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Motivated by the low efficiency and poor accuracy of the existing techniques for measuring the depth of a pavement structure, we examine the characteristics of laser speckle and speckle measurement methods in relation to the depth of a pavement structure and propose a set of methods for the 3D measurement of such structures based on laser speckle and digital image processing. First, we analyze the relevant characteristics of laser speckle measurement, and then based on the relevant region and temporal sequence correlation, a faster digital speckle correlation method is used to establish a mathematical relationship between the 3D image and the structural depth. The erosion and dilation technique and mean optimization algorithm are proposed to eliminate the noise and burr of primeval 3D reconstruction. After this, a 3D point cloud of information on the depth of the pavement structure is obtained. Finally, through fitting, a good linear relationship was found between the texture depth obtained by the sand paving method and the system. From measurements using a laboratory calibration platform, it was found that the accuracy of measurement of our method can reach the subpixel level, and the running speed is high, meaning that it can be used for highly efficient and accurate detection of the depth of a road structure.

To study the application of laser shearing speckle interferometric vibration measurement technology in acoustic resonance landmine detection, we present an experimental and theoretical study of the method for laser shearing speckle interferometric landmine detection based on time-average. The relationship between the out-of-plane displacement gradient generated by the sound wave on the ground and the phase change of the shearing interference laser is analyzed. A high-power loudspeaker is used as the source of sound wave excitation, and the laser shearing speckle interferometric vibration measurement system is used in landmine detection experiments to investigate the changes of laser shearing speckle interference fringes under different experimental conditions such as sound wave intensity, detection target, buried depth, and soil environment. The study reveals that it is feasible and effective to use laser shearing speckle interferometry to detect landmines, and it provides a theoretical and methodological basis for realizing the rapid scanning of acoustic-optic landmine detection.

Full-field optical coherence tomography (FFOCT) can be used to acquire cellular-level morphological images of nonfixed and unstained biological tissues. However, the speckle noise and vignetting of FFOCT images are drawbacks in terms of pathological application. We present a novel adaptive nonlocal image enhancement algorithm that uses a fluorescence microscopy (FM) image of the nucleus to achieve cytoplasm image enhancement under FFOCT. The algorithm comprises a denoising module and an adaptive nonlocal enhancement module. The denoising module is an end-to-end residual network with skip connections and dilated convolution. The adaptive nonlocal enhancement module enhances images to segment tissues, nontissues, and transition areas so that the images retain the details of the original cell structure but do not contain noise. The proposed algorithm was implemented on a dual-modality scanner system that combines FFOCT and FM. A total of 12 pig liver and pig kidney tissues were sectioned into 10,236 samples for experiments. The peak signal-to-noise ratio and structural similarity index value increased from 30.8 and 0.58, respectively, for the aforementioned system without the proposed algorithm to 34.2 and 0.86, respectively, for the aforementioned system with the proposed algorithm. Ten samples of human breast tissues were used to validate the proposed algorithm. The average contrast-to-noise ratio was 1.69 and 11.73 when the aforementioned system was used with and without the proposed algorithm, respectively. The results of this study revealed that the quality of scanned images considerably improved when the proposed system was used and that the proposed system can facilitate clinical pathological diagnosis.

In order to resolve the problem of speckle interference phase map noise interference, a phase denoising method of digital speckle pattern interferometry based on empirical wavelet transform (EWT) and cross correlation is proposed. First, the EWT is used to decompose the speckle phase map, and a series of components is obtained. Then the noise-free components containing phase information are extracted based on the cross correlation. After that, combined with the particle swarm optimization algorithm and sine–cosine denoising method, an improved sine–cosine denoising method is proposed to process the noise components. Finally, the noiseless components are reconstructed to obtain the denoised speckle phase map. Simulation and experimental results show that the proposed method can effectively reduce the noise interference and obtain accurate phase information.

We present a method for transferring images with high spatial resolution through highly scattering media. Rather than achieving high resolution (HR) using a larger transmission matrix (TM), which is difficult to implement in practice, we propose an image coding scheme based on compressed sensing, which is capable of recovering object information at low sampling rates, to enhance TM-based image reconstruction. The essence of the proposed scheme can be considered as an exchange of spatial and temporal resolution. By appending extra images, which are compressive encoded signals of the high-frequency information of the original object, following transmission of the low-resolution image, HR image of the original object can be reconstructed from the measured speckle signals. The feasibility of the proposed method is validated by optical experiments. The proposed method achieves a remarkable resolution improvement without relying on a complex optical system, thereby indicating promise for existing applications involving image transmission through scattering media.

A method of simultaneous and precision measurement of yaw and pitch based on digital speckle pattern interferometry is proposed. The relationship between the yaw/pitch and phase distribution is analyzed in detail, resulting in the establishment of the yaw and pitch simultaneous measurement model. A plane fitting method is proposed to separate the two angle motions from a single phase map. The proposed method enjoys the advantages of being free of cooperative target, high-resolution measurement, and compact optical setup. Experimental results show the mean absolute error of the measurement is

Digital speckle pattern interferometry (DSPI) is a widely used technique in various high-precision nondestructive testing applications. However, a coherence multiplicative noise, called speckle noise, is produced. Speckle noise degrades the quality and the resolution of the fringes, leading to difficulties in fringe analysis and phase retrieval. We aim to restore the DSPI correlation fringes and reduce the speckle noise through a multidirectional total variation method. In this method, the local fringe orientation, computed through Riesz transform, is used to increase the sensitivity of the variational regularization to the spatially varying directional intensity changes. The proposed algorithm is applied to the directional wavelet phase retrieval technique. The performance of the proposed method is evaluated with both simulated and experimental DSPI fringe patterns, obtaining satisfactory results.

A wave-front shaping optimization algorithm is proposed to image through scattering media with higher accuracy. Pearson correlation coefficient (PCC), structural similarity (SSIM), and gradient structural similarity (GSSIM) are introduced as the loss functions to recover images. We have concluded that GSSIM has the optimal detail recovery performance among complex multivalued targets, followed by SSIM and PCC. To find the optimal phase mask, we propose the squirrel search algorithm and compared it with two classical global optimization algorithms including the genetic algorithm, the simulated annealing algorithm, and the commonly used continuous sequence algorithm. The simulation and experimental results show that the proposed squirrel search algorithm has a faster convergence speed and higher robustness and stability, which indicates an improved reconstruction quality. This algorithm will help realize imaging through scattering media better and quicker, which is meaningful in dynamic biological tissue imaging and related fields such as optical detection.

The correlation-based image subset matching methods such as digital image correlation (DIC) are widely used for measuring the deformation fields. The specimen surface must be covered with speckle patterns, or more exactly, random intensity distributions with sufficient contrast, which act as a carrier of deformation information, to acquire reliable and accurate matching in the correlation calculation. The requirement of the artificial speckles on the specimen surface inevitably contaminates the specimen. To maintain the cleanliness of the specimen surface, an in situ deformation estimation method without speckle fabrication is developed and its result is demonstrated. Topological similarity measurement (TSM) is a feature vector-based displacement estimating algorithm for full-field deformations that uses feature vectors that encode the relative positions of neighboring mark spots. Compared with the DIC method, this approach makes up for its deficiency in displacement estimation of speckle-free flat surface by incorporating a topological arrangement-based feature vector, subpixel center location method, and biharmonic spline interpolation method. Experimental results indicate that the proposed TSM method is feasible to become the preferred displacement estimation method for uniform deformation. As a complement to the DIC method, the TSM method is more suitable for uniform deformation distribution, which requires high measurement accuracy but cannot use DIC.

Ghost imaging via sparsity constraints (GISC) is an advanced imaging technique. The reconstruction quality of GISC is affected by the sparse ratio of the object, the regularization parameter, and the iteration number. Influences of these parameters on the peak signal-to-noise ratio (PSNR) of the reconstructed image are discussed and evaluated. The optimal regularization parameter and iteration number at different sparse ratios are given. Then the reconstructed images of GISC using the optimal parameters at different sparse ratios are shown. The improvement of the reconstruction quality of GISC utilizing the optimal parameters is confirmed through comparison with normalized ghost imaging. Finally, the reconstruction quality of GISC with random noise is analyzed, and a method to obtain the sparse ratio of the object by analyzing the signal of the bucket detector is discussed.

This work aims to provide a quick solution for impressing a well-defined and repeatable speckle pattern on the surface of a material sample. The proposed technique is based on a water-soluble stabilizer on which the speckle pattern generated and optimized via computer is printed. To verify the application, a two-dimensional (2D) digital image correlation (DIC) system is employed to measure the full-range strain distribution at the macroscopic level during a tensile test on open-hole aluminum specimen. The experimental setup consists mainly of an action camera, a macro lens, and an open-source 2D DIC software. The measured data obtained from the DIC are compared to the other ones provided both from a traditional measurement method based on strain gauge and by a numerical simulation. The results indicate that the approach is both accurate and reliable to obtain stress-strain curves especially in the presence of plastic deformations.

Laser active imaging is widely used in many fields. The intensity image quality of laser active imaging is affected by various degradations, such as speckle effect and noise. Most existing objective image quality assessment (IQA) methods that consider only a single distorted image are not suitable for an intensity image. A multiscale full-reference intensity IQA method is presented. The proposed method is based on an improved gradient magnitude similarity deviation (GMSD) in the nonsubsampled contourlet transform (NSCT) domain. The reference and distorted images are decomposed by NSCT to emulate the multichannel structure of the human visual system. Then, the GMSD of each sub-band is computed to capture the intensity image quality. At last, the contrast sensitivity function implementation is employed in the sub-bands of the NSCT domain. All sub-bands’ GMSD is evaluated and pooled together to yield the objective quality index of a distorted intensity image. Experimental results show that the proposed method can effectively and accurately evaluate the quality of intensity images, and it is highly consistent with subjective perception.

High-speed laser shadowgraphy is crucial in range testing. We report the design, development, and application of a dual-sequence large-scale high-speed laser retroreflective shadowgraph system (DLH-LRSS) to monitor large-scale transient phenomena on a field-test range. The DLH-LRSS is based on the retroreflective technique. The narrow pulse width laser can effectively freeze ultrahigh-speed processes, thereby allowing the investigation of ultrafast phenomena. Multiple shooting modes contribute to the flexibility of the test. Three typical experiments were conducted at the Nanshan Test Center, and good quality sequential images were captured by DLH-LRSS. We demonstrate that DLH-LRSS has large-scale, ultra-fast recording and quantitative study capabilities. The DLH-LRSS can be used in the rapid diagnosis and measurement in range testing and similar observation applications.

A digital method is proposed whereby spatial speckles created when an optically rough surface is illuminated by a diffuse coherent laser beam are used to generate slope contour fringes. This is done by photographing the speckles contained in a parallel plane in front of the plate before and after deformation. The resulting photographs are then fast Fourier transformed twice and numerically compared with the digital speckle correlation software called computer-aided speckle interferometry to produce slope contours of the plate. The slope contour data are then stored in a computational array. The curvature contours are generated by numerical differentiation of the slope data; similarly, the deflection contours are generated by numerical integration of the slope data. The curvature data can then be further used in conjunction with plate theory to obtain the bending and twisting moments the plates experience as well as the stress distribution in the surface of the plate. The method is simple and straight forward. A comparison with several other techniques is presented.

With the increase in digital image correlation (DIC) applications, it is an important task to realize the fully automatic measurement and provide an automatic deformation measurement sensor. Keeping this in mind, we propose an automatic speckle region selection method for DIC. This method can be divided into three main steps: initial contour selection, active contour computation, and morphological operation. The sum of modulus of the local intensity gradient vector of the local image is used to determine the initial contour, the active contour computation is driven by improved automatic local Gaussian distribution fitting energy, and the morphological closing operation is finally conducted to achieve better robustness. Experimental results show that the root mean squared error of the localization is less than 0.8260 pixels. We further integrate the proposed method into an open-source DIC software, and the fully automatic deformation analysis is realized. The fully automatic deformation measurement will further widen the application scope of DIC, especially in the field of smart robotics and smart city.

A single-shot procedure to reduce speckle noise in numerically computed complex-valued wavefields is presented. The method is supported by the possibility of numerically producing multiple speckle realizations of a calculated complex-valued wavefield to reduce the speckle noise through a noncoherent superposition of the produced realizations. Although the method is applied to digital holographic microscopy, it could be utilized in other techniques where a numerical representation of the complex-valued wavefield of interest can be obtained. Experimental results with nonbiological and biological samples are presented to support the feasibility of the method.

The shape function and lens distortion affect the accuracy of digital image correlation (DIC). Many studies have been conducted on the shape function and lens distortion correction, and an appropriate shape function with lens distortion correction can reduce measurement errors. However, the influence of the implementation order of correcting lens distortion with known distortion parameters and matching subsets with different shape functions has not been discussed. The two implementation orders are: correcting matched grid points after matching distorted subsets and correcting distorted images before matching the subsets. The former will cause undermatching, and the latter will produce additional interpolation errors. Simulations and an experiment with known lens distortion parameters were performed to evaluate the accuracy of the two implementation orders. The forward additive Gauss–Newton algorithm with first-order and second-order shape functions was applied to the measurements. The results show that the order of correcting distorted images before matching subsets has higher accuracy as compared with another order, and a second-order shape function should always be used unless the underlying deformation is known to be lower order.

Automated extraction of the +1 term spectrum can greatly benefit the real-time measurement of dynamic deformation in the digital speckle pattern interferometry (DSPI). However, most spatial filtering methods are not satisfactory for automatic analysis because they need manual intervention or are unable to accurately extract multiple +1 term spectrums in one frequency distribution image. We propose an adaptive local threshold segmentation method for the Fourier spatial filtering of a three-dimensional DSPI system. This method first uses global thresholding to coarsely recognize the spectral regions and then adopts local thresholding to accurately determine the spatial filtering windows for each spectrum. Experimental result shows that the proposed adaptive local threshold segmentation can accurately extract multiple desired +1 term spectrums even if the intensities of different spectral regions differ greatly. In addition, experiments of the comparison between the automatic and manual spatial filtering and automatic analysis of dynamic deformation demonstrate that the proposed method can be used for the real-time measurement.

In computer-generated holograms (CGH), a random phase (RP) is usually introduced to spread the object information in the hologram and avoid the frequency-filtering effect in the reconstructed image. However, this action introduces speckle noise in the reconstructed image but also permits one to reconstruct objects with bigger size than the hologram. In recent years, alternative RPs have been proposed to reduce the speckle noise in comparison with the uniform distributed RP that is usually applied. However, its effects in different types of holograms, its capacity to reconstruct big objects, and its capability to eliminate the frequency-filtering effect have not been reviewed in detail. We present an analysis of the normal distributed, pink, brown, and Perlin phases in Fourier and Fresnel holograms coded as amplitude and phase (Kinoform and complex amplitude encoded). Only in the Kinoform holograms, the RP is required to eliminate the frequency-filtering effect. In other cases, the phase is useful to record big objects in the hologram. The Perlin phase had a better performance than the other phases, achieving the reconstruction of objects with bigger size than the hologram and reducing the speckle noise. We present a quantitative and qualitative analysis of each distribution, including simulated and experimental results that confirm the numerical analysis. We believe these results could be helpful in the selection of an adequate RP in CGH, according to the hologram configuration.

A self-contained platform for realistic modeling of digital holographic microscopy (DHM) is presented. Amplitude and phase samples, imaged in different architectures, can be modeled to produce numerical DHM holograms that include all the parameters that are present in real experiments, providing an accessible way for newcomers to have a first approach to this research field. The platform is based on considering the imaging arm of the DHM that produces the object wave as the result of the convolution process between the geometrical-optics image prediction of the sample with the point spread function introduced by diffraction. The DHM hologram is produced by the amplitude superposition of complex-valued object wave with a reference wave of arbitrary description. The sampling of the analytically produced DHM holograms is set from the input discretized image according to the specifications of the digital camera aimed to be used. The feasibility of the realistic platform is exemplified by contrasting the wrong results of a nonrealistic simulation with experimental results to show the need for using a complete realistic simulation like the one presented; further applications of the platform to numerical modeling speckle noise reduction over samples with controlled levels of roughness, and phase-shifting DHM techniques, are included.

Measuring surface deformation of objects with natural patterns using digital image correlation (DIC) is difficult due to the challenges of the pattern quality and discriminative pattern matching. Existing studies in DIC predominantly focus on the artificial speckle patterns while seldom paying attention to the inevitable natural texture patterns. We propose a recursive-iterative method based on salient features to measure the deformation of objects with natural patterns. The method is proposed to select salient features according to the local intensity gradient and then to compute their displacements by incorporating the inverse compositional Gauss–Newton (IC-GN) algorithm into the classic image pyramidal computation. Compared with the existing IC-GN-based DIC technology, the use of discriminative subsets allows avoidance of displacement computation at pixels with poor spatial gradient distribution. Furthermore, the recursive computation based on the image pyramid can estimate the displacements of the features without the need for initial value estimation. This method remains effective even for large displacement measurements. The results of simulation and experiment prove the method’s feasibility, demonstrating that the method is effective in deformation measurement based on natural texture patterns.

For many years, the use of advanced composite materials in aeronautics, automotive, and sports applications has been well established. The characteristics of composite materials in terms of weight reduction, fatigue, and corrosion resistance make them competitive in many cases with respect to conventional ones (i.e., metal alloys). On the other hand, the fabrication process of the most employed composites reinforced by carbon or glass fibers requires complex steps that are not always environmentally friendly. In fact, such composite materials are not themselves “green.” For these reasons, in the last decades, the use of natural reinforcing fibers has gained increasing attention, allowing the development of new materials sharing the same advantages with conventional composite systems while respecting the environment. Due to their structural complexity, these materials are not always compatible with the use of standard nondestructive evaluation methods, e.g., ultrasounds testing. The effective use of speckle holographic techniques as nondestructive evaluation methods in full field and noncontact modality is proved on “green” composite materials. In particular, electronic speckle pattern interferometry is tested on different kinds of specimens preliminarily subjected to flexural tests. Results show that, in most cases, the damage appears more severe on the back side of specimens, opposite to the loaded one, with the sole exception of basalt laminates.

Ferroelectric materials are of great interest because of their rich applications. Several techniques have been used to understand their behaviors. Two-beam coupling configuration using LiNbO3 crystal as a recording mediums has been extensively used for real-time in-plane displacement of a transparent rough object. Low-power He–Ne laser was used to create the index grating of the speckle pattern inside the crystal and to read the grating to give the Young’s fringes. In this technique, for the in-plane displacement measurement, the image of the transmitted object is formed in the crystal. The aim of our work is to extend the applicability of the two-beam coupling configuration using LiNbO3 as a recording medium for the evaluation of in-plane displacement of a transparent rough object using speckle photography method; measurement uncertainty of in-plane displacement was studied on a wide range of displacements using experimental and new theoretical formula for displacement measurement.

In the measurement of the 3-D shape of a large curved object using traditional 3-D-digital image correlation (DIC), the left and right camera images are relatively rotated or deformed too much, which will cause the matching to fail. We propose a 3D-DIC matching method. Compared with traditional methods having fixed-size templates, this method is robust to relative rotation and deformation between dual cameras and is suitable for 3D-DIC measurement of nonplanar objects. In the proposed method, we use polar coordinates to define the matching template as a variable circle, which is convenient for interpolation calculation and changing the template size. The zero-mean normalized cross-correlation method is used to determine the position of an entire pixel on the distorted image. This step can provide an initial value close to the true value for subsequent iterations of the inverse compositional Gauss–Newton algorithm. The performance of the proposed method is verified by experiments. Compared with the traditional matching method, the proposed method is suitable for matching large curved surfaces between 3D-DIC dual cameras and extends the application of DIC technology.

With regard to the three-dimensional (3D) shape measurement, based on speckle interferometry, of an object with a fine structure beyond the diffraction limit of the objective lens, phase distribution of each speckle region in a speckle pattern and a specklegram is discussed. The experimental results confirmed that the phase distribution with respect to the shape of an object exists spatially even in the speckle region of the speckle pattern. However, the measurement results of a superfine structure by speckle interferometry indicated that the spatial continuity of the phase distribution does not exist in each speckle but in the specklegram. This feature was used to confirm that a wide range of 3D shape measurements of an object with a fine structure can be realized by analyzing specklegrams calculated from speckle patterns before and after a lateral shift in speckle interferometry. Furthermore, it is confirmed that the shape measurement of a nonperiodical structure can be performed.

In this work, we propose a combination of the Teager–Kaiser energy operator (TKEO) and the spiral phase transform (SPT) for robust instant energy estimation of amplitude-modulated and frequency-modulated (AM–FM) signals, where the energy extraction is followed by a high-frequency component, generally considered as noise. We demonstrate that this noise component can be subtracted mathematically using the SPT transformation applied to the AM–FM signal. The improvement in demodulation is tested using a simulated AM–FM image and evaluated by the image quality index. An experimental speckle fringe pattern obtained by digital speckle pattern interferometry on a hard disk is denoised using a multiband approach and demodulated using the proposed method.

In wireless laser communication, the effects of atmospheric turbulence cause laser spots to appear as irregular speckles in the far-field plane, which may make automatic acquisition, pointing, and tracking systems unable to locate a spot position. We propose an innovative and precise anti-interference, spot-detection algorithm for far-field irregular speckles based on the intensity characteristics of the far-field spots. Before obtaining the spot center coordinates through two-dimensional vector space circle fitting, the proposed algorithm employs computer graphics combined with mathematical morphology to fill the connected domains and extracts light-spot edges using the Canny operator, thus resulting in an accurate detection of far-field spots. The algorithm validation, using an automated experimental platform (atmospheric turbulent refractive-index structure parameter C n 2 is 10 − 15 m − 2/3), yields a detection threshold accuracy of 30 to 32 pixels (each pixel represents 3 μrad) for 10.2 km far-field spot images. The corresponding deviation satisfies the two-dimensional aiming platform for a minimum step distance of 17.45 μrad, verifying the feasibility and effectiveness of the algorithm.

Digital image correlation (DIC) is a noncontact technique that is widely used for deformation measurement, but improving the calculation efficiency to achieve real-time DIC calculation has always been a big concern. A parallel temporal sequence DIC method is proposed, which chooses seed points to determine the integer-pixel displacement and applies the moving least-squares fitting technique to acquire the subpixel displacement. This method avoids traditional complex iterations and takes full advantage of the GPU parallel computing. Results of a simulation experiment and an actual experiment demonstrate the accuracy and efficiency of the proposed algorithm. The calculation speed in the simulation experiment of the proposed method achieved 463,320 POI/s, whereas the speed in the actual experiment was 432,866 POI/s, when the speed of the ICGN method was 2700 POI/s and 2074 POI/s under the same accuracy, respectively. Also, the subpixel displacement calculation made up less than 1% of the entire calculation. The computational efficiency could be further enhanced if a faster integer-pixel displacement calculation method is discovered or a parallel algorithm is used.

A common-path spatial phase shifting digital speckle pattern shearing interferometry setup is introduced for simultaneous measurement of in-plane and out-plane strain components under dynamic loading using two laser beams with different wavelengths that symmetrically illuminate the test object and a single detector. The simplicity, stability, and efficiency of the arrangement are provided by using a glass plate as a shearing device, which is capable of tuning the sensitivity continuously. The phase is recovered from a single frame by the Fourier method. In this setup, the spatial carrier frequencies can be adjusted independent of the amount of the lateral shear. Ultimately, the desired field of view can easily be achieved by simple imaging optics. To validate the feasibility and the flexibility of our technique, the proposed setup is used to evaluate the in-plane and out-plane strain maps of an aluminum plate, which is deformed under dynamic stress in its plane. The temporal phase stability of the proposed system is also investigated.

There is a need to remotely measure the full phase and amplitude information of small-scale acousto-seismic vibrations in order to detect the presence of buried objects (e.g., tunnels, etc.), or for other purposes. This remote sensing information may need to be collected with a large area coverage rate and at a safe standoff distance. To accomplish this, we have implemented a shearographic imaging system that incorporates phase stepping in a novel way, automatically separating random speckle noise from surface motion, without requiring an intermediate unwrapping step. This method, which we call surface-phase-resolved shearography, is especially effective for very low-amplitude motions that generate less than one light-wavelength of phase change. In laboratory studies, we have demonstrated sensitivity of two nanometers RMS with 532-nm-wavelength light.

Efforts to extend speckle-based focal plane array modulation transfer function measurements beyond the detector Nyquist frequency have unearthed challenging spectral estimation issues. In an attempt to better understand the task of speckle imagery spectral estimation, we explore the nuances of various estimation techniques, making comparisons using both real speckle imagery and simulated data. Parameters and features of the techniques investigated include number of image realizations, the size of image realizations, and applications of windows to speckle imagery spectral estimation. Real-world testing considerations such as laser stability and the challenge of collecting significant numbers of independent image realizations are addressed in the analysis. Results from this research show the advantage increasing the number of realizations has on estimation variance, the robustness of smaller realization segments when battling speckle field imagery spatial nonuniformities, the benefits of windowing image segments with regard to power spectral density estimation accuracy, and the impact that the increasing aperture area has on system signal-to-noise ratio.

The recognition of structural cracks and reconstruction of displacement fields around cracks are challenging tasks in engineering. By improving the standard digital image correlation (DIC) method and applying it in measuring the discontinuity of cracks, this study presents a coarse–fine search strategy based on a discontinuous DIC for rapid crack recognition and accurate reconstruction of the displacement fields. The technique introduces a split-subset model with a vector that represents the direction of crack opening to overcome the limitation of the standard DIC in analyzing discontinuous areas. A specially designed coarse–fine search strategy is applied to support the calculation model for achieving higher accuracy. The proposed approach is validated using numerically synthesized crack images as well as images obtained from notch tensile experiments in the laboratory. The results show that the proposed method can recognize the crack faces; in addition, it works well for reconstructing the displacement fields of both continuous and discontinuous (cracked) areas with average absolute errors of approximately 0.02. The proposed technique is also valid for slightly noised images.

Because the sensitivity of shearography is determined by its shearing direction, aeolotropic defects could be difficult to detect in nondestructive testing (NDT) using a digital shearography system with a single shearing direction. We report an adjustable aperture multiplexing spatial phase-shift digital shearography system for simultaneous measurement of displacement derivatives in three directions. By setting the aperture parameters properly, three spatial carrier frequencies are produced within the speckle pattern, and the information of three interferograms can be separated in the frequency domain. Phase maps of the three shearograms can be obtained by applying a windowed inverse Fourier transform, which enables simultaneous measurement of the displacement derivative. The system is suitable for NDT with a dynamic load. The capability of the triple-directional spatial phase-shift digital shearography system is described by theoretical discussions as well as experiments.

The phase obtained in the interferogram is generally not continuous real phase information. Therefore, a phase larger than 2π will have a phase jump called wrapped phase. The traditional phase unwrapping algorithm usually requires a large amount of computation, and in reality, phase unwrapping is often a complex and morbid inverse problem due to factors such as speckle noise, random perturbations, low modulation, and phase discontinuity, and the processes of unwrapping of some algorithms depend on the path chosen. Therefore, it is difficult to obtain an accurate absolute phase under actual conditions. To improve the accuracy of the absolute phase, a phase unwrapping algorithm based on the transport-of-intensity equation with two-wavelength illumination is proposed. The experimental results illustrate that the proposed method not only improves the accuracy of absolute phase but also holds a good antinoise performance.

A method based on the measurement of out-of-plane displacement by the phase-shifting digital speckle pattern interferometry is proposed to detect the edge of internal defects. The out-of-plane displacement is scanned by the scanning method of gradient streamlines first, which can provide paths through the defect edge with the best signal-to-noise ratio everywhere. Then, continuous wavelet transform (CWT) combined with the weighted structural intensity of the wavelet maxima is applied to detect the hidden singularities of the gradient streamlines that form the defect edge later. Moreover, the weighted structural intensity method is optimized to adapt to the detection of the hidden singularity by CWT. The results of theory, simulation, and experiments agree well, and the proposed method is efficient in internal defect detection.

Wavelets shrinkage is the most illustrative of wavelets transform for speckle noise reduction. We aim to study the performance of a monogenic wavelet transform to reduce the speckle noise in digital speckle pattern interferometric fringes. The proposed method is implemented on simulated and experimental speckle fringe patterns and its performance is appraised on the basis of peak signal-to-noise ratio (PSNR) and quality index (Q). The ability to reduce the speckle noise by the proposed method is compared with other classical speckle denoising methods. The obtained results corroborate the effectiveness of the proposed method for speckle noise reduction in speckle fringes in terms of PSNR and Q. It is also observed that the method provides better qualitative and quantitative results. Furthermore, the proposed method preserves the edge information of the speckle fringes, a feature that is quantified by the edge preservation index.

A type of distortion-free single-pixel ghost imaging without lenses is successfully realized by using a mobile phone. The proposed method is not sensitive to imaging distance. When the ratio of imaging distance to the size of the imaging object is close to 1 ∶ 2, clear and distortion-free imaging can still be achieved. This imaging entails low requirements for system hardware. A clear image can be realized when the brightness of a mobile phone is 70 lumens. Benefiting from the small volume, high portability, and wide application of mobile phones, as well as the broad application prospects of ghost imaging, the ghost imaging based on mobile phones can provide a portable and distortion-free imaging and detection method with significant applications in biological recognition and health monitoring.

With the development of artificial intelligence technology, intelligent fringe processing is a goal of relevant researchers in optical interferometry. We propose an intelligent method to achieve fully automated extraction of the fringe skeletons in electronic speckle pattern interferometry (ESPI) based on U-Net convolutional neural network. In the proposed method, the network is first trained by the samples that consist of the noisy ESPI fringe patterns and the corresponding skeleton images. After training, the other multiframe ESPI fringe patterns are fed to the trained network simultaneously; the corresponding skeleton images can be obtained in batches. Using our method, it is not necessary to process fringe patterns frame by frame. Our method is especially suitable for multiframe fringe patterns processing. We apply the intelligent method to one computer-simulated and one real-dynamic ESPI measurement, respectively. For the simulated measurement, it takes just 40 s to obtain the skeleton images of 20 noisy ESPI fringe patterns using our method. Even for low-quality experimental obtained ESPI fringe patterns, our method can also give desired results.

Compressive sensing ghost imaging (CSGI) is an imaging mechanism that can nonlocally obtain an unknown object’s information with a single-pixel detector by the correlation of intensity fluctuations. In the practical research and application of CSGI, object detection plays a crucial role in real-time monitoring and dynamic optimization of speckle pattern. We demonstrate, for the first time to our knowledge, how to solve the low-resolution and undersampling problems in CSGI object detection. The method we use is to combine generative adversarial networks (GANs) with object detection systems. The robustness of the object detection model can increase by generating reconstructed images of different resolutions and sampling rates for training. The experiment results have verified that the mean average precision of CSGI object detection using GANs has been improved 16.48% and 2.98% on MSCOCO 2017 compared with two traditional learning methods, respectively.

We experimentally demonstrate an iterative method for a vectorial optical field generator (VOF-Gen) to achieve accurate amplitude modulation in creating arbitrary complex beams. The method could converge rapidly in several steps and is effective to optimize the patterns applied to the liquid crystal spatial light modulators on a pixel-by-pixel basis to obtain arbitrary desired amplitude distribution. Meanwhile, this method could also mitigate the speckles caused by the defects of the optical components used in the VOF-Gen system and calibrate the wavefront related to the amplitude distribution. Several kinds of optical fields with different intensity distributions are successfully generated to verify the capability and versatility of the presented technique. This effective method may find many important applications in optical tweezers, microscopy, and unidirectional coupling.

Triangulation-based three-dimensional (3-D) optical metrology utilizing structured illumination requires the projector and camera to be geometrically calibrated. The projection of laser speckle structures in temporal correlation-based methods has been found to be useful for high-speed 3-D measurement. However, it requires the use of a stereo-camera configuration as speckle projection does not utilize conventional projection optics. Previously, we have shown a model-less method to calibrate these unconventional projection systems. This re-enabled single-camera setups where stereo systems used to be required. Due to the random nature of speckle production, speckle structures were not suitable as a set of repeatable illumination structures is needed for the model-less calibration method to succeed. A projection apparatus is proposed, which enables the production of a repeatable set of speckle structures. This device is then utilized in a single-camera optical metrology 3-D measurement system.

A polymer optical fiber strain sensor with extended dynamic range is reported. The proposed algorithm resets the reference fiber status depending on the magnitude of the specklegram deviation so the correlation coefficient never saturates, yielding a continuous response over the full range for both positive and negative strains. The technique was evaluated on the measurement of axial strains using a ZEONEX core, poly(methyl methacrylate) cladding multimode fiber, presenting reproducible results with 3 × 10 − 3 μϵ − 1 sensitivity (∼15 μϵ resolution) within a 22,600 μϵ interval. In contrast to the available approaches, the presented method can retrieve the strain direction and does not require intensive image processing, thus providing a simple and reliable technique for mechanical measurements using multimode optical fibers.

A resampling ring-mask method in spatial spectral domain (RRM-SSD) is proposed for speckle reduction in one-shot digital holography. Spatial spectrum of a hologram with speckle is divided into several subspectra using the resampling ring masks. Different subspectra are individually used to obtain the corresponding reconstructed amplitude images by the angular spectrum method. With the random distribution of the speckle, averaging these uncorrelated amplitude images would successfully enable speckle-reduced reconstruction. No specific operations for hologram recording or complex process for reconstruction are required in this method. Comparison with the previous rectangular mask method in spatial domain proves the advantage of the RRM-SSD method for both speckle reduction and high-resolution reconstruction. Experimental results show that the RRM-SSD method is efficient and simple for speckle reduction, which can be widely used in holographic imaging.

We propose an optical method for uroflowmetry, exploiting the laser speckle contrast imaging (LSCI) technique onto an intermediate tubing apparatus having an elastic wall that can sensitively respond to flow-induced shedding vortices. Based on the method, we devised and fabricated an elastic-walled U-shaped tubing apparatus (EWUSTA), using the three-dimensional printing technique. We utilized the spatiotemporal contrast scheme for the LSCI as a fast and reliable computational algorithm. We investigated three different materials of flex-vinyl, ninja-flex, and natural rubber latex for the elastic wall of the EWUSTA in steady flow conditions, and verified that their optimal operational ranges could extend up to 7, 15, and 25 ml/s, respectively. We characterized the natural-rubber-latex-based EWUSTA in dynamic flow conditions in comparison with a commercial reservoir-weight-transducer-based gravimetric flowmeter, and verified its feasibility. We stress that the proposed method can offer precise and accurate information on flow dynamics. In addition, we found that the upper limit of the optimal operational range of the proposed apparatus had strong correlation with the tensile strength of the elastic-wall material. We reckon that the proposed and demonstrated method has great potential not only for uroflowmetry but also for other flow-related medical and industrial applications.

We propose a fast algorithm to extract the phase in the center of an interferogram with circular fringes obtained in a simple Michelson interferometer setup with one rough surface. The phase in the center is a linear function of the displacement of an object, which is very advantageous. It avoids the uncertainty in displacement associated with intensity measurements near a maximum or minimum. Also the direction of movement is always clear. The method used also overcomes the problems associated with speckle, unavoidable with a rough surface, common in industrial applications, when using laser as a light source. Using a large number of points in the interferogram to obtain one value largely reduces noise and also increases the accuracy. Displacement is measured with an accuracy

Ghost imaging is an indirect system that allows the imaging of an object without directly seeing the object. The speckle pattern that contains the information about light and objects has increasingly become a popular topic in pseudothermal light ghost imaging. However, existing research still has encountered problems of poor imaging quality and slow sampling speeds. We propose a ghost imaging method based on N-order speckle patterns to recover the object (NSGI). The N-order speckle patterns combine N independent laser speckles individually produced by passing an expanded and collimated He–Ne laser through a digital micromirror device (DMD). The sampling frequency can be improved by controlling the trigger signals of different DMDs. The results of the simulation and experiment have verified that our method can increase sampling speed and reconstruction accuracy. In addition, NSGI can be applied to more applications by designing multiple independent speckles with different properties.

A beam-smoothing method that combines induced spatial incoherence (ISI) and continuous phase plate (CPP) is presented. A method designing special CPPs for ISI is proposed. Then, spatiotemporal character of focal spots with different ISI parameters and different phase plates was numerically analyzed. Using special phase plate consisting of small CPPs designed separately, with 10-nm bandwidth 527-nm light, focal spot nonuniformity at spatial wavelength 10 to 50 μm could be reduced to 2% in 130 ps and was further reduced to 0.5% in 2.3 ns. A two-lens aperture system is added to lower the nonuniformity of the near-field in the light path. Using ISI + CPP, the shape of the focal spot could be controlled perfectly, and the characteristic that the ISI + CPP focal spot changes randomly with time is beneficial to the control of laser and plasma interactions.

The problem of optimizing the distribution of integration time for the degree of linear polarization (DOLP) polarimetry with different types of noise statistics (Gaussian additive, Poisson shot, and gamma speckle) is investigated, in order to decrease the variance of estimator. It is found that if the total integration time of intensity measurements is fixed, the variance of DOLP estimator depends on the distribution of integration time in the presence of Gaussian noise and Poisson shot noise, whereas for the gamma noise, the variance does not depend on the distribution of integration time. We deduce the closed-form solutions of the optimal distribution of the integration time for Gaussian additive and Poisson shot noise within a common theoretical framework. In addition, we investigate the consequence of misjudging the noise statistics, and it is found that the degree of optimization only decays slightly if the noise statistics are misjudged. This characteristic indicates that it is not essential to know the statistics of the noise in advance.

Many works around the digital speckle correlation (DSC) are to improve the computational efficiency and measurement accuracy in recent years. This work aims to improve the efficiency and accuracy of DSC for both single-point and full-field points used in mechanical properties test of materials. For this purpose, first, a subpixel initial guess for the inverse compositional Gauss–Newton algorithm (IC-GN) with the first-order shape function is introduced for single-point image registration. Then, with the aid of strategy of subset image edge extend interpolation (SIEEI), the efficiency and accuracy of full-field displacement and deformation measurement are improved simultaneously. The SIEEI is employed to reduce mean squared difference errors caused by traditional bicubic interpolation algorithm. Comparative studies between the traditional IC-GN algorithm and the proposed algorithms are presented using simulated speckle images and CCD images. The proposed method achieves more executive efficiency and more accuracy for single-point and full-field points image registration. The computational efficiency of the proposed algorithms increases 7.5% for full-field registration using CCD images. The mean of squared difference errors of the SIEEI method is less than the traditional bicubic interpolation algorithm. The presented approach shows great potentials for challenging application, such as mechanical properties test of materials.