Telecentric Imaging Research Articles

Electron beam welding residual height measurement by line laser illumination and telecentric imaging

Residual height significantly affects the fatigue life of electron beam welding (EBW) joints. Therefore, accurate measurement of the EBW residual height is necessary for grinding and removing it, to improve the fatigue resistance of the joint. However, there is no specific method for the EBW residual height measurement, and existing methods for other regular welds are often unsuitable due to their limitations. This paper proposes a novel EBW residual height measurement method, to the best of our knowledge, based on line laser illumination and telecentric imaging [line laser telecentric imaging (LLTI)]. Unlike regular line-structured light methods based on near-centered imaging, telecentric imaging has advantages on the simple imaging model, minimal aberration, and can avoid a complex calibration process and improve the detection accuracy. In this paper, residual height calculation algorithms for both flat and cylindrical bases of LLTI are discussed, and, for cylindrical base welds, an error analysis is conducted and a corresponding solution is proposed. A series of experiments is conducted, and the accuracy is under 0.02 mm, which proves the effectiveness of the LLTI method.

Single-shot dual-wavelength telecentric in-line-and-off-axis hybrid digital holography with non-prior reconstruction

Dual-wavelength in-line-and-off-axis hybrid digital holography (iohDH) can achieve high-resolution holographic dynamic imaging. However, it requires the prediction of the diffraction distance and the complex amplitude of the reference beam, which is time consuming and results in complications and accuracy limitations. While telecentric imaging technique can obtain nondiffractive images without predicting the diffraction distance, it also can even eliminate spherical aberration and astigmatic aberration. Therefore, a dual-wavelength telecentric iohDH is proposed to realize non-prior high-resolution reconstruction in a single shot. Employing the dual-wavelength telecentric iohDH, our approach acquires the focused in-line-and-off-axis hologram using a color camera in a single shot. In this case, we perform wavelength conversion on the phase and low-frequency information about the off-axis hologram as constraints for in-line iteration. Then, the in-line amplitude constraints are performed in the spatial and frequency domains until the algorithm converges. Compared to the state-of-the-art dual-wavelength iohDH, our approach can streamline the reconstructed processes without demanding a priori information of the diffraction distance and the complex amplitude of the reference beam. More importantly, our approach enables higher quality and efficient reconstruction under the telecentric system. We verified our approach using simulations and experiments, and the results indicate that our approach can allow the amplitude and phase reconstruction with high resolution in a single shot.

Double vision: 2D and 3D mosquito trajectories can be as valuable for behaviour analysis via machine learning

BackgroundMosquitoes are carriers of tropical diseases, thus demanding a comprehensive understanding of their behaviour to devise effective disease control strategies. In this article we show that machine learning can provide a performance assessment of 2D and 3D machine vision techniques and thereby guide entomologists towards appropriate experimental approaches for behaviour assessment. Behaviours are best characterised via tracking—giving a full time series of information. However, tracking systems vary in complexity. Single-camera imaging yields two-component position data which generally are a function of all three orthogonal components due to perspective; however, a telecentric imaging setup gives constant magnification with respect to depth and thereby measures two orthogonal position components. Multi-camera or holographic techniques quantify all three components.MethodsIn this study a 3D mosquito mating swarm dataset was used to generate equivalent 2D data via telecentric imaging and a single camera at various imaging distances. The performance of the tracking systems was assessed through an established machine learning classifier that differentiates male and non-male mosquito tracks. SHAPs analysis has been used to explore the trajectory feature values for each model.ResultsThe results reveal that both telecentric and single-camera models, when placed at large distances from the flying mosquitoes, can produce equivalent accuracy from a classifier as well as preserve characteristic features without resorting to more complex 3D tracking techniques.ConclusionsCaution should be exercised when employing a single camera at short distances as classifier balanced accuracy is reduced compared to that from 3D or telecentric imaging; the trajectory features also deviate compared to those from the other datasets. It is postulated that measurement of two orthogonal motion components is necessary to optimise the accuracy of machine learning classifiers based on trajectory data. The study increases the evidence base for using machine learning to determine behaviours from insect trajectory data.Graphical

Design method for a telecentric multiscale imaging system to avoid mechanical interference

Multiscale imaging systems are widely employed for high-resolution imaging purposes. However, research in this area often lacks comprehensive theoretical analysis of these systems and discussions regarding potential mechanical interference among adjacent optical paths, leading to directional inadequacies and complexities in optical design. A novel method for designing multiscale imaging systems, to our knowledge, is proposed to address this gap. This method effectively guides optical design processes and mitigates mechanical interference. First, a theoretical analysis of the field of view distribution in multiscale imaging systems is conducted alongside the proposition of a method for calculating the field of view size for individual imaging systems. Second, telecentric optical paths are introduced, establishing an optical model and deriving a calculation formula for the system’s critical aperture, thereby identifying conditions to prevent mechanical interference. Third, a telecentric multiscale imaging system with satisfactory imaging performance is designed, featuring a focal length of −200mm and a telecentricity of less than 0.1 mrad. The system achieves diffraction-limited performance in the diffuse spot’s radius and modulation transfer function (MTF) curve without encountering mechanical interference. This design methodology presents a novel approach for subsequent multiscale imaging system designs, offering theoretical insights and practical implications.

Focal plane coincidence method for a multi-view telecentric 3D imaging system.

Multi-view microscopic fringe projection systems, which use high-resolution telecentric lenses and the Scheimpflug condition, face challenges in coinciding focal planes accurately, resulting in inconsistent measurements between views. In this Letter, we developed a sharpness evaluation function based on the total power of the line-spread function, which was subsequently used to generate a full-field sharpness distribution map. Then we employed the correlation between the sharpness map and orientation of the focal plane to precisely coincide the focal planes. Experimental results validate the proposed method and demonstrate its improved consistency in 3D reconstruction.

Telecentric phase imaging at extended depth of focus using digital holographic microscopy

Digital holographic microscopy (DHM) is a well-known technique utilized by researchers to precisely examine the 3D topography of microscopic objects. Until now, DHM systems have offered researchers a trade-off between either achieving telecentricity or maintaining a limited depth of focus, presenting a challenge for comprehensive 3D imaging. Here, we propose a very stable DHM system that utilizes a customized microscope objective and electrically tunable lens to enable rapid axial scanning within the extended depth of focus range, continuous field of view, and telecentricity at each image plane. An extended depth of focus up to 5.5 mm axially is obtained with consistent magnification and resolution with a minimal magnification error of 2.2 % over the entire axial range. Additionally, a thorough investigation into the temporal stability of the system was conducted, with multiple holographic recordings taken over a period of 7 s. The minimal deviation in object height, measuring just 0.31 nm (without the use of vibration isolation), underscores the good temporal stability inherent in this proposed DHM system.

Collimated phase measuring deflectometry

Phase measuring deflectometry has been applied for free-form specular surface metrology, but its measured slope results are sensitive to the depth of sample positioning, which is also called the height-slope ambiguity. The objective of this work is to tackle this height-slope ambiguity problem. The main idea is to introduce collimated camera rays using a telecentric imaging lens and collimated structured-light illumination with a Fourier lens. This setup makes the fringe phases become only sensitive to the surface slopes and insensitive to the depth of the sample positioning. In this way, the slope calculation is theoretically independent of the sample depth. We call this new deflectometry technique Collimated Phase Measuring Deflectometry (CPMD). With our developed CPMD experimental setup, the measurement is insensitive to the depth of sample positioning, e.g., the measured height dispersion is less than 30 nm RMS within a 10 mm depth range when measuring a 50-mm-diameter spherical mirror with a 200 mm radius of curvature. The merits and limitations of the proposed CPMD technique are discussed, revealing its prospects in practical metrology applications and potential future investigations.

Multi-angle Scheimpflug projection 3D microscope: Design, calibration, and three-dimensional reconstruction

3D measurement of complex micro-structures has becoming increasingly important for precision manufacturing. In this paper, a multi-angle Scheimpflug projection 3D microscope (MASP-3DM) is proposed, and the detailed design procedure is given. The system deployed four Scheimpflug projections and one telecentric imaging to overcome the shortcomings of limited depth of field (DOF), occlusion and reflection. Then, an improved and practical system calibration method is proposed, which utilizes pinhole projection model to calibrate the telecentric imaging. Meanwhile, local homography is combined with perspective-n-point (PnP) strategy to improve the calibration accuracy. Further, a point-cloud registration error correction model is established to achieve complete and high-precision measurement. The experimental results indicate that the re-projection error is within ±0.5μm and the registration error is less than 2μm for system calibration, the maximum of mean deviation, measurement bias, standard deviation, and type A uncertainty are respectively less than 3μm, 7μm, 4μm, and 1μm for 3D measurement, within the 10(H)mm×50(W)mm×40(D)mm volume.

A single‐camera telecentric imaging stereo‐digital image correlation system and its application to carotid pulse signal detection

AbstractCarotid pulse signal (CPS) is one of the most important biomarkers for evaluating cardiovascular risk. To realize easy‐to‐operate and accurate CPS detection, a single‐camera telecentric imaging stereo‐digital image correlation (stereo‐DIC) system is established. The system uses a specially designed colour separation device and a single colour camera attached with a telecentric lens to record images of blue and red colours from different optical paths. By processing the recorded two sub‐channel colour images using regular stereo‐DIC algorithm, full‐field 3D displacements of the skin on the neck directly over the carotid artery are obtained. The maximum fluctuations in the out‐of‐plane displacement direction are extracted as CPS, based on which velocity, acceleration and frequency can be further extracted. The performance of the established system was demonstrated by detecting the pulse signal of a volunteer before and after exercise.

Sphericity measurement based on telecentric imaging mechanism with image distortion correction and eccentricity error compensation

Sphericity is an essential geometric parameter for determining the processing manufacturing quality of spherical components. This study presents a sphericity measurement method based on telecentric imaging mechanism with image distortion correction and eccentricity error compensation. A sphericity measurement system is designed, and the projection images of the cross-section on the equatorial plane of the measured sphere are acquired using a line scan camera with bilateral telecentric lenses. An analytical camera model for this sphericity measurement system is established by considering major sources of image distortion. The subpixel edge points of the projection images are extracted and converted to cross-sectional profiles on the equatorial plane according to the camera model. An eccentricity error compensation strategy for contour matching of the obtained cross-sectional profile on the equatorial plane is also developed to obtain the three-dimensional contour point coordinates of the sphere. Sphericity is then calculated according to the least square criterion. The effectiveness of the proposed method is verified by measuring two steel balls with diameter in 20 mm and 30 mm. The sphericity measurement error is reduced by 0.3383 mm and 0.4653 mm respectively after image distortion correction and is reduced by 0.2268 mm and 0.0795 mm respectively after eccentricity error compensation. The proposed method can be extended to form error measurements for opaque components with rotary structure on the outer contour.

Nonlinear optimization considering target feature points for bitelecentric camera calibration: experimental study.

The telecentric camera has found extensive application in microscopy imaging due to its remarkable attributes of maintaining constant magnification and minimal distortion within its depth of field. In telecentric imaging technology, the accuracy of measurements frequently hinges upon the calibration precision of the telecentric camera. In real-world scenarios, the shallow depth of field characteristic of telecentric cameras often leads to out-of-focus targets during the capturing process, which in turn results in the inability to accurately extract pixel coordinates of feature points, making it difficult for optimization algorithms to converge to the optimal value. We propose a nonlinear optimization algorithm based on pixel coordinates of optimized feature points for bitelecentric cameras. Incorporating pixel coordinates into the optimization process yields the theoretically optimal solution based on bitelecentric camera model. The obtained pixel coordinates are used for second initial value estimation, followed by the optimization of all parameters. Compared to existing methods, the proposed approach significantly reduces reprojection errors under both blurry and clear target conditions. Experimental results demonstrate superior performance in processing blurry defocused images.

Development of an upgraded motional Stark effect diagnostic system on EAST tokamak

A new multi-channel motional Stark effect (MSE) diagnostic system has been developed on the upgraded EAST tokamak, which was installed on the port C to observe a tangential neutral beam. A telecentric imaging lens was deployed to ensure uniform illumination from the core to the boundary. A square fiber head which contained 23 fiber bundles was mounted to this imaging lens; each fiber bundle contained 19 fibers and two of them were assigned to CXRS and BES spectrometer, respectively. The angle tuning method was used for matching the Doppler shift of the σ component’s wavelength which was caused by the beam voltage. At the present stage, the MSE system only contains ten channels that would be extended to 23 channels in the future, covering a measurement range from R = 1.8 to R = 2.27 m with a temporal resolution of 10 ms and a spatial resolution of 3 cm. The polarization angle-constrained q profiles and current density profiles were reconstructed with EFIT equilibrium reconstructions. In the sawtooth discharges, the q = 1 surface position was validated by the ECE signals, which further verified the rationality of the MSE measurement.

Accurate and flexible calibration method for a 3D microscopic structured light system with telecentric imaging and Scheimpflug projection.

3D imaging and metrology of complex micro-structures is a critical task for precision manufacturing and inspection. In this paper, an accurate and flexible calibration method for 3D microscopic structured light system with telecentric imaging and Scheimpflug projector is proposed. Firstly, a fringe projection 3D microscopy (FP-3DM) system consisting of a telecentric camera and a Scheimpflug projector is developed, which can take full advantage of the depth of field (DOF) and increase the measurement depth range. Secondly, an accurate and flexible joint calibration method is proposed to calibrate the developed system, which utilizes the established pinhole imaging model and Scheimpflug distortion model to calibrate telecentric imaging, and fully considers the correction and error optimization of the Scheimpflug projection model. Meanwhile, the optimized local homography is calculated to obtain more accurate sub-pixel correspondence between the camera and the projector, and the perspective-n-point (PnP) method make the 3D coordinate estimation of the feature point more accurate. Finally, a prototype and a dedicated calibration program are developed to realize high-resolution and high-precision 3D imaging. The experimental results demonstrate that the re-projection error is less than 1µm, and the 3D repeated measurement error based on feature fitting is less than 4µm, within the calibrated volume of 10(H)mm × 50(W)mm × 40(D)mm.

High-Precision Line Laser Three-Dimensional Measurement Method Based on Telecentric Imaging

High-Precision Line Laser Three-Dimensional Measurement Method Based on Telecentric Imaging

In Situ Measurement of 2-D Crystal Size Distribution During Cooling Crystallization Process via a Binocular Telecentric Imaging System

In this paper, a novel in-situ measurement method with stereoscopic image analysis is proposed for monitoring crystal length and width distributions during a cooling crystallization process, based on a binocular telecentric imaging system. Firstly, a stereoscopic imaging calibration model is established for using binocular telecentric cameras to conduct in-situ measurement during a cooling crystallization process. Secondly, an enhanced algorithm is presented to improve matching crystal images from binocular image pairs, based on in-situ image preprocessing and segmentation. Thirdly, four key corners related to crystal length and width are determined by using the boundary features of matched crystal image projections. Finally, the length and width of each crystal are computed based on the reconstructed key corners in three-dimensional (3D) coordinate space, by using the established stereoscopic calibration model. Experimental validation on the proposed stereoscopic imaging calibration model and 3D reconstruction of key corners is carried out via in-situ measurement of a microscale checkerboard plate inserted into a cooling crystallizer. In-situ measurements of crystal length and width distributions during the cooling crystallization process of β-form L-glutamic acid are conducted to verify the effectiveness and advantage of the proposed measurement method.

Measurement of Gloss Unevenness with Different Reflection Angles

In this study, we introduced a measurement method for gloss unevenness as a function of reflectance angles. Gloss is one of the most important qualities of materials, and it is evaluated subjectively by the intensity of reflected light and gloss unevenness. People can estimate the texture of a material using gloss unevenness, which is found in the small peripheral part of the gloss area and can easily be recognized by observing the reflected light while moving the object or eyes. However, it is not easy to photograph and quantify gloss unevenness. One reason is the area where gloss unevenness is observed is a small area near specular reflection. Second, the appearance of gloss unevenness changes depending on the reflection angle. We developed a measurement apparatus to measure gonio-photometric gloss unevenness. We introduced two solutions: a wide-area gloss unevenness measurement technology using telecentric imaging and a rotating mirror optical system that defects reflected light at an angular resolution of 0.02°. We analyzed three materials—mirror, plastic, and paper—and proposed three indicators as a quantitative evaluation method for gloss: intensity of reflected light at the specular angle, full width at half maximum (FWHM) of Bidirectional Reflectance Distribution Function (BRDF), and gloss unevenness image at the FWHM.

Flexible and simple telecentric camera calibration method.

The measurement accuracy of telecentric imaging technique straightforwardly depends on the calibration of the telecentric camera. We present a flexible and simple calibration method based on a telecentric imaging model and orthogonality of rotation matrix. First, we use the orthogonality of rotation matrix to solve the magnification. Second, the external parameters are solved by the imaging model, and the ambiguity of sign is solved. Finally, we use the LM nonlinear optimization method to solve the distortion parameters. Experimental results show that the reprojection error is 0.7 pixels, which represents the actual dimension of 6.37µm. In addition, the standard measuring block and real objects are measured, and the results verify the measurement accuracy and robustness of the proposed method.

Design of a Concentric Multi-Scale Zoom Optical System Based on Wide Object Distance and High-Precision Imaging.

To effectively balance the trade-off between a large field of view (FOV) and high resolution of an optical system, as well as to solve the problem of image stitching misalignment after focusing, firstly, this paper conducts a theoretical analysis of the design principle of the concentric multi-scale optical system and the causes of image stitching misalignment after focusing. Secondly, the design idea of using a combination structure of a two-layer front concentric imaging group and an image-space telecentric relay imaging array and then a joint full-motion zoom relay imaging system is proposed. Finally, an image-space telecentric two-step zoom concentric multi-scale optical system with a 7 × 7 relay imaging array is designed. The FOV of this optical system is 60° × 45°; the focal lengths are 50 mm and 100 mm for the center channel and 50 mm for the other channels. This concentric multi-scale zoom system has the advantages of both high-precision imaging stitching with a wide object distance and high-resolution imaging, which makes up for the defects of the conventional concentric multi-scale optical system, making it a promising application in the fields of aviation and security.

Optimal focal length search and design of a compact zoom lens based on a telecentric intermediate image.

The design of a compact zoom lens requires a designer's most abundant and professional skills and experience, which increases the difficulty of zoom lens miniaturization. In this paper, an automatic optimal focal length search method for a catadioptric zoom lens is proposed. After 7 h of searching for the initial structure and further optimization, a 3× compact zoom lens based on a telecentric intermediate image is obtained, which is applicable for 1/4 inch (6.35 mm) CMOS, with a focal length range of -4.8∼-14mm and a F-number range of 2.7-8.0. The depth and total length are within 6 mm and 30 mm, separately. The proposed method helps to reduce researchers' difficulty in designing compact zoom lenses and can provide some reference for the development of the mobile zoom lens industry.

Orientation-selective sub-Rayleigh imaging with spatial coherence lattices.

The Rayleigh resolution criterion sets the minimum separation for two-point objects to be distinguishable in a classical optical imaging system. We demonstrate that the sub-Rayleigh resolution can be achieved in a telecentric imaging system with the help of a partially coherent illumination whose spatial coherence has lattice-like distribution. We show that the orientation-selective sub-Rayleigh imaging can be realized by controlling the spatial distribution of the coherence lattice into different symmetries. We carry out a proof-of-principle experiment to demonstrate the orientation-selective sub-Rayleigh imaging for a 1951 USAF resolution target. Our results indicate a flexible orientation-selective high-resolution imaging with spatial coherence engineering of the partially coherent light.