing methods usually acquire the fringe orders and wrapped phase independently. Therefore, more patterns are needed to project onto the object, which decreases the measuring speed a lot. We propose an inner shifting-phase method that codes the fringe orders and the wrapped phase into the same four patterns. With the unwrapping algorithm, we obtain the fringe orders and the wrapped phase simultaneously, which decreases the number of required patterns a lot. The proposed method calculates the absolute phase pixel-by-pixel independently without any preacquired information or geometric constraints. Experiment results show that the proposed method achieves almost the same accuracy with only a third of the patterns required by the conventional three-frequency four-step method. Experiment results also verify the validity of the proposed method in measuring both complex and separated objects. [ABSTRACT FROM AUTHOR]

Laser scanners have emerged as a powerful instrument for high-precision 3-D geometry measurement. However, such attractive laser scanners have failed to address to drop of receiving power due to laser’s interaction with the surface of the object under measurement. This is a serious weakness of laser scanning systems since it can result in data deterioration and data loss in the acquired 3-D point clouds. Numerous studies have been carried out on laser illumination on rough surfaces and have shown that interference between backscattered lights within the laser beam spot forms speckles that affect the level of the receiving power. In this article, we propose a laser scanner having a rapid and continuous polarization scrambling function and cope with the drop in the receiving power. This is the first report of speckle reduction with the depolarization approach in laser scanning used for industrial measurement. [ABSTRACT FROM AUTHOR]

Fringe projection profilometry (FPP) is widely used in the 3-D noncontact measurement. In FPP, one of the most important steps is obtaining the absolute phase through fringe orders and wrapped phase. The existing methods usually acquire the fringe orders and wrapped phase independently. Therefore, more patterns are needed to project onto the object, which decreases the measuring speed a lot. We propose an inner shifting-phase method that codes the fringe orders and the wrapped phase into the same four patterns. With the unwrapping algorithm, we obtain the fringe orders and the wrapped phase simultaneously, which decreases the number of required patterns a lot. The proposed method calculates the absolute phase pixel-by-pixel independently without any preacquired information or geometric constraints. Experiment results show that the proposed method achieves almost the same accuracy with only a third of the patterns required by the conventional three-frequency four-step method. Experiment results also verify the validity of the proposed method in measuring both complex and separated objects. [ABSTRACT FROM AUTHOR]

Precise and contactless shape measurements of rotating objects is important, e.g., for monitoring and controlling the manufacturing quality in lathes. For this purpose, multisensor and single-sensor approaches based on optical distance and surface velocity measurements are state-of-the-art techniques. Two- and single-sensor systems are particularly promising to measure the angular-dependent radius of the cross section of the rotating object in a scanning regime with minimal optical access. Since a comparison between the different sensor systems is missing, the potential of these underdetermined sensor systems is unclear. In addition, displacements of the rotational axis and sensor misalignments are suspected to be crucial error sources, but the error is unknown. For this reason, an error analysis is performed regarding the resulting systematic error and the random error for the two- and single-sensor systems. As a result, the different sensor systems have an equal cross-sensitivity with respect to lateral displacements of the rotational axis from the sensor axes, but the two-sensor approach has the lowest sensitivity regarding sensor misalignments. For the studied measurement conditions, the systematic error dominates the sensor noise for the two-sensor system and the single-sensor system with combined distance and velocity measurement at an object mean radius >6 mm. The smallest total measurement uncertainty is obtained with the two-sensor system. Finally, the relevance of systematic error depends on the utilization, i.e., for instance on the absolute rotor radius, the stability of the rotor axis, the sensor position, the accuracy of the sensor alignment, and the uncertainty of the distance and/or velocity measurements. [ABSTRACT FROM AUTHOR]

The icing on transmission line conductor usually causes accidents such as conductor galloping, conductor breaking, line tripping, and even network paralyzing, which could bring serious threats to the safe operation of the power system. The conductor icing shape not only has a great influence on the estimate to melting ice time or melting ice current, but also directly affects the mechanical calculation of conductor galloping. Therefore, it is paramount to determine the conductor icing shape. Many available artificial measurements are inaccurate and even ineffective enough in some cases. This paper proposes a novel online technology for measuring icing shape on transmission line conductor based on vision and force sensors. The proposed technique consists of three parts, namely, the online device based on vision and force sensors, the communication network, and the monitoring center. The images of the iced conductor are captured from two cameras, and the processing algorithms are used to extract the average diameters of the iced conductor. The equivalent cross-sectional area is also obtained by the force sensors and a built-in mechanics model. The icing shape description algorithm is proposed to accurately represent the icing shape. We thoroughly analyze the performance of the technology by a series of experiments under artificial icing simulations and natural conditions. The experimental results demonstrate that the proposed method can measure the icing shapes with the overlap ratio of greater than 0.85. In addition, the proposed measuring technology shows satisfactory results on the field tests carried out on operating transmission lines. [ABSTRACT FROM AUTHOR]

A vision-based measurement approach for pill shape detection is presented along with other applications. Rapid and accurate pill identification is needed by medical and law enforcement personnel during emergencies. But real-world pill identification is challenging due to varied lighting conditions, minor manufacturing defects, and subsequent pill wear. Surmounting these challenges is possible using multiple inputs: pill color, imprint, and shape. Of these different inputs, pill shape is the most important and difficult parameter due to its variations. In this paper, we describe a novel technique to accurately detect the complex pharmaceutical pill shapes using measurements derived from a superimposed adaptable ring centered automatically on either the shape’s centroid or its bounding box midpoint determined based on the measurements from two other rings, namely the inner ring and the outer ring. It is shown that the measurements from the overlays of the adaptable ring suffice to successfully classify the shapes of the pills currently in the Pillbox database (U.S. National Library of Medicine, 2014) with an accuracy of 98.7%. Our method demonstrated higher accuracy when compared with Hu-moments on the same data set. Using logistic regression techniques, Hu-moments provided an accuracy of 96.6%. Though developed for the domain of pharmaceutical pill shapes, we discuss how the measurements from the adaptable ring can also be used in other industrial applications to increase the level of accuracy with the help of this real-time less computationally complex method. [ABSTRACT FROM AUTHOR]

A novel depth from defocus (DFD) measurement system is presented, where the extension of the measurement range is performed using an emergent technology based on liquid lenses. A suitable set of different focal lengths, obtained by properly changing the liquid lens supply voltage, provides multiple camera settings without duplicating the system elements or using moving parts. A simple and compact setup, with a single camera/illuminator coaxial assembly, is obtained. The measurement is based on an active DFD technique using modulation measurement profilometry for the estimation of the contrast at each image point as a function of the depth range. Two different measurement methods are proposed, both based on a combination of multiple contrast curves, each derived at a specific focal length. In the first method (intensity contrast method), the depth information is recovered directly from the contrast curves, whereas in the second (differential contrast method), the depth is measured using contrast curve pairs. We obtained a measurement $\sigma _{{{0}}}$ of 0.55 mm over a depth range of 60 mm with the intensity contrast method (0.92% of the total range) and an $\sigma _{{{0}}}$ of 0.76 mm over a depth range of 135 mm with the differential contrast method (0.56% of the total range). Thus, the intensity contrast method is within the state-of-the-art DFD systems, whereas the differential contrast method allows, $\sigma _{{{0}}}$ being almost equal, a remarkable extension of the depth range. [ABSTRACT FROM AUTHOR]

3-D vibration measurement is achieved using a single laser scanning vibrometer (LSV) and laser scanner (LS) by moving them to three arbitrarily different locations from the principle that vibration analysis based on the frequency domain is independent of the vibration signal based on time domain. The proposed system has the same effect as using three sets of LSVs, and has an advantage of reducing equipment costs. Analytical approach of obtaining in-plane and out-of-plane vibration of surface is introduced using geometrical relations between three LSV coordinates and vibrations measured at three different locations. The proposed algorithm is verified by comparing the experimental results obtained by a three-axis accelerometer and a developed optical system with an LSV and an LS combined together. [ABSTRACT FROM PUBLISHER]

This paper presents an instrument for the measurement of the 3-D surface geometry of disks and circular strips. Furthermore, tensor product surface approximations and analysis techniques are implemented, which enable a mode analysis of the data. The measurement device consists of a motorized rotary stage and a set of six tactile sensors which measure the displacement of the surface during rotation. The data from all six sensors are acquired synchronously, yielding six tracks of data lying on a circular field with a constant number of samples per track. This delivers data in polar coordinates. Each tactile sensor is individually calibrated, and a linearizing polynomial determined. This improves the achievable accuracy of the displacement measurements. It should be noted that the selection of six sensors is arbitrary and the methods presented work for any number of sensors. Furthermore, the sensors may be repositioned should a smaller separation be required to obtain additional resolution when measuring smaller devices. A data analysis method is implemented that enables the computation of individual surface mode models. The analysis tool is also applicable to cylindrical objects, as demonstrated. A tensor product approximation procedure for surfaces of revolution is presented. The method uses complex anisotropic moments, yielding both the magnitude and phase of specific surface modes. A covariance propagation analysis is derived for the tensor product approximation. This method is used in conjunction with a Kolmogorov–Smirnov test to determine how many surface modes are required to obtain a satisfactory fit. The results for a set of four test objects are presented, showing that a standard deviation of the approximation residual of \sigma \approx 10~\mu m is obtained for objects with diameters of \Phi=200~mm. [ABSTRACT FROM PUBLISHER]

Quality evaluation of rolling processes in the metal industry involves an inspection of the shape of the outgoing products in real time during manufacturing. Shape measurement systems are usually based on 3-D reconstructions of the surface of rolled products. As surface properties are crucial, these systems favor contactless techniques. Using 3-D measurements of the surface of rolled products, several geometric properties can be analyzed. In this paper, we analyze how uncertainty is propagated in a contactless shape measurement system designed and developed by the authors and presented in previous published works. This measurement system is based on active triangulation, and it is able to provide inline width and flatness measurements of long, flat-rolled products in harsh industrial environments. The camera model used to calibrate the vision system is described, and it is used to estimate the uncertainty of the reprojected 3-D points on the scene. The system uses the reprojected 3-D points, and the speed of the product movement in the production line to reconstruct its surface. Thus, the uncertainty of the speed is also estimated. Finally, the propagation of both the uncertainty of the 3-D reprojection, and the uncertainty of the speed into the final width and flatness measurements is analyzed. This paper comprises a detailed uncertainty propagation analysis in 3-D shape measurements computed indirectly through functional relationships. [ABSTRACT FROM AUTHOR]

The determination of flame or fire edges is the process of identifying a boundary between the area where there is thermochemical reaction and those without. It is a precursor to image-based flame monitoring, early fire detection, fire evaluation, and the determination of flame and fire parameters. Several traditional edge-detection methods have been tested to identify flame edges, but the results achieved have been disappointing. Some research works related to flame and fire edge detection were reported for different applications; however, the methods do not emphasize the continuity and clarity of the flame and fire edges. A computing algorithm is thus proposed to define flame and fire edges clearly and continuously. The algorithm detects the coarse and superfluous edges in a flame/fire image first and then identifies the edges of the flame/fire and removes the irrelevant artifacts. The autoadaptive feature of the algorithm ensures that the primary symbolic flame/fire edges are identified for different scenarios. Experimental results for different flame images and video frames proved the effectiveness and robustness of the algorithm. [ABSTRACT FROM AUTHOR]

Two new calculable cross capacitors are being constructed by the National Measurement Institute of Australia and the Bureau International des Poids et Mesures, with the aim of limiting sources of uncertainty to 1 nF/F, where achievable. In particular, the main electrodes, which are four stainless steel bars 470 mm long and 50 mm in diameter, must be geometrically accurate to 100 nm. An automated system based on capacitance probes has been constructed for the purpose of measuring the geometry of the bars with an uncertainty of around 20 nm. Two sets of electrodes have been finished to the required accuracy, although further improvement may be possible. [ABSTRACT FROM AUTHOR]

An original procedure is presented for the calibration of fringe-projection-based 3-D vision systems. The proposed approach estimates both the phase-to-depth and transverse relationships by directly measuring the phase maps for only three planes placed within the calibration volume and then estimating the phase maps for a number of other "virtual planes." Experimental tests conducted on a fringe projection system show the effectiveness of the proposed procedure. [ABSTRACT FROM AUTHOR]

In this paper, a methodology to process fringe patterns is presented. The core of the signal processing technique is the use of the direction information, which is a modulo 2π quantity that locally indicates the direction along which fringes grow at a maximum rate. By using this information, it was possible to perform adaptive and direction/orientation-based operations on fringe images to remove unwanted effects (mainly distortions at the image borders), to enhance the contrast, and to extract the phase information encoded. The method has been applied on shadow-moire interferograms with carrier fringes in order to measure the surface of small objects. The developed algorithm allows processing of. fringes whose phase information is not monotonically increasing everywhere and to more accurately estimate the phase itself. The results from two experimental fringe patterns are presented. These results are compared with the measurements that are performed by means of a conoscopic system that is used as a reference. Hence, it was possible to calculate the uncertainty of the performed measurements. Aside from the possibility of decoding fringe patterns that present closed fringes, the proposed method provides measurements that are characterized by an uncertainty that is more than halved compared with Fourier-based methods. [ABSTRACT FROM AUTHOR]

This paper presents a generalized analysis model for fringe pattern profilometry. We mathematically derived a new analysis model that gives a more general expression of the relationship between projected and deformed fringe patterns. Meanwhile, based on the proposed generalized model, a new algorithm is presented to retrieve 3-D surfaces from nonlinearly distorted fringes. Without any prior knowledge about the projection system, we still can obtain very accurate measurement results by using a generalized analysis model and a proposed algorithm. Computer simulation and experimental results show that the generalized model and the proposed algorithm can significantly improve the 3-D reconstruction precision, especially when the projected fringe pattern is nonlinearly distorted. [ABSTRACT FROM PUBLISHER]