Telecentric Imaging Research Articles

Measurements of Local Flow Characteristics in a Gas–Liquid–Solid Circulating Fluidized Bed with Immersed Telocentric Photography

The gas bubble size, local phase holdup, and other local flow characteristic parameters of the gas–liquid–solid circulating fluidized bed play an important role in the design and operation of this type of reactor. However, due to the limitation of measurement techniques of three-phase fluidization, the local flow characteristics have not been well measured and studied. In recent years, telecentric imaging technology has been applied in other multiphase systems, and a new telecentric probe method has been used to measure the local flow characteristics of a gas–liquid–solid expanded bed. In this work, the measuring method was introduced to measure the local phase holdups and bubble size distributions of the three-phase circulating fluidized bed. Concurrently, the influence of the superficial liquid and gas velocities on the particle circulation rate in the three-phase circulating fluidized bed was studied. The method reliability of gas holdup and bubble size measurements was validated by a conductivity probe method. The results provided basic data of such a reactor for future flow numerical simulations and industrial applications.

An Accurate and Flexible Calibration Method for 3d Microscopic Structured Light System with Telecentric Imaging and Scheimpflug Projection

An Accurate and Flexible Calibration Method for 3d Microscopic Structured Light System with Telecentric Imaging and Scheimpflug Projection

Research on Surface Defect Detection Technology Based on the Zonal and Time-Sharing Computational Imaging

In order to solve the misjudgment problem of different information in common gray images due to the very similar features such as morphology and gray value, and to avoid the high cost of using the 3-D sensor in defect detection, a zonal and time-sharing computational imaging (ZTSCI) is proposed in this paper. Firstly, a four-zone and time-sharing exposure visual imaging system is constructed by monocular telecentric imaging and lights in different directions. Then, according to the gray images under four groups of illumination conditions, and the direction vectors of incident lights, five feature images are calculated, such as fused images of maximum and average, enhanced images of normal vector, gradient and relative height. On the one hand, the image background fluctuation caused by the calculation error introduced by the ideal hypothesis is weakened, and the interference of surface texture to defect detection is reduced. On the other hand, the difference of easily-confused information can be increased in different feature spaces. In this paper, the ZTSCI is further applied to defect detection on the surface of new energy batteries. It solves the problem that defects and interference information are difficult to distinguish in gray images captured with common machine vision methods, reduces the difficulty of subsequent defect recognition, and lays a foundation for improving the accuracy of recognition.

A New Method for Auto-Inspecting Both Opposite Surfaces of Tec Components Based on Equal-Optical-Path Imaging System

An new optical method for simultaneously inspecting opposite surfaces defects of thermoelectric cooler (TEC) components that meets condition of equal optical paths for both surfaces imaging has been proposed. The optical apparatus for surfaces defects inspection has been designed and established with “confocal” imaging system consisting of two trapzoid reflection prisms and one optical images combiner. Experimental investigations on defects inspection with the “confocal” imaging system have been carried out. The results showed that the proposed optical method can be used to simultaneously inspect the defects of opposite surfaces of TEC components without need to employ a tele-centric imaging lens with large depth of focus. It was concluded that the optical inspection method can meet the technical requirements for inspecting opposite surfaces (both side surfaces, or both top and bottom surfaces) defects of TEC components and has advantages of equal good imaging quality, increased inspection accuracy and throughput, simplified system configuration and improved system reliability etc.

A calibration-free two-dimensional spectral soot emission platform for temperature and soot measurements

A simple and cost-effective temperature and soot volume fraction measurement technique that does not rely on spectral or absolute irradiance calibration is developed. The technique is based on a three-sensor CMOS camera, where the broadband sensitivity of RGB sensors were narrowed down to representative wavelengths using a tri-bandpass filter. A telecentric imaging lens was used to improve the accuracy of the tomographic deconvolution of the axisymmetrical ethylene laminar diffusion flames stabilized on a co-flow burner. Temperature and soot concentration measurements have been compared with results previously reported from the same burner. Good agreement for temperature measurements and excellent agreement for soot concentration measurements have been shown. Additionally, to demonstrate the dynamic range of the system, two-dimensional temperature and soot volume fraction fields were measured for four cases of dilution corresponding to various soot loadings. As the dilution rate was increased, the peak soot volume fraction was drastically reduced and temperatures were increased in accordance with the reduced dissipative heat losses from broadband soot radiation.

Compound eye image scanner optimally arranged for image combining and suppressing axial chromatic aberration by wavefront coding

A compound eye optical system design allows us to construct a compact image scanner with a large field of view and a large depth of field, both of which are beneficial for use in a copier’s flatbed scanner. Our prototypes are constructed of 32 or 28 imaging units, which are ommatidia, and the representations from the imaging units are electrically combined. Since the arrangement of the imaging units is a key factor in combining the images correctly without any loss at the boundaries, we discuss two types of arrangements along with results of their prototypes: a one-line arrangement of varying magnification imaging units and a two-line staggered arrangement of telecentric imaging units. We also discuss the effectiveness of wavefront coding (WFC) in suppressing chromatic aberration, which is caused using cheap refractive imaging units, along with experimental results of applying both cubic- and quartic-phase-modulated WFC.

Simulation and analysis of variable numerical aperture wide-field microscopy for telecentricity with constant resolution

In this work, we propose simulation and analysis of a novel combination of a variable numerical aperture wide-field microscope objective with an Electrically Tunable Lens (ETL) for axial scanning with a telecentric image space. This work resolves the challenges of the conventional ETL based wide-field microscopic systems viz. variable magnification, field-of-view (FOV), an inconsistent lateral and axial resolution. The proposed combination performs the tuning of the object plane by changing ETL current and effectively modifies the object space numerical aperture using an aperture controller to provide a stationary telecentric image plane. Moreover, with the proposed combination, the lateral resolution of the system is preserved over an axial range of 7.586mm with a magnification error of 2.55 %. The proposed work witnesses the advantages of providing fast axial scanning, extended axial scanning range, autofocusing, invariant magnification, constant FOV, and constant resolution along with a compact optical setup.

Fluorescent 2D Digital Image Correlation With Built‐in Coaxial Illumination for Deformation Measurement in Space‐constrained Scenarios

An advanced two-dimensional digital image correlation (2D-DIC) using active imaging and telecentric imaging has been proposed to overcome the problems of out-of-plane motion and ambient light variation. Here, the technique is further improved by combining built-in coaxial illumination and telecentric fluorescent imaging to provide quality 2D-DIC measurement even in space-constrained scenarios. This improved 2D-DIC firstly excites the fluorescent speckles on the specimen surface using an ultraviolet light source through built-in coaxial illumination, and then employs a monochromatic telecentric imaging system to capture the images of the excited speckles. By tracking the movements of fluorescent speckles using DIC, full-field displacements and strains can be retrieved. For validation, the quality of speckle images captured by three different 2D-DIC systems was firstly evaluated, proving that the proposed fluorescent 2D-DIC can eliminate the specular reflections and acquire speckle images with the best quality. A uniaxial tensile test of an Al alloy specimen in an environmental chamber and thermal strain measurement of a polymethylmethacrylate sample in a water bath device were performed using the established system, further confirming the practicality of the proposed method. With the improved performance, compactness, and ease of use, this fluorescent 2D-DIC is expected to be widely applied in future research.

Telecentric in-line-and-off-axis hybrid digital holographic high-resolution reconstruction method

In-line digital holography usually employs a phase retrieval algorithm to decouple the phase information but fails to eliminate the unwanted DC and twin image terms when the measured sample does not agree with the sparsity. While the off-axis digital holography can efficiently remove the unwanted image terms but can not reserve the high frequencies of the sample to realize high resolution. The in-line-and-off-axis hybrid digital holography was then developed to provide a relatively high resolution digital holographic imaging without considering the effect of the unwanted terms. In other words, the in-line-and-off-axis hybrid digital holography merges all of the best virtues of the mentioned-above methods in an efficient and elegant way. However, this state-of-the-art method requires prior knowledge about the diffraction distance, which results in time-consuming and low accuracy. In other sense, telecentric technology can realize non-diffractive imaging without the knowledge about the diffraction distance or spherical aberration or defocusing aberration. Therefore, in this paper, a novel in-line-and-off-axis hybrid digital holography is proposed by introducing telecentric imaging architecture, and the corresponding reconstruction method is further proposed by utilizing constrained iterative approach. In this method, telecentric in-line-and-off-axis hybrid digital holography is first used to acquire focused off-axis and in-line holograms, respectively. The low resolution phase information is reconstructed from the off-axis hologram by using Fourier transform method with the help of the sample-free off-axis hologram, and then multiplexed with the amplitude information obtained from the in-line hologram to act as the initial complex amplitude in the iterative recovery process. As a result, constrained iterations are carried out in the spatial domain and frequency domain to realize high resolution and high speed reconstruction. After simulations, we build an experimental setup and demonstrate the operation of the method with USAF resolution target, onion cells and bee wings. Both the simulation and experimental results show that the proposed method can require no prior knowledge to suppress the phase disturbance caused by the unwanted image terms and optical aberrations, resulting in high speed and full utilization of spatial bandwidth product of the digital camera to yield high resolution reconstruction. We hope that the proposed method will have most practical applications in the case where large resolution, high speed and good quality are needed.

Design and characterization of telecentric f-θ scanning lenses for broadband terahertz frequency systems.

Telecentric beam scanning using f-θ lenses offers nearly uniform spot size, linear beam displacement, and normal incidence angle over a planar surface. These unique properties allow for the minimization of imaging distortion over a wide field-of-view. In this article, we present a numerical method for designing custom f-θ lenses in the THz regime. We fabricated three lenses made from different commonly used polymer materials in the THz optics. We demonstrated their optical performance metrics compared to a conventional plano-convex lens over the broadband 0.3 THz–1 THz range. We find that the f-θ lens designed using the optical properties of high-density polyethylene achieved superior performance by maintaining a constant phase over a wide field of view of about 34°. We demonstrate this isophase property by measuring a constant time of arrival of the THz time-domain pulses over a reference mirror with a standard deviation of ∼19 fs, in excellent agreement with simulation predictions. This work will pave the way for the design and implementation of highly precise and fast telecentric imaging systems in the THz frequencies.

A convenient 3D reconstruction model based on parallel-axis structured light system

Conventional high-accuracy three-dimensional (3D) reconstruction methods usually involve complicated and time-consuming system calibration. This paper presents a novel and simple 3D reconstruction model based on a parallel-axis structured light system. Compared to the traditional structured light system, the proposed system contains a perspective projection unit and a telecentric imaging unit. Instead of obtaining the complicated structured light system parameters, the proposed model only requires the retrieval of a product matrix which could be easily calibrated by using an ordinary white plane. For an arbitrary point A on the object surface, it is mathematically proved that the product matrix of its phase map and height distribution is proportional to the distance between A and projector optical center in XY plane, and independent of the depth in Z direction. Experiments were carried out to measure a step-height object and a cylindrical surface, and the results verify that the proposed method can achieve high-accuracy 3D shape measurement.

High-precision video extensometer based on a simple dual field-of-view telecentric imaging system

We present a novel dual field-of-field (dual-FOV) video extensometer for high-precision tensile/compressive strain measurement in material testing. The present dual-FOV video extensometer, based on the combined use of a bilateral/object-space telecentric imaging system and a cube prism, can orthogonally capture two separated FOVs on the sample surface. For any two gauge points selected in the two separated FOVs, the gauge length can be theoretically computed from the systematic parameters of the imaging system. By tracking the movement of the gauge points using advanced subset-based digital image correlation (DIC) algorithm, temporal changes of the gauge length, and consequently the average strains of the test sample can be obtained. Due to the enlarged gauge length, logitudinal strains can be measured with higher sensitivity and precision compared with previously reported video extensometers that use only a telecentric lens. The precision, accuracy, and practicability of the proposed video extensometer are validated by real tensile tests.

Airy-beam tomographic microscopy.

We introduce Airy-beam tomographic microscopy (ATM) for high-resolution, volumetric, inertia-free imaging of biological specimens. The work exploits the highly adjustable Airy trajectories in the three-dimensional (3D) space, transforming the conventional telecentric wide-field imaging scheme that requires sample or focal-plane scanning to acquire 3D information. The results present a consistent near-diffraction-limited 3D resolution across a tenfold extended imaging depth compared to wide-field microscopy. We anticipate the strategy to not only offer a promising paradigm for 3D optical microscopy, but also be translated to other non-optical waveforms.

Spectral Domain Optical Coherence Tomography Imaging Performance Improvement Based on Field Curvature Aberration-Corrected Spectrometer

We designed and fabricated a telecentric f-theta imaging lens (TFL) to improve the imaging performance of spectral domain optical coherence tomography (SD-OCT). By tailoring the field curvature aberration of the TFL, the flattened focal surface was well matched to the detector plane. Simulation results showed that the spot in the focal plane fitted well within a single pixel and the modulation transfer function at high spatial frequencies showed higher values compared with those of an achromatic doublet imaging lens, which are commonly used in SD-OCT spectrometers. The spectrometer using the TFL had an axial resolution of 7.8 μm, which was similar to the theoretical value of 6.2 μm. The spectrometer was constructed so that the achromatic doublet lens was replaced by the TFL. As a result, the SD-OCT imaging depth was improved by 13% (1.85 mm) on a 10 dB basis in the roll-off curve and showed better sensitivity at the same depth. The SD-OCT images of a multi-layered tape and a human palm proved that the TFL was able to achieve deeper imaging depth and better contrast. This feature was seen very clearly in the depth profile of the image. SD-OCT imaging performance can be improved simply by changing the spectrometer’s imaging lens. By optimizing the imaging lens, deeper SD-OCT imaging can be achieved with improved sensitivity.

Enhanced laser speckle optical sensor for in situ strain sensing and structural health monitoring.

In this Letter, an enhanced laser speckle optical sensor (LSOS) for nondestructive, noncontact, and high-accuracy strain measurement has been developed. Subsystems of laser beam shaping and telecentric imaging were incorporated into the LSOS to achieve optimized speckle patterns, and a field-of-view (FOV) separation was introduced to extend sensor gauge length. Validation tests confirmed that the LSOS achieved consistent results with resistive strain gauges in laboratory conditions with maximum RMS error (RMSE) of $ 9.44\;\unicode{x00B5} \unicode{x03B5} $9.44µε. Sensing practicality was demonstrated in field tests. The results showed that the LSOS is capable of achieving accurate strain measurements in an external environment with maximum RMSE of $ 13.34\;\unicode{x00B5}\unicode{x03B5} $13.34µε.

Towards converged statistics in three-dimensional canopy-dominated flows

Multi-plane telecentric 2D particle image velocimetry (PIV) measurements were performed over and within the interstices of rough beds in an open-channel flow with low relative submergence. Two rough beds constituted of cubes and elongated cuboid obstacles in a square array were investigated with the same modified relative submergence. The telecentric method allowed the complete fluid space within the interstices and above to be measured without disturbing the flow. Nine planes were measured yielding a total of 17 planes by symmetry consideration. This allowed double-average computations to be performed with a 17 $$\times$$ 17 horizontal spatial resolution yielding allowing to resolve adequately the shear stress terms contributing to the total shear stress. It is shown that subsampling with a coarse grid of 1 $$\times$$ 5 yields acceptable spatially converged results, but only for the double-averaged longitudinal velocities. Together, the spatially averaged turbulent shear stresses and the dispersive stresses require a sampling of at least $$9\times 9$$ within the canopy and $$5\times 5$$ above for the errors relative to the total shear stress to remain below 5%. Above the canopy, the dispersive shear stress depends largely on the aspect ratio of the roughness obstacles, being much higher for the cubes than for elongated cuboids. Finally, the converged total shear stress is shown to yield the vertical integrated drag stress as well as the repartition of the flat-bed shear stress to the total shear stress.

Solution to the slope-height ambiguity problem in phase measuring deflectometry based on a co-axial telecentric optical path

In recent years, phase measuring deflectometry (PMD) has become a popular method to measure the shapes of specular surfaces. However, the inherent slope-height ambiguity problem, which indicates the phase of the recorded fringe pattern is determined by both the height and slope of the test surface, causes considerable ambiguity errors in the measurement results. Existing error reduction methods usually require the initial shape information of the test surface or additional equipment to assist the measurement. These requirements make measurement inflexible in some situations. In this paper, the slope-height ambiguity problem in PMD is modelled and analyzed analytically for the first time, and the system geometry requirements of minimizing the ambiguity error are carried out accordingly. To meet these requirements, a co-axis optical path setup with a telecentric imaging system is proposed. In our simulation and experiment, this system reduces the phase and height errors by up to two orders of magnitude, compared to those from the normally employed system geometry setup.

Ultrasensitive video extensometer using single-camera dual field-of-view telecentric imaging system.

Video extensometer assesses mechanical properties of materials by measuring their length changes under different loadings with combined use of video recording and image processing. In this Letter, an ultrasensitive video extensometer implemented on a single-camera dual field-of-view (dual FOV) telecentric imaging system is proposed. The method integrates the idea of FOV separation and telecentric imaging, which can increase the strain resolution with an order of magnitude and enhance the strain accuracy significantly. Validation experiments indicate that the root mean square error of a video extensometer was effectively decreased from 25.2 to 4.4 microstrains after employing the proposed method. Time-varying thermal strains of Al alloy and alumina ceramic samples were also precisely measured by the established ultrasensitive video extensometer, confirming its accuracy and practicality.

Design of UV LED illumination system for direct imaging lithography

UV LED, which can be used as the illumination source in the direct imaging (DI) exposure equipment, has the advantages of rich wavelengths and low cost. It has a good application prospect in the field of printed circuit board (PCB) manufacturing. In practical application, there are high requirements for output power density, stray light control, and luminance uniformity of DI system to improve the productivity and exposure performance. We present an optical structure of the illumination system for DI lithography to provide uniform illumination low stray light for digital micromirror device (DMD), which combines Kohler illumination and double telecentric imaging. In this design, the high luminance illumination can be obtained by LED etendue analysis, fly’s eye condenser is not needed, and the stray light on the DMD caused by light outside the effective angle of the LED can be mostly filtered out. Based on this idea, a lithographic illumination system with the numerical aperture of 0.1 is designed and fabricated for 0.95-in. DMD. According to experimental measurements, the effective illumination power is up to 10 W, the stray light accounts for 10.7%, and the illuminance uniformity is about 90%, which meet the requirements of DI lithography equipment used in PCB manufacturing.

Alignment-tolerant single-shot digital holographic microscopy based on computer-controlled telecentricity.

An alignment-tolerant telecentric digital holographic microscopy (AT-T-DHM) system based on computer-controlled telecentricity is proposed. It consists of a three-step process-optical recording, computational compensation, and retrieving processes. With a tube-lens-based two-beam interferometer, phase information of the object is recorded on the hologram, where another optical quadratic phase error (O-QPE) due to the misalignment of the tube lens happens to be added. In the computational compensation process, this phase error can be estimated, by which the O-QPE is balanced out from the recorded hologram. Then, only the phase information of the object can be retrieved from the O-QPE-compensated hologram. This computational compensation process makes the proposed system virtually operate in a telecentric imaging mode, which enables implementing a practical AT-T-DHM. Wave-optical analysis and experiments with a test object confirm the feasibility of the proposed system.