Fringe projection profilometry (FPP) has become one of the most powerful techniques for three-dimensional (3D) non-contact measurement. However, in practical scenarios, the various reflectivity of the unknown measured objects often greatly makes the system unable to achieve the theoretical precision under the same system parameter settings. Therefore, the adaptively system parameter setting is essential to be developed. In this paper, we propose a novel metric model, i.e. the accuracy quality function, for initial accuracy evaluation using in-situ acquired images under the current parameter settings. The causes that potentially affects the ultimate accuracy are analyzed via theoretical derivation and further adopted within the evaluation model. In addition, an optimal exposure selection method based just two images is carried out to fast adjusting. Experimental results demonstrated that the proposed accuracy quality model aligns well with the actual condition. Under optimal exposure, it achieved a significant reduction in phase error by 36.15% and by 21.39% in low- and high- exposure, highlighting its strong performance and potential for high-accuracy and in-situ 3D shape measurement applications.

Cost-efficient multi-objective synchronous ranging solutions are highly desired in many fields such as manufacturing and infrastructure. However, traditional laser ranging methods have limitations in absolute ranging and multi-channel expansion. In this paper, a multi-channel absolute distance measurement method based on optical carrier-based microwave scanning interferometry (OCMSI) is proposed. After microwave scanning and synchronous demodulation, the amplitude spectrums and phase spectrums of interference signals are established. The transmission signals of each channel can be separated and reconstructed by using the discrete Fourier inverse transform. Additionally, it is demonstrated that the designed global optimization algorithm for extracting free spectral range can effectively reduce the impact of detection errors and channel interference. Existing interferometers can achieve multi-channel parallel absolute distance measurement without the need for additional modulation, demodulation, and optoelectronic detection devices. Experimental results have shown that the system structure is simple, and the ranging accuracy of for 3 channels is higher than ± 60 μm within at least 35 m optical path.

With the sustained development of railway transportation, the urgent necessity to improve train operation safety makes obstacle detection become the research focus. However, existing railway obstacle detectors still face challenges in balancing detection accuracy and speed during the shunting process. In addition, they are not robust enough in real-world railway environments, especially in complex scenes involving small obstacles. To address these problems, this paper presents a real-time and high-accuracy railway obstacle detection model using lightweight CNN and improved transformer (RH-Net) for detecting railway obstacles efficiently to guarantee traffic safety. First, the Lightweight Feature Extraction Module (LEM) is designed to minimize the model’s computational load while maintaining its feature extraction ability. Then, the Improved Transformer Module (IFM) is developed to boost the model’s ability about stably extract global contextual information. Finally, the Enhanced Multi-Scale Feature Fusion Module (EFM) is proposed to optimize the detection of obstacles with different sizes, especially small objects. In the experiments on railway dataset, RH-Net achieves optimal detection performance on GeForce GTX 1080Ti (96.99% mAP and 135 FPS) and Jetson Xavier NX (97.02% mAP and 43 FPS), which is significantly superior to the existing detection models. Experimental results show that RH-Net has excellent detection ability, which can accurately and efficiently detect obstacles in complicated railway environments. Moreover, the experiments on MS COCO indicate that RH-Net can achieve more satisfactory detection performance than existing state-of-the-art methods. Therefore, the proposed model can be well-applied to more complex real-world scenes for multiple object detection.

High-speed tracking technology has wide applications in the military and aerospace industry. However, existing approaches, such as camera arrays or Doppler radar systems, suffer from high cost and inconvenience. This paper reports a high-speed target tracking control system based on short-time rotational reflection imaging, specifically aimed at overcoming certain limitations. In the system we designed, a high-speed camera coupled with a rotating reflector is used to achieve reliable high-speed target tracking. This paper first introduces the working principle and mathematical model of the system, then analyzes the key technologies, including motor response delay time and rotational speed curve fitting, and, finally, verifies the feasibility of the system and the correctness of the theory based on a series of experiments. Experimental results demonstrated that our work is efficient and accurate in target tracking and image clarity. The developed system demonstrates significant potential for widespread use across military and aerospace sectors. Furthermore, the insights gained from our investigation into key technologies could act as a reference point for fellow researchers in related scientific areas.

A microwave photonic mixing approach to laser Doppler frequency shift (DFS) measurement employed for high-velocity estimation is proposed. Based on optical carrier-based microwave interferometry, the DFS of the light wave frequency domain is mapped to the microwave frequency domain. With a modulated broadband light wave, a sufficiently small coherence length of a light wave is produced, avoiding the influence of optical interference on photonic mixing. Compared to conventional laser DFS measurement, the frequency of intermediate frequency (IF) signal can be reduced by 4 and 5 orders of magnitude. Microwave photonic mixing based on a heterodyne interferometer is also performed to eliminate the direction ambiguity of motion. In a proof-of-concept experiment, the frequency shifts from 0 to 100 kHz with a frequency shift error of <30 Hz at a wide bandwidth can be achieved. For radial velocity estimation, these results reveal a range from 0 to 3000 m/s and a resolution of <0.9 m/s at the 5-GHz frequency band.

An absolute distance measurement approach based on microwave photonics is proposed to provide a highresolution and non-ambiguity range measurement solution by microwave frequency scanning. The microwave interference spectrum containing optical path difference information can be generated through two rounds of intensity modulation. The approach combines the characteristics of easy focusing and collimation of light waves, as well as easy manipulation of microwaves, to achieve accurate absolute distance measurement. The experimental results show that high-precision measurements can be achieved within the frequency scanning range of only 2 GHz. The error of the distance measurement is less than +/- 0.079 mm. In addition, the effectiveness of the scanning pattern and the data processing algorithm are verified.

Objective The developing modern economy and manufacturing industry have put forward a higher demand for related measuring instruments. However, the requirements for orthogonality of internal structures increase the difficulty of instrument manufacturing, and the production efficiency is reduced. To break through the limitations in the production and manufacturing of large-scale measuring instruments in China, the concept of non-orthogonal shafting measuring instruments is proposed. This kind of instrument does not require the shafting to be perpendicular or intersecting with each other, and the non-orthogonal shafting laser theodolite (N-theodolite) is a typical non-orthogonal shafting instrument. Much research has explored its measurement performance and related theories. However, due to the lack of the reference end on the laser axis for a single N-theodolite, the inverse kinematics model fails to be established accurately. The inverse kinematics model is necessary for precision theory research, which is convenient for data simulation of error spatial distribution. Besides, the guidance technology based on the inverse model can help improve the automatic measurement function of the N-theodolite. Therefore, to address the difficulty of calculating the rotation angle for the N-theodolite in inverse motion without a reference end, the linear model for the inverse motion to achieve fast and high-precision calculation of the rotation angle is proposed in this paper. Method In this paper, the basic theory of Lie groups and Lie algebras is introduced to achieve fast and high-precision calculation of the rotation angle. First, based on the theory of Lie groups and Lie algebras, the kinematics model of the N-theodolite can be constructed. The coordinate transformation matrix is represented as the product of the exponentials formula (POE) with clear physical meanings. Second, the error model of N-theodolite can be obtained through the corresponding differential calculations, and the parts related to the rotation angle error component are preserved. Then, the constraint relationship between the spatial target point and the laser axis pose parameters is constructed, and the linear equations for solving the error correction value of the rotation angle can be established. Besides, the initial values can be quickly obtained through trigonometric functions. Finally, the high-precision rotation angle values are obtained by linear addition of the initial estimation values and the error correction values. The efficient and accurate linear inverse kinematics model of N-theodolite is established. Results and Discussion In this paper, the simulation and real experiments are carried out to verify the proposed linear inverse kinematics model of the N-theodolite. The simulation results show that the rotation angle error calculated by the proposed method approaches 0 (Table 2 and Fig. 7). The proposed method is completely proved to be feasible in principle, and the inverse rotation angles are calculated with extremely high accuracy. However, the interference from multiple error sources is reflected in the experimental results, and the mean rotation angle error in the actual experimental is less than 0.02 mrad (Table 3). The parameters in the proposed linear inverse kinematics model include shafting parameters and the coordinates of spatial points, which would inevitably affect the performance of the N-theodolite. The two-dimensional turntable is used to provide reference values for rotation angle, and the official data of the manufacturer shows that the angle error of the turntable is +/- 3 '', approximately +/- 0.014 mrad. The calculated angle error by the proposed method is slightly higher than the angle error of the turntable. Therefore, the results of the proposed linear inverse kinematics model approximate the angle error output by the high- precision two-dimensional turntable under the existing experimental conditions, which can prove the feasibility and accuracy of the method proposed in this paper. Conclusions A linear inverse kinematics model is proposed to address the difficulty in the inverse motion angle calculation of N-theodolites. Based on the basic theory of Lie group and Lie algebra, the forward motion model and theoretical rotation angle error transmission model for the N-theodolite are established. By combining the constraint relationship between the measured target point and the spatial laser axis pose parameters, a linear equation for calculating the error correction value of the rotation angle is constructed, achieving high-precision calculation of the rotation angle. The feasibility of this linear inverse kinematics model has been verified through simulation experiments, and the average rotation angle error calculated from real experiments is 0.019 mrad and 0.013 mrad. Due to the influence factors such as spatial coordinate errors of laser points, internal shafting errors, and turntable angular errors, the accuracy of related calculations is limited, but the current problem of no reference end is solved by the proposed method, and the requirements of research related to error distribution are met. Further research will be carried out on the accuracy theory of N-theodolite measuring systems, and improving the accuracy of shafting parameters and rotation angles simultaneously will be a key focus.

Fringe projection profilometry (FPP) has been widely applied to three-dimensional (3D) shape measurement with the advantages of non-contact and high accuracy. However, measuring the scenes of multi-reflective variations (MRVs) is still a tricky task for conventional FPP because the dark-field and saturation could lead to phase errors and reconstruction errors. In this paper, a novel high dynamic range (HDR) technique based on accurate pixel-by-pixel phase-error adjustment strategy is proposed, which is mainly focus on both accuracy and quality of the reconstruction. A reflection model and an error transformation model are built to analyze the disturbances induced by MRVs in the captured fringe patterns. By setting a proper phase error threshold, the exposure time range of each pixel can be calculated. Finally, a greedy algorithm is introduced to globally minimize the number of exposures. Experiments results have verified that our proposed method can improve both the measurement accuracy and accurate point cloud integrality (APCI) compared with conventional FPP and several HDR methods, making it suitable to be further applied in the HDR 3D shape measurement.

Fringe projection profilometry (FPP) is widely used in the field of 3-D shape measurement with the advantages of noncontact, high speed, and nondestructive. However, measuring the complex reflective surface is a severe challenge for the FPP system. In this article, an active optimization method based on the reflection characteristics of the surface is proposed. We first design an ultrasonic atomization optimization measurement (UAOM) device. It is capable of uniformly and stably covering the measured surface with a microscopic water mist layer. The water mist layer is used to change the reflection characteristics, thereby reducing the impact of overexposure and underexposure on the measurement results. Apart from that, a bidirectional reflectance distribution function (BRDF) model and projector-object-camera radiation transfer model are established, which could be used to quantitatively describe the changes in reflection characteristics caused by the microscopic water mist layer. Comparative experimental results demonstrate that the proposed method yields improved measurements of complex reflective surfaces without the need for intricate algorithms. The final 3-D result of the points cloud based on our method is more complete and accurate, which improves the efficiency and versatility of 3-D measurement.

The imaging quality of a rotational reflection high-speed tracking system is greatly affected by the optical characteristics of the reflector and the depth of field limitations of the imaging system, especially for tracking systems working in small distances. In order to improve the imaging quality, this paper focused on two factors that affect the imaging quality: double vision caused by the optical characteristics of reflectors and blurring caused by the depth of field of imaging systems. This paper quantified the impact of these two factors on imaging through theoretical analysis, proposed a method of changing the hardware position, and conducted a simulation and experiments. The results show that the proposed solution in this paper can effectively improve the imaging quality of the system. The content studied in this paper has certain significance in the field of high-speed tracking of rotating reflectors and can provide reference for relevant researchers.

Motivated by the increasing demands on the precision of 3D large-scale measurement, the extrinsic parameters calibration with high accuracy of the bistatic non-orthogonal shafting laser theodolite (N-theodolite) system is required. A two-step method is proposed to achieve the extrinsic parameters calibration with high accuracy in this paper. In the first step, by analyzing and setting the approximate emitted point during the motion of the laser axis in local space, the calculation of the initial extrinsic parameters can be simplified. In the second step, the above results are taken as the initial values of optimization, and the distances between the spatial laser points provided by PSD sensors with high accuracy in global space are used to construct the unconstrained optimal objective function. The proposed method is validated with the measurement experiment of the bistatic N-theodolite system, the average error of 3D coordinate measurement is less than 0.4 mm, and the average error of distance measurement is less than 0.3 mm within 5 m.

Digital fringe projection (DFP) is widely applied in three-dimensional (3D) shape measurements. However, its performance is severely disturbed while measuring complex surfaces with an extensive range of reflectivity. In this paper, to enhance the underexposed regions and reduce the saturated regions, an automatic and effective method is proposed for the multi-exposure determination of the DFP system. During the whole process, only one unsaturated image must be captured preliminarily, which could establish an adaptive strategy based on the intensity value distribution function. Raw absolute phase maps captured with different exposure times are synthesized at the pixel level for phase retrieval and 3D reconstruction. To evaluate the effectiveness of our method, a high dynamic range method and a global optimal exposure method are introduced for comparison. Verification experiments demonstrate that our method performs better for the 3D reconstruction results of complex surfaces with a large reflectivity range.

Discrete point interpolation has emerged as a prime candidate technology for dual-station accurate intersection in 3-D precise measurement. However, the operation is low-efficiency for capturing multiple reference points to measure a single target point. The quantitative model of linear small angle in the interpolation is proposed by analyzing the movement trajectories of laser beam and laser spot. An efficient method for 3-D precise measurement using laser spot traversal and target point interpolation is proposed to achieve accurate intersection without multiple approximations. The laser spot traverses the field of view of the camera to establish an interpolation grid. The target point inserts the traversal grid and can be aimed by laser beam in arbitrary position. The traversal step size is determined by the determining equation of linear small angle. The approach is time-efficient for capturing all measured points in the field of view. The experimental results show that the proposed method is suitable for multiple points and surface measurement.

The research about non-orthogonal shafting laser theodolite (N-theodolite) is motivated by the growing demand for instrument. The reference pose of laser axis in motion state cannot be obtained due to the absence of reference end, and the error compensation of the N-theodolite is lack of basis. Therefore, the shafting error parameters identification and compensation method without reference end is proposed in this paper. Based on the derived kinematics model of the N-theodolite, the laser axis pose error in motion state can be described accurately. Then the aiming error is defined as the key reference parameters to construct the linear equations for effective identification and iterative compensation of shafting error. The experimental results have shown that the average aiming error of the compensated N-theodolite is less than 0.04 mm within 10 m. Compared with the existing calibration approaches, the shafting parameters adjustment performance of the proposed method is more prominent.

The parameter calibration of a digital fringe projection profilometry (DFPP) system is a fundamental step and directly related to 3D measurement accuracy. However, existing solutions based on geometric calibration (GC) suffer from the weakness of limited operability and practicality. In this Letter, a novel, to the best of our knowledge, dual-sight fusion target is designed for flexible calibration. The novelty of this target is the ability to directly characterize control rays for ideal pixels of the projector, and to transform the rays into the camera coordinate system, which replaces the traditional phase-shifting algorithm and avoids the error from the nonlinear response of the system. Attributed to the excellent position resolution of a position-sensitive detector within the target, the geometric relationship between the projector and camera can be easily established by projecting only one diamond pattern. Experimental results demonstrated that the proposed method using only 20 captured images is capable of achieving comparable calibration accuracy to the traditional GC method (20 images versus 1080 images, 0.052 pixels versus 0.047 pixels), which is suitable for rapidly and accurately calibrating the DFPP system in the 3D shape measurement field.

The laser sensing technique with visual laser beam has been an increasing interest in large-scale 3D measurement. However, a major limitation has been the high-accuracy positioning of laser beam in 3D space. An efficient calibration method of spatial laser beam in large-scale measurement is proposed in this paper. A novel PSD-based laser detecting device is presented to accurately collect the laser beam data. The 3D coordinates of points on the laser beam can be transformed from the PSD coordinate system to the world coordinate system by the pre-calibrated reference points and spatial transformation analysis. Furthermore, the optimized method of PSD data processing is proposed to reduce the influence of ambient light on detecting laser point coordinates. Finally, the spatial posture of laser beam can be obtained by linear fitting from the highaccuracy acquired points. The calibration process is greatly simplified compared with the existing methods. The experimental results show that the average deviation between the obtained spatial points and the fitted line is less than 0.015 mm within 5 m. The proposed method is applied to non-orthogonal shafting theodolites, and the point measurement average error is less than 0.17 mm, and the distance measurement average error is less than 0.16 mm.

Laser stripe extraction serves to be a crucial technique in the line-structured light system, and its accuracy and speed are directly related to the measurement performance. However, the traditional Hessian matrix method may produce redundant centers and missing centers, which will limit its accuracy and robustness. Besides, the complex calculation of the method makes it difficult to be applied in real-time measurement. In order to overcome these issues and achieve real-time center extraction, an improved FPGA-friendly laser stripe center extraction method is proposed. A novel judgment function is designed to replace the maximum eigenvalue, and the numerical difference between the centers and the other domains is more salient. The center judgment criteria are modified and the non-maximum suppression is used to deal with the redundant and missing centers. Furthermore, the proposed method is implemented in FPGA to achieve real-time processing. The calculations are rationally optimized to reduce resource utilization and delay time without reducing the accuracy. The mean absolute errors are 0.0039 pixel, 0.0373 pixel, 0.0520 pixel, and 0.0646 pixel, and the root mean square deviations are 0.0068 pixel, 0.0469 pixel, 0.0654 pixel, and 0.0811 pixel, respectively, in the accuracy experiment with the noise deviations of 0, 0.01, 0.02, and 0.03. The running and delay time of the proposed method in FPGA are 14.89 ms and 216.42 μs. The experimental results verify that the proposed method is highly accurate, robust, and time-efficient.

Compared to a conventional fixed focus lens, a zoom lens is more flexible and adaptable. However, the challenges involved in precise calibration for a zoom camera prevent its widespread use in close-range photogrammetry. A practical calibration method for a zoom camera is proposed. The zoom-focus model is established through dimension reduction of the setting variables, which is represented as a set of functions of the zoom setting. The zoom and focus settings are updated in real time for objects at different measurement depths. The calibration process only requires the zoom camera to observe the control points distributed in the designed calibration field with several combinations of zoom and focus settings. All the coefficients of the zoom-focus model can be solved by a nonlinear joint optimization. Experimental results have proved that the proposed method is effective for close-range photogrammetry.

Perspective projection model is the common measurement method for laser sensor to calculate the 3D coordinates of spatial points. However, the error of mathematical model is introduced, which inevitably decrease the measurement accuracy of laser sensor. In order to improve the accuracy, an optimization measurement method is proposed in this paper. Firstly, the structural design and measurement principle of laser sensor is developed. Secondly, the error propagation model is established, and the effect of the structural parameters in perspective projection model is probed in detail. Next, an optimization measurement method is presented by adjusting the structural parameters, which is the projection angle of laser beam in measurement coordinate system. Finally, the effectiveness of the proposed optimization method is evaluated by the simulative and practical experiments. The experiment results show that the maximum error can be reduced to 0.589 mm with optimization from 1.090 mm of previous method. Enough theoretical research is offered for optimization design of measurement model of the laser sensor.

Laser is becoming an increasingly important tool in 3-D precise measurement. To reduce the measurement error, the size of the laser spot should be as small as possible. Aiming at the problem mentioned above, a calibration and measurement method based on optical lenses is proposed. Limited to manufacturing and installing errors, the decentration, and tilt among the laser beam, planoconcave lens (PL) and liquid lens (LL) cannot be avoided exactly. The analysis of the optical path with the decentration and tilt is performed. Regarding the complicated calculation, MATLAB is utilized to obtain the motion law of the target spot. The calibration experiment shows that the root mean square error (RMSE) of points used for line fitting is no more than 0.116 mm at a range of 10 m. Moreover, the measurement experiment shows that the RMSE of point measurement and distance measurement of the system is 0.425 and 0.219 mm, respectively. We demonstrate that the calibration and measurement method with optical lenses is feasible for 3-D measurement.