Understanding and quantitatively characterizing the hidden distribution and evolution of the whole-field stress in fractured coal is crucial for accurately predicting crack propagation and connection that affect the transport capability of coalbed methane in reservoirs during fracture stimulation. However, it is challenging to directly reveal and quantify the hidden stress field using traditional methods because of the difficulties associated with extracting and identifying the stress distribution around randomly distributed, discontinuous, irregular-geometry cracks as well as with fabricating a model with embedded cracks. This study presents an optical method to reveal and quantify the continuous evolutions of the whole-field principal stress difference and shear stress in fractured coal by combining three-dimensional (3D) printing technology and digital photoelasticity. 3D printing technology is used to replicate the transparent model of fractured coal. The ten-step phase-shifting method and quality-guided phase unwrapping algorithm in photoelasticity are used to estimate the whole field photoelastic parameters (isoclinic and isochromatic) in the fractured coal model. The comparison between experimental and numerical results is provided to further prove the efficacy of this method in handling the natural fractured coal structure. The effects of crack irregularity and discontinuity on the stress evolution in fractured coal are evaluated as well.

It is crucial for sparse aperture systems to preserve imaging quality, which can be addressed when fast corrections of pistons within a fraction of a wavelength are available. In this paper, we demonstrate that only a single deep convolutional neural network is sufficient to extract pistons from wide-band extended images once being appropriately trained. To eliminate the object characters, the feature vector is calculated as the input by a pair of focused and defocused images. This method possesses the capability of fine phasing with high sensing accuracy, and a large-scale capture range without the use of combined wavelengths. Simple and fast, the proposed technique might find wide applications in phasing telescope arrays or segmented mirrors.

In composite plates, there are generally amounts of residual stresses that can have adverse effects in final parts. So, it is very important to know about the residual stress's type, location and amounts in composite plates. The measurement methods of the residual stress are commonly indirect and the displacement and strain is calculated at first. Two-dimensional digital image correlation (2D DIC) is a practical and effective tool for quantitative in-plane deformation measurement of a planar object surface and now widely accepted and commonly used in the field of experimental mechanics. In the hole drilling method, the residual stress is measured immediately just after drilling and the other researchers have paid less attention to the amount and pattern of changes in released stress around the hole after drilling at different times. It can be stated that the novelty of this research may identify the optimal measurement time after the drilling and the holes with different diameters is considered and the displacement and strain fields are measured at different times. So in this research, the residual stress measurement of a composite plate in the time elapsed is discussed. Also, the effects of hole's diameter and elapsed time after drilling in amount of released residual stresses are investigated by 2D DIC method. The point-wise local least-squares fitting technique (PLS) is used for strain estimation from displacement fields. The residual stress is calculated by the estimated strain, determination of calibration constants in orthotropic materials and considering the properties of composite plate that has already been calculated. The finite element method (FEM) is used for determination of calibration constants and the correction factor in FEM is obtained by comparing of the FEM and experimental tensile test results.

Despite the recent advances in augmented reality (AR), which has shown the potential to significantly impact on our daily lives by offering a new way to manipulate and interact with virtual information, minimizing visual discomfort due to the vergence–accommodation conflict remains a challenge. Emerging AR technologies often exploit focus-tunable optics to address this problem. Although they demonstrated improved depth perception by enabling proper focus cues, a bulky form factor of focus–tunable optics prevents their use in the form of a pair of eyeglasses. Here, we propose a novel optical configuration for a compact vari-focal AR display which deliberately utilizes the zeroth and first diffraction orders of the LC lens to produce two foci: one for a real object and the other for a virtual object with addressable focal planes. The prototype AR glasses can adjust the accommodation distance of the virtual image, mitigating the vergence–accommodation conflict without substantially compromising the form factor or image quality. In addition, we describe the design, fabrication, and characterization of an ultrathin, polarization-insensitive focus-tunable liquid crystal (LC) diffractive lens with a large aperture, a low weight, and a low operating voltage. We show that the polarization dependence of the lens, which is an inherent optical property of LC lenses, can be insensitive using the trilayer birefringent materials and by aligning the optical axes of each birefringent material at a specific angle. The polarization insensitivity eliminates the need for a polarizer, thus further reducing the form factor of the optical system. This novel approach offers significantly reduced complexity for designing AR glasses with addressable focal planes. These technologies for ultrathin lens and AR display show promising potential for developing compact optical systems in various applications.

A new approach to perform 2D Digital Image Correlation (DIC) on objects that are subjected to large out-of-plane displacements is presented. It employs out-of-plane information to distort the speckle images of the reference and the deformed surface, increasing their similarity and therefore their sub-set cross-correlation peaks. The new approach is implemented using the combined Fringe Projection and 2D-DIC technique on two experiments: The buckling of an initial flat plate, and the rotation of the reference surface towards the camera from 0° to 60°. In both cases large speckle distortions are produced due to two issues, the lens magnification and the surface tilting. The first speckle distortion is applied to the deformed image to perform the lens magnification correction, and the second speckle distortion is applied to the reference image. The obtained distorted speckle images become very similar, allowing to successfully compute corresponding pixels between both images in a simple way (i.e. without nonlinear minimization processes). For this propose, an analytical iterative equation is derived to determine pixel displacements due to out-of-plane surface tilting. The resulting displacements are compared with those obtained by the conventional procedure, applying 2D-DIC (using open source Ncorr software) on the non-distorted speckle images and then making the lens magnification correction. The novel procedure proposed makes it possible to successfully find the corresponding pixels at the reference surface for out-of-plane rotations of 55° and 60° for which Ncorr failed. 3D-DIC (using commercial VIC software) is also employed to validate the results.

In this research, a novel, precise and cost-effective method of 2-dimensional (2D) temperature measurement based on lifetime technique is introduced without using a high-speed camera. The setup is simple and includes only a blue excitation light-emitting-diode (LED) and a low frame-rate CMOS camera. Instead of multiple frame capturing techniques in a single decay curve, the proposed technique takes a single image per excitation pulse in a stroboscopic mode. Two-dimensional thermographic imaging of a printed circuit board as a test sample was done based on the transient luminescence characteristics of magnesium fluoro-germanate (MFG) thermographic phosphor (TP). A cost effective, blue LED (425 nm) excitation source was used to illuminate the circuit board at a repetition rate of 33.33 Hz. The integration time of the camera was 10 ms and the detection window was applied after turning off the LEDs with an increasing delay during the successive periods. 100 pictures covering 4 milliseconds of the decay curve were then used to calculate the lifetime and temperature of each pixel of the image of the circuit board. Combined pixel-to-pixel and shot-to-shot uncertainty was only ±2.5 °C with a high spatial resolution of 50 µm.

Sapphire is widely used in civilian and military equipment owing to its superior optical and mechanical properties. Femtosecond laser has been demonstrated to be an effective tool to process sapphire material. However, the direct processing of sapphire by femtosecond laser still meets some challenges, such as poor ablation morphology and low laser energy absorption. In this work, femtosecond laser processing of sapphire coated with a 12-nm-thick gold film (Au-coated sapphire) has been investigated. The experimental results have revealed that the ablation morphology of Au-coated sapphire has been improved, featuring fewer molten materials and thermal cracks, as well as regular crater shape and uniform periodic surface structures. It has also been found that, under 100 shots condition, the threshold fluence of Au-coated sapphire is reduced by about 56% compared to that of uncoated one. Meanwhile, the incubation effect of Au-coated sapphire is stronger than that of uncoated one. We also illustrate that the material removal rate of Au-coated sapphire is increased up to about two times higher than that of uncoated one. In order to reveal the effective mechanism of the gold film in the laser processing of sapphire, the energy transfer process among incident photons, free electrons and sapphire lattice phonons was studied. Our study provides a guidance for improving the laser ablation capacity of sapphire.

Surface defect detection of strip steel is still a challenging task for its complex variances, e.g., intra-class defects exist large differences in appearance while inter-class defects contain similar parts. To address these issues, we regard the defect object as the salient part of the image and propose a novel, effective saliency propagation algorithm based on multiple constraints and improved texture features (MCITF). Firstly, we deliberately design 83-dim texture features that are used to generate label matrix (among which the label information viewed as the important basis of diffusion process) in the framework of multiple-instance learning. Then we resort to Laplacian regularization viewed as smoothness constraint for enlarging the gap between defect objects and background, and high-level prior (background, object, and mid-level feature) constraints for constraining the label information propagation process locally in order to uniformly highlight the complete defect objects while effectively suppress the non-salient background. Finally, we observe that the superpixel segmentation algorithm based on spectral clustering can adequately capture the edge information of defects, thus promoting to yield high-quality pixel-level saliency maps. Experimental results implemented on the real challenging strip steel benchmark database demonstrate that our MCITF model outperforms state-of-the-art methods with large margins and strong robustness in terms of eight evaluation metrics.

By illuminating targets with a modulated light signal, a time-of-flight (ToF) camera can calculate depth maps based on the phase shifts generated by the round-trip travel of the light from the targets back to the sensor. The accuracy of such depth imaging depends on the quality of the returned light. However, the calculation can be disturbed by various factors, including the external environment and the internal structure of the camera. Multiple coupled interferences can introduce noise into the depth data collected by a ToF camera in the spatial domain. It is difficult to express the relationship between the noise in depth maps and these mixed disturbances in a mathematical model. Based on the theory of differential entropy and a large amount of depth data from a ToF camera, this paper analyzes the characteristics of depth imaging entropy, proposes an evaluation method for depth image quality, and presents a multilayer perceptron model with information entropy (E-MLP) trained to optimize the accuracy of depth imaging. Experimental results show that this method can significantly improve the depth accuracy in the case of mixed noise.

A novel helical laser hole-cutting (HLHC) technology based on coupled assistance of water-based ultrasonic vibrations and external longitudinal/transverse magnetic fields is reported for cutting holes in nickel super-alloy GH4049 sheets. Effects of external unidirectional transverse and longitudinal magnetic fields on HLHC performance were correspondingly analyzed and compared with and without water-based ultrasonic assistance. The influence of the loading direction of an external magnetic field and the magnetic field intensity of an external longitudinal/transverse magnetic field on HLHC performance is first reported based on systematic corresponding comparisons with and without water-based ultrasonic assistance. The effects of external longitudinal/transverse magnetic fields and beam expanding ratios on actual laser focal spot sizes were analyzed and discussed. It was demonstrated that the latest reported HLHC technology assisted by water-based ultrasonic vibrations and external longitudinal magnetic fields could generate deeper holes with relatively high cutting efficiency, smaller hole taper, higher hole aspect ratio, smaller hole circularity deviation, better hole wall quality, much better performance for relieving residual stress, thinner recast layer, more uniform refinement of grains, and better performance for improving micro hardness. Generally, a stronger external longitudinal magnetic field coupled by water-based ultrasonic vibrations was more helpful for cutting deeper and higher-quality holes. It was shown that the change of loading direction of an external magnetic field with/without ultrasonic assistance did not greatly alter recast layer formation and micro hardness performance for the workpiece helically cut. Compared to the transverse magnetic assistance, a corresponding longitudinal magnetic field coupled with water-based ultrasonic vibrations was more effective for micro hardness improvement for this reported HLHC technology. When using the same water-based ultrasonic vibration assistance, a stronger longitudinal magnetic field was more effective for relieving residual stress for the workpiece helically cut (a stress reduction percentage of around 96.5% was reported in this work). The average laser focal spot size obviously decreased with laser beam expanding ratio, but the external longitudinal/transverse magnetic assistance did not greatly alter the actual laser focal spot size.

In this paper, we have fabricated and characterized a bimetallic optical fiber surface plasmon resonance (SPR) sensor, followed by molybdenum disulfide (MoS2) nanosheets have been coated on the surface of metal film. The performance of the sensor is discussed in terms of the sensitivity and the figure of merit (FOM) theoretically and experimentally. The effect of different thickness of Au/Ag film on the performance of the sensor was studied. The results indicate that the FOM of sensor with silver film is much higher than that with gold film, although gold film is conducive to the sensitivity improvement. Two types of sensors are fabricated, which are the sensor with silver layer and the sensor with bimetallic layers of silver and gold. The sensitivity of the sensor with silver film and the sensor with bimetallic layers are 2141 nm/RIU and 2487 nm/RIU, respectively; and the FOM values are 19.44 RIU−1 and 18.47 RIU−1, respectively. Then, it is theoretically confirmed that MoS2 has the potential to promote the sensitivity of SPR sensor, and the sensitivity is tuned by the number of layers of MoS2 nanosheets. MoS2 nanosheets successfully bridged the gold film with the help of chemical bonds. The sensitivity of the sensor with MoS2 nanosheets is 3061 nm/RIU, and its FOM value is 23.29 RIU−1.

Laser hot-wire cladding (LHWC) is a hybrid deposition process where the wire is preheated by resistant heat during the deposition process. This process can dramatically increase the deposition efficiency and the material utilization rate. In this study, a cobalt-based metal cored wire was deposited by using laser hot-wire cladding. The CMOS camera assisted by a green laser illumination was used to monitor the stability of the deposition process and to investigate the molten pool width and length. The effects of processing parameters such as wire feed rate and scanning speed on the clad geometrical characteristics (height, wetting angle, and dilution rate) were studied in detail. The variation of the secondary dendrite arm spacing (SDAS) and microhardness with different processing parameters was discussed. The microstructure and corrosion resistance of the deposited alloy were finally analyzed. It was found that the resistant heat applied on the wire was a dominate factor influencing the stability of the deposition process. The clad height could be predicted by measuring the molten pool width. The molten pool length could be used as an indicator to reflect the cooling rate of the process. The clad height, wetting angle, and dilution rate were all sensitive to the wire feed rate and the scanning speed. The microhardness was mainly determined by the dilution rate. A high dilution rate decreased the clad hardness. The microstructure of the deposited alloy had a typical hypo-eutectic structure. An improved corrosion resistance of the deposited coating was obtained when compared to the substrate.

Measuring the vibration frequency of structures with image processing methods is one of the non-contact measurement methods. However, the frequency to be measured should be lower than the camera capture speed. In this study, a method was developed to extract and to compare pattern characteristics of data acquired from a pixel of a low-speed camera. At the first stage, images of a rotating disk were recorded at known frequencies to find out the camera’s response. After that, distinctive features were extracted both from the raw and clustered intensity data acquired from the pixels of the recorded images. At the second stage, pixel intensity data of a rotating disk with an unknown frequency was also recorded with the same camera. Similarly, features were extracted from these data, and they were compared with the features at known frequencies. A frequency range was found for the unknown frequency value in this way. Finally, the unknown frequency value was estimated by interpolation in the frequency range. The method was verified on an experimental setup, and five unknown frequency values were successfully estimated. Consequently, higher frequencies than the capture speed of the camera were correctly estimated using the developed method. On the other hand, it was also noticed that the clustering of the discrete intensity data was also significant.

We propose a wrapped phase denoising method based on convolutional neural networks (CNN), which can effectively denoise a noisy wrapped phase. The noisy numerator and denominator of the arctangent function are firstly denoised by CNN, and then the filtered numerator and denominator use the arctangent function to obtain the clean wrapped phase. We experimentally verify the denoising performance using various wrapped phase that contains different noise conditions, where the denoised wrapped phase can achieve a satisfactory unwrapping performance using the existing simple unwrapping method. In addition, the proposed method is further demonstrated through the comparison of the existing methods, and shows an accurate denoising result without adjusting any parameters.

Chaotic systems have been widely applied in digital image encryption due to their complex properties such as ergodicity, pseudo randomness and extreme sensitivity to their initial values and parameters. An image encryption algorithm based on a hidden attractor chaos system and Knuth–Durstenfeld algorithm is proposed. First, a hidden attractor chaos system is used to encrypt digital image. Compared to a self-excited attractor, the hidden attractor's attracting basin does not intersect with any small neighbourhoods of the equilibria. It is difficult for attackers to reconstruct the attractor by finding equilibrium points. Therefore, the hidden attractor chaotic system is difficult to decrypt. Meanwhile, the hidden attractor chaos system is very sensitive to initial values and parameters. Second, the Knuth–Durstenfeld algorithm has good randomness. In addition, the Knuth–Durstenfeld algorithm can reduce the time complexity and the space complexity of the permutation while achieving good permutation effects. Thus, Knuth–Durstenfeld algorithm is used to permutate the digital image. Finally, DNA sequence operations are used to diffuse image pixels values. Some experimental analyses have been applied to measure the new scheme, and the experimental results illustrate the scheme possesses better encryption performances. This method can be applied in secure image communication fields.

Digital holographic microscopy (DHM) is actively investigated in the field of bio-imaging as a technique for quantitative phase microscopy. During recent years, the digital holographic technique for enhancing the spatial resolution using speckle patterns has been reported by several research groups. In this study, we have enhanced the spatial resolution and extended the measurement range of the shape measurement by applying a two-wavelength method to DHM using speckle illuminations.

Mach-Zehnder and conoscopic interferometry are used to explore photoelastic properties of anisotropic crystal materials. In a number of cases an application of both techniques significantly improves an accuracy of piezooptic and photoelastic measurements. The performance of such combined approach is demonstrated on tetragonal lithium tetraborate (LTB) single crystals, as an example. Special attention is paid to methodological and metrological aspects, such as measurement accuracy and the quantitative error analysis of the resulting measurements. Performing the interferometric measurements for different geometries of piezooptic coupling the full sets of piezooptic and photoelastic tensor constants of LTB crystals have been determined. The acoustooptic efficiency, on the other hand, has been evaluated using the magnitudes of photoelastic constants derived from the piezooptical measurements. For the geometries with strong photoelastic coupling LTB demonstrates quite large acoustooptic performance with figure of merit value, М2, achieving 2.12 × 10–15 s3/kg. It is several times larger than that of strontium borate crystals, nowadays the best acoustooptic material in deep-ultraviolet spectral region.

A novel one-layer micrometre-sized TiC/TiN-based spectrally selective coating was designed and fabricated via spray method-laser cladding hybrid deposition in air. The laser cladding was used to obtain TiC/TiN-Ni/Mo cermet coatings with desirable optical properties. In particular, an absorptance (α) of ~84% and thermal emissivity (ε) of ~5% at 300 K were calculated at a TiC/TiN weight ratio of 1:1. In addition, the thermal stability of the coating was outstanding after heat treatment at 650 °C for 200 h. The absorptance and emissivity of the 30 wt.% TiC-30 wt. %TiN-20 wt.% Ni-20 wt.% Mo cermet coatings were respectively 82% and 4%, respectively, at 650 °C. Furthermore, the cermet coating also exhibited excellent weatherability, indicateing that TiC/TiN-based cermets are suitable for spectrally selective materials. Moreover, the laser cladding was demonstrated as an improved and novel preparation technology for manufacture of solar selective absorbing coatings.

The use of light encoding techniques is widespread in the field of 3D surface reconstruction. This paper presents a stereo-camera calibration methodology, which integrates structured light encoding with an active digital device. The structured light encoding approach is proposed to unambiguously solve the stereo matching issue for stereo-camera setups. A sequence of vertical and horizontal binary striped patterns, combined with a checkerboard pattern, is displayed by a high-resolution LCD screen, which is used as calibration board. A bundle adjustment technique is adopted to simultaneously adjust both camera parameters and screen geometry, as part of the stereo-camera calibration process, thus taking into account the possible inaccuracies of the digital display. The same structured light approach, with small variants, is projected by a multimedia digital projector to carry out 3D surface reconstruction. The proposed methodology defines a comprehensive framework for the development of a 3D optical scanner, from calibration to 3D acquisition, which has been validated by measuring primitive surfaces and reconstructing free-form shapes with different stereo-camera setups.

High-resolution scanning electron microscope (SEM) is widely used for microscale material and structure characterization. However, the distortion in SEM imaging causes serious error for the quantitative measurement of optical methods. In previous work, the distortion calibration requires to set up a mathematic model of SEM imaging and then to determine the model parameters by calculation. The distortion filed is hard to directly extract from the SEM image by classic methods. In order to tackle this problem, we developed a direct sampling moiré method (DSM) for simple and fast analysis of grating distortion. In DSM, the distortion fields can be directly presented visually through processing a single grating image. The measurement accuracy of DSM was verified by numerical simulation to be better than the classic sampling moiré. In the experiment, a standard grating with pitch of 6.5 μm was used in DSM to calibrate the distortion fields at different magnifications (50 × , 100 × , 150 × , 300 × ) in SEM. The fitting results of the calibrated distortion field agreed well with the existing distortion model, which demonstrated the feasibility of DSM for distortion calibration in SEM.

High-performance optics puts stringent requirements on the defect control of transparent optical components (TOCs). In order to accurately and reliably detect the surface and internal defects of TOCs, this paper proposes a three-dimensional (3D) defect distribution detection method based on coaxial transmission dark-field (CTDF) microscopy. The illumination and imaging light paths are coaxial, and a high-pass filter in front of the microscope objective is applied to remove the illuminated background light on the defect images and improve the imaging contrast. Based on the finite depth of focus (DOF) of the microscope objective, the focal plane position of defects can be determined by axial light intensity analysis, and the 3D defect distribution reconstruction is performed. Simulations and experiments show that the method can realize high-contrast defect imaging and has the ability to detect the 3D distribution of surface and internal defects of TOCs.

Checkerboard represents the best pattern for in-plane displacement measurement in terms of sensor noise propagation because this pattern maximizes image gradient. It also exhibits other interesting properties in terms of pattern-induced bias for instance. Digital Image Correlation (DIC) is not the best option to extract displacement fields from such periodic patterns, and spectral methods should be used instead. In this paper, it is shown that three different spectral techniques initially developed for classic bidimensional grids can be adapted to process checkerboard images. These three techniques are the Geometric Phase Analysis (GPA), the windowed version of the Geometric Phase Analysis (WGPA), and the Localized Spectrum Analysis (LSA), which can be regarded as the ultimate version of WGPA. The main features of these three techniques as well as the link between them are given in this paper. Their metrological performance are compared in terms of displacement resolution, spatial resolution and bias. Synthetic checkerboard images deformed with a suitable reference displacement field are considered for this purpose. It is shown that GPA is the fastest method. According to the metric used in this paper, the best metrological performance is obtained with WGPA with suitable settings. LSA followed by a deconvolution algorithm is just behind, but the calculation time is approximately 10 times lower than that of WGPA for the examples considered in this paper, which makes it a reasonable choice for the determination of in-plane displacement fields from checkerboard images.

The fringe projection profilometry (FPP) has obtained increasing popularity in the field of industrial automation, inverse engineering and graphics. Recent literature reveals that the non-ideal system point spread function (PSF) in FPP will cause phase error in the area around the discontinuous edge. Existing error reduction methods need to detect the location of all the edges accurately first, which are hard to accomplish when the camera is defocused. Meanwhile, the corrected data in the error area relies heavily on the data in its nearest unaffected area, which makes the corrected data unreliable. We prove that the non-ideal system PSF will also induce phase error in the area of surface details, and this is a problem seldom discussed in FPP. In this work, the relationships between the PSF-induced phase error and system parameters are deduced mathematically and numerically. Additionally, a deconvolution-based method is proposed to reduce the PSF-induced phase error in this paper. The proposed method can overcome the shortcomings of the existing approaches. Both simulation and experiment results show that our proposed method can reduce the Root Mean Square phase/height error by up to 4 times, and can also improve the measured 3D details significantly.

In this paper, we propose Fourier Volumetric Computational Reconstruction (FVCR), which is a new computational integral imaging reconstruction (CIIR) algorithm. It handles physical pixel overlapping by refocusing in the Fourier domain. Thus, the longitudinal resolution of the reconstructed 3D image is the same as optical reconstruction. It also considers the lateral resolution limit of integral imaging and minimizes the computational load in CIIR. To verify the improvements in both lateral and longitudinal resolution of the reconstructed 3D image, in this paper, we implemented optical experiments. To the best of our knowledge, this is the first report of the efficient CIIR algorithm that considers the longitudinal and lateral resolution limits of integral imaging.

The traditional phase-coding (TPC) method has been widely used for three-dimensional (3D) shape measurement. However, the fringe order determined from the coding patterns cannot be precisely aligned with the wrapped phase calculated from the sinusoidal patterns when measuring dynamic objects. Consequently, phase unwrapping errors will occur around the 2π phase jumps of the wrapped phase. To address this problem, the present work proposes a complementary phase-coding (CPC) method for dynamic 3D shape measurement. Specifically, the projection of the sinusoidal and coding patterns forms a cycle and the se patterns are shifted by a half-period between even and odd cycles. Combining two fringe orders of adjacent even and odd cycles, the absolute phase map can be correctly recovered. Moreover, to improve the measurement speed, all 8-bit gray patterns are converted into 1-bit binary patterns by a binary dithering technique. Both simulations and experiments demonstrate the robustness and efficiency of the proposed method.

A multiple-image encryption method based on compressed coded aperture imaging and pixel location scrambling operation is proposed. In the encryption process, each plaintext image is compressively encoded by its corresponding random matrix, which is generated via a row and column displacement transformation operation on an original random matrix (ORM) and some auxiliary keys. With the help of charge coupled device (CCD), the superimposed image are then generated by recording and grouping all the encoded images; finally, the ciphertext image is obtained by pixel location scrambling operation for the superimposed image. During decryption, the participant who possesses his/her correct key-group, can successfully reconstruct the corresponding plaintext image by location index key, auxiliary keys, scrambling operation, compressed coded aperture imaging retrieval algorithm. Theoretical analysis and numerical simulations validate the feasibility of the proposed method.

A dual-channel interferometer (DCI) is proposed for vibration-resistant optical measurement. The DCI employs a dual-channel interference optical path design, wherein an assistant channel is used to generate dense fringe patterns by introducing a spatial carrier frequency, and the coefficients of the vibration tilt phase plane are then extracted. The sparse fringe patterns of the main channel are combined with the coefficients of the tilt phase plane to calculate the measured phase distribution. This study reports the optical path and principles of the interference system. Simulations are performed using the proposed DCI phase extraction algorithm. To verify the DCI performance, phase measurements are carried out in a vibrating environment, and the results obtained using the proposed method are compared with those obtained using spatial phase-shifting interferometry (SPSI). The experimental results demonstrate that the DCI can effectively overcome the effect of vibration. Unlike spatial carrier frequency interferometry, although the spatial carrier frequency is also introduced in the assistant channel in the proposed DCI method, the dual-channel information extraction algorithm helps eliminate the calibration process of the retrace error before measurement, making this a calibration-free measurement. Moreover, the DCI utilizes environmental vibration to generate a ‘phase shift’, thus not only eliminating the need for a vibration isolation platform but also the need for a precise phase shifting device, with advantages such as structural simplicity, ease of operation, and low cost.

The only known approach that can break the diffractive-imaging-based encryption (DIBE) was proposed by Li and Shi in 2015. However, their approach works under the assumption that the phase distribution of the random phase masks (RPMs) is within [0, π]. In other words, it is no longer effective when such requirement is not fulfilled. In this paper, we propose a universal method, referred to as learning-based chosen-plaintext attack (L-CPA), to break DIBE. The L-CPA enables one to recover the plaintext from the ciphertext by aid of a well-trained artificial neural network (ANN), regardless of the phase distribution of the RPMs. Furthermore, the proposal can be accomplished with no need of knowing the details of the optical arrangement of DIBE. To our best knowledge, this is the first paper that reveals the absolute insecurity of DIBE against CPA. Numerical simulations are presented to demonstrate the effectiveness and feasibility of the proposal.

An incoherent digital holographic imaging system was constructed in this study based on the Michelson interferometer, where the distance of the observed object can be calculated via mathematical modeling. We obtain the composite hologram by generalized phase-shifting method, then utilize the contrast evaluation function to determine the focus distance of composite hologram. The relationship between the focus distance and observation distance is deduced accordingly to complete the measurement process. Experiments show that the uncertainty of the proposed method decreases as observed distance increases in the range of 508 mm–660.4 mm. The results presented in this paper may provide a theoretical foundation for incoherent holographic imaging and three-dimensional measurement.

A commercial stereo light microscope, such as the OLYMPUS SZX7, has nonnegligible lens distortion for three-dimensional microscopic image correlation. To tackle the problem, an effective calibration method of using a stereo-vision system is proposed. It is implemented using non-coplanar feature points with weighted radial constraints, where the weights are incorporated into the objective function to estimate the internal and external parameters of the vision system iteratively. In the calculation of the distortion coefficients, a non-parametric distortion model expressed by a bicubic spline surface is used. The iterative convergence of the objective function and the influence of the attitude number of the calibration target on the calibration results are analysed, and verified with a digital stereo light microscope established with OLYMPUS SZX7. Both shape and displacement measurements have been performed to demonstrate the proposed method. The experimental results show that the proposed method can achieve higher accuracy compared with Zhang's classical method and the unweighted two-step method.

In the process of computer vision-based seam tracking, although strong noise interference exists such as that arising from arc and splash in the welding process, the tracking effect has been significantly improved by noise reduction, feature point probability estimation and other methods. However, in the process of automatic tracking of a seam tracking system, the robot's pose cannot be adapted to various welding conditions in real time, resulting in decreased weld quality. To enhance the adaptability and real-time estimation of the robot's pose during welding, this paper proposes a real-time pose estimation method for a seam tracking system. In a welding environment with strong noise interference, the real-time pose estimation of the welding workpiece is carried out, and the robot's pose is changed in real time. The pose estimation is realized by building point cloud data, constructing a tool coordinate system in real time and obtaining rotation angles. To accurately acquire the point cloud data, efficient convolution operators (ECO) for tracking and the morphological intersection method integrated with a support vector machine (SVM) are adopted to classify the images with strong noise to better suppress the tracking model drift. The offline tracking test shows that compared with the original tracking algorithm, the proposed method can significantly suppress the peak value of pixel error and reduce its mean value. The welding experiment results show that the proposed method can be adapted to various welding conditions and achieve adaptive and real-time robot pose goals, which improves the welding precision and quality.

Particle size distribution (PSD) measurement is important for improving product quality, inhibiting environmental pollution, and safeguarding human health. The near-field scattering (NFS) technique offers many substantial advantages over conventional techniques. However, the reconstruction equations are ill-posed and it is difficult to get a reliable solution. In this paper, we propose a new method that can solve the reconstruction equations. This method combines the Backus-Gilbert eclectic theory with truncated singular value decomposition (TSVD) method to obtain a non-negative approximation solution as the initial value of Chahine algorithm for fine calculation. Our method exhibited good performance with several numerical cases of unimodal and bimodal PSDs. It was then used for PSD measurement of two standard particles (GBW(E) 120,028 and 120,047) using the NFS techniques. The relative errors of the length mean diameter, with respect to the peak size of these two standard particles, were 0.18% and 0.76%, respectively.

In this paper, a new image encryption algorithm was proposed based on the Chua's multi-scroll chaotic circuit system. In comparison with other classical discrete systems, the new multi-scroll system has higher complexity, thus it was more suitable for the design of the image encryption algorithm. The correlation of the adjacent pixels was greatly eliminated by using the Arnold cat transformation. According to the scroll number of the multi-scroll chaotic system on x-y direction, parameter p and q of the Arnold transformation can be determined. Especially, the pixel value of the ciphered image of the new random diffusion algorithm depends on two random non-adjacent pixels and the interference value of the multi-scroll chaotic system, it significantly reduced the correlation between the adjacent pixels, and it greatly increases the ability to resist deciphering. The experimental results show that the encryption effect of the ciphered image were significantly higher than that of other encryption algorithms. It provided a data support and theoretical direction for the research of the related cryptography fields.

Microstructured surfaces over large areas are typical in diffractive optics and biomimetic surfaces. The topography measurement is challenging due to the multi-scale nature. In this paper, an efficient stitching method is proposed for fusion of multiple subapertures to produce a large area topography. The first step is detecting edge features and then projecting them on the OXY plane. The next step is correction of lateral coordinates by registering the edge curves in overlapping regions. The change of lateral coordinates is linearly related to the lateral shifts and clocking angle which are estimated by simply solving a linear least-squares problem. In the final step, height change is also linearly related to the piston and tip-tilt of subapertures. Hence it is corrected by minimizing the height difference in overlapping regions, which is again modeled as a linear least-squares problem. The stitching method is experimentally verified on a holographic sample. Totally seven by seven subapertures are measured with an interference microscope and seamlessly stitched together.

The geometrical characteristics of dental implants, such as thread depth and width, facing angle, pitch, and surface roughness, are important to assess their stability and osseointegration after implant surgery. Herein, we demonstrate the potential use of depth-resolved swept-source optical coherence tomography (SS-OCT) to assess the structural quality of widely used dental implants. The implemented SS-OCT system was centered at a wavelength of 1300 nm with a 100 nm full-width at half-maximum. Four dental implants with different structural formations fabricated using either titanium or ceramic were visualized. Qualitative assessments were performed using boundary flattening with an amplitude-profiling algorithm to emphasize and compare the thread depths, surface roughness, and inner structures of the experimental samples. Cross-sectional and volumetric OCT data clearly revealed the depth, width, and pitch of the dental implants, and especially, the quantitative assessment of axial and lateral thread depth unobtainable using conventional inspection methods was successfully conducted. The depth of thread was measured using a depth-directional intensity profile. In conclusion, the high-resolution SS-OCT system could be utilized to improve the quality assurance of dental implant products through multi-plane assessment.

The track of moving target by single photon lidar (SPL) is investigated in marine aerosol environment. The aerosol-induced clutter severely limits the detection performance of SPL. Conventional time-correlated single-photon counting histogram threshold method can only detect static targets. We introduced track algorithm to deal with the discrete scattering point signal of SPL. Furthermore, a two-stage (TS) dynamic model with an adaptive strong Kalman filter was proposed in SPL tracking technique. The proposed method, referred to TS-SKF, has a better performance than the current statistical model with Kalman filter (KF) and the TS model with KF in simulated marine aerosol environment. Finally, a series of field experiments were conducted with a self-established SPL system to track the moving cargo ships over the sea at Qingdao (35.9°N, 120.3°E), China. Both the moving and static targets can be effectively detected with the TS-SKF method. The results show that the proposed real-time processing algorithm can provide a new sight to promote the practical application of SPL.

Colour adaptation techniques are useful in extending the method of Twelve fringe photoelasticity (TFP) to solve problems in industries as one can minimise the generation of specific calibration tables for each experiment. Image artefacts affect the functioning of existing colour adaptation techniques affecting the fringe order demodulation. Colour transfer is a popular technique in the field of image processing to tackle colour mismatch between images. In this paper, the applicability of two colour transfer strategies in the context of TFP is explored, of which one is selected for implementation. A comparative study on the performance of the novel colour transfer method with the existing colour adaptation techniques is carried out.

The active controllable metasurfaces based on electromagnetically induced transparency (EIT) have attracted great interest in the research of terahertz functional devices. We demonstrate a dual-mode controllable terahertz metasurface based on complementary plasmon-induced reflection structure. Both photosensitive silicon and monolayer graphene are integrated into the unit cell to realize the active control of the EIT-like reflection window and the whole resonance, modulated by optical pumping and extra electrical voltage, respectively. The excellent properties of the modulation depth and the group delay highlight this metasurfaces in the applications of terahertz modulators and slow light devices. Moreover, this dual-mode modulation in one design provides a good way of commercial terahertz multi-functional devices.

This paper presents the design, fabrication and testing of a single-mode fiber optic ultrasound transducer. This transducer can work as an ultrasound generator and a receiver, based on principles of photoacoustic (PA) effect and Fabry–Pérot (FP) interference. PA effect is a process in which ultrasound waves are generated from optical energy by a PA material, and an FP structure is used for ultrasound receiving. The transducer was manufactured by coating a material called gold nanocomposite directly onto a single-mode optical fiber tip. The gold nanocomposite acted as the PA material and formed the FP structure as well. The transducer had the same size as the single-mode fiber, whose diameter was only 125 μm. It demonstrated its ability to generate ultrasound signals with a bandwidth of 5.6 MHz at an efficiency of 3.2 × 10−5 and function as an ultrasound receiver with a sensitivity of 2.81 mV/MPa. The generator and receiver built together can currently only work separately. This new transducer has substantial potential to work in restricted space and harsh environments for non-destructive testing.

Current iterative digital image correlation (DIC) algorithms can efficiently converge at the deformation vector with high accuracy when they are fed with reliable initial guess. Thus, the adaptability of DIC method is dominated to a large extent by the estimation of initial guess. In recent years, image feature-based technique, especially the scale-invariant feature transform (SIFT), was introduced to DIC for the estimation of initial guess in the case of large and complex deformation, due to its robustness in handling the images with translation, rotation, scaling, and localized distortion. However, feature extraction and matching in SIFT are very time consuming, which limits the applications of the SIFT-aided DIC. In this study, we developed a SIFT-aided path-independent DIC method and accelerated it by introducing the parallel computing on graphics processing unit (GPU) or multi-core CPU. In our method, SIFT features are used to estimate the initial guess for the inverse compositional Gauss-Newton (IC-GN) algorithm at each point of interest (POI). The experimental study shows that the developed method can deal with large and inhomogeneous deformation with high accuracy. Parallel computing (especially on GPU) accelerates significantly the proposed DIC method. The achieved computation speed satisfies the need for real-time processing with high resolution for the images of normal sizes.

We propose an optical encryption scheme for multiple images based on angle multiplexing and computer generated hologram(CGH) in this paper. In the encryption process, the original images are firstly modulated by two random phase keys in Fresnel transform with different diffraction distances. Secondly, the modulated images are coherent superposition with solid angle multiplexed reference beams to generate interference fringes and form a compound image. Finally, the compound image is encoded to a CGH by Roman coding method. In the decrypted process, the corresponding experimental system using SLM is built, in which multiple images can be encrypted synchronously with high efficiency. Except random phase key, the proposed encryption system has multiple kind of keys including diffraction distance and wavelength to improve the security. Due to the characteristics of high storage efficiency and simple calculation, this optical multiple-image encryption scheme has important application prospect in improving the efficiency of information transmission and multi-user authentication.

Whispering gallery mode (WGM) microcavities have a small mode volume, a high quality (Q) factor and great application potential in the field of integrated optics. The coupling of light into microcavities has attracted wide attention in the development of optical filters, sensors, and low-threshold lasers. WGM microcavities based on different coupling methods have promising applications for lasing and high-precision sensing. In a WGM coupling system, the coupling method determines the coupling efficiency, Q factor and sensor integration and even helps to determine the WGMs that are excited in the microcavity. This paper briefly reviews the coupling methods used for WGMs, summarizes their characteristics, and introduces the coupling methods of recent advances in the field, all of which are instructive for improving the coupling methods in the future.

We present two novel techniques in single-pixel imaging (SPI): the first one is Hadamard-Bayer SPI in which the Hadamard-Bayer illumination patterns are employed for fast full-color SPI, with a sampling rate and computational complexity that are one-third of the full-sampling condition; the second one encodes spatial information of an image into time-varying factors in SPI and loads them into the illumination patterns, enabling high-quality single-pixel image fusion. Furthermore, we fuse these two techniques to achieve full-color visible watermarking while imaging. Theory, simulations, and experiments confirm the feasibility of the schemes. These techniques have high scalability and wide application prospects, and possess the potential to be applied in LED structured illumination systems, single-pixel multi-spectral imaging systems, and single-pixel broadcast systems.

This paper proposes a new kind of non-contact optical measurement system, which combines with a polygon mirror and a conical lens to achieve the purpose of simultaneously measuring six-degree-of-freedom (6 DOF) geometric errors for a rotary axis of a machine tool. At the beginning, we used the Zemax software to construct the proposed optical structure and observe the change of light spots on the detectors. Then, we built the proposed optical structure on the Matlab software by using the skew-ray tracing method, which used to present the actual condition of light spot change and compare with the simulation results. Then, we designed the appropriate fixtures to place the system structure on a rotary axis in a laboratory environment. After completing the laboratory-built prototype, we analyzed the effects of different geometric errors on the detectors at different angular positions of a rotary axis of a machine tool. The proposed measurement system simultaneously measured 6 DOF geometric errors of the rotary axis, and the experimental results were compared with those of two dial gauges by using the measurement method of ISO 230–7. The experimental results show that the maximum deviation ranges of the radial motion error in X axis, the radial motion error in Y axis, the tilt motion error around X, and the tilt motion error around Y are ±4.7 µm, ±6.0 µm, ±3.6 arcsec, and ±8.1 arcsec, respectively.

Divided-aperture (DA) microscopy is an imaging method that, in addition, improves the anti-scattering capability. The present state-of-the-art imaging methods have the drawbacks of requirement of scanning or low spatial resolution. In this paper, we propose a DA digital holographic microscopy (DADHM) to enable single shot quantitative phase imaging, which, to our knowledge, is the first-of-its-kind. The proposed system is built using a Linnik-type interferometric module associated with the DA technique. In this method, interestingly, an element tilt is not required to produce an off-axis angle between the sample and reference beams because of the employment of DA. Some analyses and experiments were conducted to validate the efficiency of the proposed method, and the results indicate that a high anti-scattering capability with a mean error of 2.42 nm were achieved by the proposed method.

CMSX-4 nickel superalloy specimens, in single crystal form, were prepared for in-situ machining trials using femtosecond pulse laser processing in a focussed ion beam scanning electron microscope. Preliminary laser ablation trials were carried out to establish the optimum focus distance. Various pillars were then prepared using the femtosecond laser, varying rectangular hatch pattern, scan speed, laser line separation and frequency. Polishing trials used helical laser hatch patterns to refine pillar shape and produce circular pillars with high aspect ratio. Example pillars were characterised by FIB-SEM cross-sectional examination to assess microstructural changes and re-deposition of vaporised material. The final selected laser parameters were able to produce a stable high-aspect ratio pillar in 4501 s, 1600 µm high with tip diameter 138 µm. An area 655 × 656 µm was cleared around the pillar with over 0.65mm3 of metal removed.

Direct energy deposition (DED) is an effective method to fabricate complex metal thin-wall structures. The geometrical dimensions of the cladding track have significant influence on the dimensional precision of final components. In this study, a powder-scale multi-physics model using the Finite Volume Method (FVM) is developed to study the direct energy deposition process. The mass transfer, phase transformations and heat transfer in the DED process are incorporated and the geometrical characteristics of a single cladding track can be rapidly predicted. The influences of the process parameters including laser power, powder feed rate and scanning speed on the track width and height are analyzed in detail using an analysis of variance (ANOVA) method. Based on the simulation results, a Gaussian process regression (GPR) model is developed to predict the geometrical characteristics of cladding tracks under different manufacturing parameters. Finally, both the multi-physics model and the GPR model are validated by single track deposition experiments. The results show that the proposed multi-physics simulation results are in good agreement with the experimental results and can reveal the qualitative relationship between parameters and track geometry. The GPR model is able to predict the geometrical characteristics of single cladding tracks.

A single input state, single mode fiber (SMF) based spectral domain polarization sensitive optical coherence tomography (SD-PS-OCT) system using a single linear-in-wavenumber spectral camera is presented. With the proposed dual channel linear-in-wavenumber spectral camera and the polarization alignment procedure, the orthogonal polarization components of the spectral interference signal can be detected using the all SMF based SD-PS-OCT system without any bulk polarization optical elements. To evaluate the performance of the developed system, the polarization properties of a Babinet-Soleil compensator and a quarter waveplate are quantitatively measured with high accuracy. Furthermore, the depth-resolved images of the intensity, the phase retardation and the optic axis orientation of the ex-vivo tendon tissue and the chicken muscle tissue are reconstructed to verify the feasibility of the proposed method.

Reconstructing 3D human body models for real people benefits a variety of applications. However, it is challenging to reconstruct high-fidelity human body models with high-accuracy. This study presents an accurate and efficient 3D human body reconstruction system based on digital image correlation (DIC), which can simultaneously capture high-quality 3D human body model and high-fidelity 2D color texture. The 3D reconstruction system is set up with twenty-seven cameras and nine speckle-pattern projectors, which is able to acquire 3D shape data and 2D color data. Specifically, the nine binocular stereo vision systems are adopted to reconstruct 3D shape, where the 3D reconstruction is carried out based on stereo-DIC with a speckle-pattern projector. The other nine color cameras are utilized to capture 2D color texture. It only needs a single-shot to record the images required for 3D reconstruction, which takes about 1.25 ms to ensure the high-accuracy and high-efficiency of the proposed 3D reconstruction system. A high-precision multi-camera calibration method is employed to calibrate the proposed multi-camera system. The mapping relationship between the reconstructed 3D object point and the 2D color image is determined by the calibrated result, which makes the color texture acquisition precisely and effectively. The proposed system can achieve accurate and high-quality 3D reconstruction of the whole human body surface with high-fidelity color texture. Experimental results demonstrate the high-performance of the proposed 3D reconstruction system.

This paper presents the method, implementation and validation of a borescope probe design tool devised for the challenges of optical fluid measurement techniques. The design tool is capable of predicting the path and power distribution of the laser beam through the probe and into the region interest, ensuring a cost and time-efficient design process that removes the need for experimental trials. The associated code is available as supplementary material. Optical measurement techniques have become established methods within fluid dynamics research. In contrast, their application to turbomachinery rigs is usually limited due to the restricted optical access. A small number of studies have circumvented this problem by employing borescopes to introduce the laser beam into the measurement region but wider application is inhibited because these probes are difficult to design, expensive and usually require several iterations until a suitable design is achieved. The first part of the paper presents the structure of the software program and the mathematical modelling of the optics for predicting the beam path into the measurement region. The second part presents different design options and the manufacture of a typical probe with validation in a wind tunnel facility using volumetric velocimetry. The borescope results agree very well with measurements acquired using direct illumination through a window demonstrating the efficacy of the method.

We propose a sectional hologram reconstruction method through complex deconvolution. By taking into account of both amplitude and phase (or real and imaginary parts) of wavefront propagation into the inverse problem model, we can perform more robust and better imaging quality in the sectional holographic reconstructions. The sectional capacity is about six times the diffraction-limited resolution, and the experimental results show that our method can achieve the best imaging quality compared to the state-of-the-art techniques.

With the development of aerospace and fusion engineering, understanding the mechanical behavior of materials under high-temperature conditions has become increasingly important. However, few studies are devoted to the ultra-high temperature range of 2000–3000 °C. In this study, with the aim of developing non-contact measuring techniques of mechanical deformation under ultra-high temperature, a high heat flux (~300 MW) comprehensive experimental platform is established, which includes a vacuum chamber, a three-dimensional digital image correlation (3D-DIC) system, infrared radiation thermometers and an electron beam heating system. Using the electron beam heating technique, the tungsten specimen can be heated to over 3000 °C. Owing to the use of a vacuum chamber, the thermally induced airflow disturbance at high temperature can be completely removed. Tantalum carbide (TaC) powder is chosen as the speckle material and speckle fabrication technology is developed to adapt ultra-high temperatures under vacuum conditions. In order to suppress the blackbody radiation at high temperature, three schemes based on blue light sources, self-radiating light sources and a dual wavelength optical filter technique are designed for three temperature ranges from room temperature to 3067 °C. Afterwards, full-field thermal deformation of the tungsten specimen above 3000 °C was determined based on the above strategies using the 3D-DIC technique. The feasibility and accuracy of the proposed methods are verified by comparing the measurement results with the thermal expansion strain data and model from available databases and literature. The standard deviations in different temperature intervals are 50 με for 25–1200 °C, 100–200 με for 1200–1800 °C and less than 500 με for 1800–3067 °C. The proposed methods and technologies are expected to lay a foundation for further developments in strain field measurements at ultra-high temperature.

In this study we investigated the human red blood cell (RBC) dynamics by means of biospeckle laser analysis. Blood samples from healthy donors were introduced in a 0.8 mm internal diameter capillary tube, and illuminated with a He-Ne laser in order to obtain the biospeckle pattern from both side and forward scattered light. Experiments were carried out for different concentrations of red blood cells in plasma, from 25% to 50%. Biospeckle parameters such as Correlation Coefficient and Inertia Moment, were calculated for different frequency bandwidths in order to assess their sensitivity and versatility. A filter based on the Discrete Wavelet Transform was used to decompose the registered sample activity. A relation between Inertia Moment and the RBCs to plasma volume ratio was observed. The Correlation Index that measures the level of correlation of biospeckle images was defined and analyzed. This work inquires in a technique that is suitable for the development of novel non-invasive optical tools for clinical diagnosis in vascular pathologies.

Hybridly polarized beam is a new type of vector beam different from conventional cylindrical vector beam. In our work, we present a simple and effective method to generate hybridly polarized beams by using a cascade of a super structured polarization converter and a quarter-wave plate. The transformation of light field was analyzed by using Jones vector and Jones matrix, and the Stokes parameters of the generated vector beams were measured to verify the states of polarization. Various novel multiscale micro-/nanostructures were fabricated on surfaces of 4H-SiC by using the hybrid vector beams, which are different from that induced by linearly polarized beam or conventional cylindrical vector beam. Experimental results show that the multiscale characteristics can be adjusted with a high degree of freedom by designing states of polarization. This research provides a new idea for preparation of multiscale micro-/nanostructures that may be useful in the fields of nano-/micro-electromechanical systems, thermal photovoltaics, and thermal management.

Digital image correlation (DIC) is widely used in macroscopic and mesoscopic mechanical tests because of its advantages of non-contact, high precision, full-field measurement and simple experimental equipment. The application of various nonlinear optimisation algorithms greatly reduces the computation time of the DIC iteration process. Thus, efficiently obtaining a reliable initial value is crucial. Particularly, in cases when the surface of the test object is substantially rotated or the deformation involves a large rotation, the initial value estimation can have a major influence on the execution speed of the algorithm. Some scholars have proposed initial value estimation methods for large-rotation objects, but they all sacrificed the speed of calculation. This study deals with improving the efficiency of the initial value estimation algorithm for large-rotation objects from two aspects. Firstly, we decomposed deformed and reference images into multiresolution layers to create wavelet pyramids, and the correlation coefficients between the two compared images were calculated at the low-resolution layer. The multiresolution processing of the image data provides an efficient method for registering large image data sets because the full-size data sets does not require matching. Secondly, we propose a local ring pattern (LRP), which is invariant to object rotation, to convert the 2D template into a 1D grey value sequence for calculating the correlation coefficients. The advantages of the LRP feature include the characterisation of its rotation invariance and the reduction of computational complexity. Experimental results indicate that the proposed method can be used to estimate an initial value where reference and deformed subsets are related by translational and rotational motions and the speed of the proposed method is higher than that of the traditional method.

The multi-parameter distributions of non-premixed combustion were investigated in this work by using deflection tomography and particle image velocimetry (PIV). An experimental apparatus composed of a burning system, a deflection tomography system, and a PIV system is initially designed and constructed. Two charge-coupled diode cameras are controlled by a synchroniser to capture Moiré fringes and tracer particles simultaneously, and the velocity contour and streamline map are obtained from the tracer particle distributions and flame images. The temperature distribution is reconstructed from the Moiré fringes based on the wave-front recovery method and Abel-inversion algorithm, and the velocity and temperature distributions are matched in the combustion field by using the pixel registration method. The matching results show that the high-temperature and high-speed zones of the combustion flow field do not coincide under the experimental conditions. Finally, the coupling imaging effect and measurement errors in the multi-parameter measurement are discussed.

Within the context of Digital Image Correlation (DIC), the optimal treatment of color images is considered. The mathematical bses of a weighted 3-field image correlation are first introduced, which are relevant for RGB encoded images. In this framework, noise characterization methods are developed as noise properties dictate the best suited metric to compare images. Consistent ways to process an image from elementary Bayer matrices are derived. Last, a case study on uncertainty quantification is performed.

We report on a novel layout providing variable zoom in digital in-line holographic microscopy (VZ-DIHM). The implementation is in virtue of an electrically tunable lens (ETL) which enables to slightly shift the illumination source axial position without mechanical movement of any system component. Magnifications ranging from ~15X to ~35X are easily achievable using the same layout and resulting in a substantial variation of the total field of view (FOV). The performance of the proposed setup is, first, validated using a resolution test target where the main parameters are analyzed (theoretically and experimentally) and, second, corroborated analyzing biological sample (prostate cancer cells) showing its application to biomedical imaging.