Grating Imaging Research Articles

Retinal Image Quality Through an Operating Microscope With Wavefront Shaping Extended Depth of Focus Intraocular Lens in Model Eye

To compare the quality of images of gratings placed in a model eye viewed through an extended depth of focus (EDoF) intraocular lens (IOL) to that of diffractive bifocal IOL or monofocal IOL. Experimental laboratory investigation. Nondiffractive wavefront shaping EDoF (CNAET0, Alcon Laboratories), echelette-designed EDoF (ZXR00V, Johnson & Johnson Vision), diffractive bifocal IOL with low power addition (SV25T, Alcon Laboratories), or monofocal IOL (CNA0T0, Alcon Laboratories) was placed in a fluid-filled model eye. A United States Air Force Resolution Grating Target was glued to the posterior surface of the model eye and viewed through a flat or a wide-angle contact lens. The contrast of the gratings viewed through the EDoF or multifocal IOLs was compared to that through the monofocal IOL. A wavefront analyzer was used to measure the spherical power of the central 4.5 mm optics of the EDoF, multifocal, and monofocal IOLs. The distribution of the dioptric power and the dioptric power map were compared. The gratings observed through the flat contact lens with CNAET0, ZXR00V, or SV25T were slightly blurred when viewed through the multifocal optics. The blurred area was in the circumferential area of CNAET0, the central area of SV25T, and the peripheral area of ZXR00V. The mean contrast was 0.258 ± 0.020 for CNAET0, 0.227 ± 0.025 for ZXR00V, and 0.221 ± 0.020 for SV25T for the 16.0 cyc/mm grating. The contrast was significantly lower for ZXR00V (P = .004) and SV25T (P = .004) than 0.303 ± 0.015 for CNA0T0 but the differences were not significant for CNAET0. For the wide-angle contact lens, the contrast for CNAET0 was 0.182 ± 0.009, for ZXR00V was 0.162 ± 0.011, and for SV25T was 0.163 ± 0.007 for the 16.0 cyc/mm grating, and none was significantly different from 0.188 ± 0.012 for CNA0T0. The dioptric variations of CNAET0 indicated a ring-shaped area of higher power corresponding to the circumferential blurred zone observed through the flat contact lens. The wavefront shaping and echelette-designed EDoF-IOLs reduce the contrast of the grating more than the monofocal IOL when viewed through the flat contact lens. The degree of reduction depended on the design of the extended-focus optics. The difference was less through the wide-angle contact lens.

Theory and verification of moiré fringes for x-ray three-phase grating interferometer

Dual-phase and three-phase grating x-ray interference is a promising new technique for grating-based x-ray differential phase contrast imaging. Dual-phase grating interferometers have been relatively completely studied and discussed. In this paper, the corresponding imaging fringe formula of the three-phase grating interferometer is provided. At the same time, the similarities and differences between the three-phase grating interferometer and the dual-phase grating interferometer are investigated and verified, and that the three-phase grating interferometer can produce large-period moiré fringes without using the analyzing grating is demonstrated experimentally. Finally, a simple method of designing three-phase grating and multi-grating imaging systems from geometric optics based on the thin-lens theory of gratings is presented. These theoretical formulas and experimental results provide optimization tools for designing three-phase grating interferometer systems.

Development of an intelligent multi-scale defect detection system for aspheric lenses with high accuracy and efficiency

Existing methods struggle to fulfill the demand for fast and precise detection of multi-scale faults in aspheric lenses. A high accuracy and efficiency intelligent detection system for aspheric lenses is developed, which utilizes four-step phase-shifted grating optical imaging and Fourier low-pass filtering as image acquisition and preprocessing systems. Accurate and efficient defect feature extraction methods are designed based on regression loss and lightweight feature selection networks, and the model's ability to learn multi-scale defect information is improved. Finally, experimental tests were conducted based on the self-constructed multi-scale defect datasets of aspheric lenses. The results show that the detection system achieves a global accuracy of 96.3 %, an accuracy of 63.8 % for tiny defects detection, as well as a single-image detection time of 24.9 ms, which is a significant improvement in terms of accuracy and efficiency compared with existing methods. Therefore, the intelligent detection system can be widely applied to defect detection in aspheric lenses.

Direct design method for holographic grating imaging spectrometer and corresponding recording system

The combination of holographic gratings with freeform surface technology has led to a significant breakthrough in terms of the performance of imaging spectrometers. In this paper, a general direct design method that is capable of designing imaging spectrometers that include holographic gratings based on a complex-shaped surface substrate and their corresponding recording systems is proposed. The most important feature of this method is that the recording and imaging systems are designed within one optical system model, thus allowing for an optimal balance to be achieved between the aberration correction capability and the manufacturability of the holographic grating. To illustrate the effectiveness of the proposed method, three imaging spectrometers were designed that used holographic gratings manufactured by a recording system without auxiliary mirrors, a system with a spherical auxiliary mirror, and a system with a freeform auxiliary mirror. Ultimately, these three systems all achieved diffraction-limited imaging quality with minimal distortion.

Monitoring shear deformation of sliding zone via fiber Bragg grating and particle image velocimetry

Monitoring shear deformation of sliding zones is of great significance for understanding the landslide evolution mechanism, in which fiber optic strain sensing has shown great potential. However, the correlation between strain measurements of quasi-distributed fiber Bragg grating (FBG) sensing arrays and shear displacements of surrounding soil remains elusive. In this study, a direct shear model test was conducted to simulate the shear deformation of sliding zones, in which the soil internal deformation was captured using FBG strain sensors and the soil surface deformation was measured by particle image velocimetry (PIV). The test results show that there were two main slip surfaces and two secondary ones, developing a spindle-shaped shear band in the soil. The formation of the shear band was successfully captured by FBG sensors. A sinusoidal model was proposed to describe the fiber optic cable deformation behavior. On this basis, the shear displacements and shear band widths were calculated by using strain measurements. This work provides important insight into the deduction of soil shear deformation using soil-embedded FBG strain sensors.

Curvature-based force estimation for an elastic tube

AbstractContact force is one of the most significant feedback for robots to achieve accurate control and safe interaction with environment. For continuum robots, it is possible to estimate the contact force based on the feedback of robot shapes, which can address the difficulty of mounting dedicated force sensors on the continuum robot body with strict dimension constraints. In this paper, we use local curvatures to estimate the magnitude and location of single or multiple contact forces based on Cosserat rod theory. We validate the proposed method in a thin elastic tube and calculate the curvatures via Fiber Bragg Grating (FBG) sensors or image feedback. For the curvature feedback obtained from multicore FBG sensors, the overall force magnitude estimation error is$0.062 \pm 0.068$N and the overall location estimation error is$3.51 \pm 2.60$mm. For the curvature feedback obtained from image, the overall force magnitude estimation error is$0.049 \pm 0.048$N and the overall location estimation error is$2.75 \pm 1.71$mm. The results demonstrate that the curvature-based force estimation method is able to accurately estimate the contact force.

Target-independent dynamic wavefront sensing method based on distorted grating and deep learning

A real-time wavefront sensing method for arbitrary targets is proposed, which provides an effective way for diversified wavefront sensing application scenarios. By using a distorted grating, the positive and negative defocus images are simultaneously acquired on a single detector. A fine feature, which is independent of the target itself but corresponding to the wavefront aberration, is defined. A lightweight and efficient network combined with an attention mechanism (AM-EffNet) is proposed to establish an accurate mapping between the features and the incident wavefronts. Comparison results show that the proposed method has superior performance compared to other methods and can achieve high-accuracy wavefront sensing in varied target scenes only by using the point target dataset to train the network well.

On the influence of underlying elevation data on Sentinel-5 Precursor TROPOMI satellite methane retrievals over Greenland

Abstract. The Sentinel-5 Precursor (S5P) mission was launched on October 2017 and has since provided data with high spatio-temporal resolution using its remote sensing instrument, the TROPOspheric Monitoring Instrument (TROPOMI). The latter is a nadir viewing passive grating imaging spectrometer. The mathematical inversion of the TROPOMI data yields retrievals of different trace gas and aerosol data products. The column-averaged dry-air mole fraction of methane (XCH4) is the product of interest to this study. The daily global coverage of the atmospheric methane mole fraction data enables the analysis of the methane distribution and variation on large scales and also to estimate surface emissions. The spatio-temporal high-resolution satellite data are potentially particularly valuable in remote regions, such as the Arctic, where few ground stations and in situ measurements are available. In addition to the operational Copernicus S5P total-column-averaged dry-air mole fraction methane data product developed by SRON, the scientific TROPOMI/WFMD algorithm data product v1.5 (WFMD product) was generated at the Institute of Environmental Physics at the University of Bremen. In this study we focus on the assessment of both S5P XCH4 data products over Greenland and find that spatial maps of both products show distinct features along the coastlines. Anomalies up to and exceeding 100 ppb are observed and stand out in comparison to the otherwise smooth changes in the methane distribution. These features are more pronounced for the operational product compared to the WFMD product. The spatial patterns correlate with the difference between the GMTED2010 digital elevation model (DEM) used in the retrievals and a more recent topography dataset, indicating that inaccuracies in the assumed surface elevation are the origin of the observed features. These correlations are stronger for the WFMD product. In order to evaluate the impact of the topography dataset on the retrieval we reprocess the WFMD product with updated elevation data. We find that a significant reduction of the localized features when GMTED2010 is replaced by recent topography data over Greenland based on ICESat-2 data. This study shows the importance of the chosen topography data for retrieved dry-air mole fractions. The use of a DEM that is as accurate and as up to date as possible is advised for all S5P data products as well as for future missions which rely on a DEM as input data. A modification based on this study is planned to be introduced in the next version of the WFMD data product.

Microsphere-assisted Fabry-Perot interferometry: proof of concept.

We propose a microsphere-assisted Fabry-Perot interferometry (MAFPI) for microstructure measurement. We stretch the single-mode fiber and combine it with microspheres of different sizes and refractive indices, which can form super-focused spots with different characteristics, that is, a photonic nanojet phenomenon. As a proof of principle, we performed scanning imaging of optical discs and holographic gratings by MAFPI. The optical disc image obtained by MAFPI is consistent with the result obtained by a scanning electron microscope, and the obtained grating image is consistent with the actual result.

Online Deformation Measurement of Laser Repair Substrate Based on Orthogonal Sampling Moiré

As an important branch of metal additive manufacturing, laser repair has broad application potential in the aerospace, automobile and ship building industries. In the process of laser repair, metal powder is introduced into the laser action area and heated to a molten state to form a molten pool, which moves on the component surface according to the preset scanning path. Additionally, the temperature of the repaired component changes dynamically with the movement of the molten pool, leading to the time-evolution of stress and deformation. Therefore, online deformation measurement for the repair process is essential for understanding the evolution mechanism of stress and deformation in laser repair. However, extreme environments such as ultra-high temperature, strong laser radiation and metal powder splashing exist during the laser repair, which bring great challenges for dynamic optical measurement. In this paper, an online deformation measurement system based on orthogonal sampling moiré for laser repair environment is developed, which is applied to measure the deformation field of the metal substrate during laser repair. In the measurement, laser repair is performed on a cantilever beam substrate and orthogonal grating is prepared on the side surface of the cantilever beam. The real-time grating images are recorded by an optical imaging system, in which a group of filter components are used to obtain optical images with high signal-to-noise ratio. Finally, the deformation field of the substrate during laser repair is calculated with the orthogonal sampling moiré method. The results show that the scanning path in the repair process has a significant influence on the distribution of residual deformation, which offers reference for optimizing parameters of laser repair.

Effect of nonuniform pit structure on self-imaging of periodical gratings.

Self-imaging possibilities for periodical gratings that have nonuniform pit structure are theoretically investigated. The diffraction of periodic arrays in the deep Fresnel region is analyzed according to the scalar diffraction theory. The expressions of the diffraction intensities of three different gratings that have binary square, binary circle, and Gaussian pit structure are considered. Talbot images of gratings with nonuniform pit structure are predicted to appear at multiple certain distances. The present paper shows that even a structure in short-range disorder may take on the self-imaging effect in a Fresnel field.

Neurons in the pigeon visual network discriminate between faces, scrambled faces, and sine grating images

Discriminating between object categories (e.g., conspecifics, food, potential predators) is a critical function of the primate and bird visual systems. We examined whether a similar hierarchical organization in the ventral stream that operates for processing faces in monkeys also exists in the avian visual system. We performed electrophysiological recordings from the pigeon Wulst of the thalamofugal pathway, in addition to the entopallium (ENTO) and mesopallium ventrolaterale (MVL) of the tectofugal pathway, while pigeons viewed images of faces, scrambled controls, and sine gratings. A greater proportion of MVL neurons fired to the stimuli, and linear discriminant analysis revealed that the population response of MVL neurons distinguished between the stimuli with greater capacity than ENTO and Wulst neurons. While MVL neurons displayed the greatest response selectivity, in contrast to the primate system no neurons were strongly face-selective and some responded best to the scrambled images. These findings suggest that MVL is primarily involved in processing the local features of images, much like the early visual cortex.

Computational Three-Dimensional Imaging System via Diffraction Grating Imaging with Multiple Wavelengths.

This paper describes a computational 3-D imaging system based on diffraction grating imaging with laser sources of multiple wavelengths. It was proven that a diffraction grating imaging system works well as a 3-D imaging system in our previous studies. The diffraction grating imaging system has advantages such as no spherical aberration and a low-cost system, compared with the well-known 3-D imaging systems based on a lens array or a camera array. However, a diffraction grating imaging system still suffers from noises, artifacts, and blurring due to the diffraction nature and illumination of single wavelength lasers. In this paper, we propose a diffraction grating imaging system with multiple wavelengths to overcome these problems. The proposed imaging system can produce multiple volumes through multiple laser illuminators with different wavelengths. Integration of these volumes can reduce noises, artifacts, and blurring in grating imaging since the original signals of 3-D objects inside these volumes are integrated by our computational reconstruction method. To apply the multiple wavelength system to a diffraction grating imaging system efficiently, we analyze the effects on the system parameters such as spatial periods and parallax angles for different wavelengths. A computational 3-D imaging system based on the analysis is proposed to enhance the image quality in diffraction grating imaging. Optical experiments with three-wavelength lasers are conducted to evaluate the proposed system. The results indicate that our diffraction grating imaging system is superior to the existing method.

Robust Catheter Tracking by Fusing Electromagnetic Tracking, Fiber Bragg Grating and Sparse Fluoroscopic Images

Endovascular catheterization is an intervention which offers a low risk alternative to open surgery in many patients. Today’s interventions rely heavily on fluoroscopic imaging to guide interventionalists. Fluoroscopy only produces 2D visualization of the catheter and also exposes both the patient and interventionalists to harmful radiation. Different approaches have been proposed to overcome the limitations of fluoroscopy. Fiber Bragg Grating (FBG)-based shape sensing is becoming popular to reconstruct the catheter shape. Multi-core fibers with parallel optical cores are interesting as they allow 3D shape reconstruction with a single fiber. A common issue with FBG-based shape sensing is its sensitivity to variations in twist. Even small amounts of twist can significantly impact the overall shape reconstruction accuracy. This work proposes a novel approach which combines electromagnetic tracking (EMT), FBG-based shape sensing, and sparse fluoroscopic images. The method provides real-time 3D visualization of the catheter without the need of continuous fluoroscopy. A unique feature of the proposed method is the selective use of imaging for dynamic twist-compensation of the FBG sensor. The proposed sensor-fusion method improved 3D reconstruction accuracy. Real-world in-vitro experiments promising results. For a catheter with an embedded fiber length of 170 mm, the proposed approach the 3D shape with a median root-mean-square (rms) error of 0.39 mm and an interquartile range of 0.10 mm in the 2D experiment in which the catheter was bent in a plane. A median rms error of 0.54 mm and an interquartile range of 0.07 mm were achieved in the 3D experiments.

Performance evaluation of buried pipe under loading using fiber Bragg grating and particle image velocimetry techniques

Accurate and real-time estimation of underground pipelines’ response to external loads is essential, in which the fiber Bragg grating (FBG) technique has shown great potential. A series of 1-g plane strain model tests were conducted to investigate the feasibility of using FBG sensors for monitoring the behavior of the soil-pipe system under surcharge loadings, while particle image velocimetry (PIV) was employed to measure the soil shear strain fields. The test results show three types of soil failure surfaces, i.e., logarithmic spiral, linear, and logarithmic. Regarding various failure modes, two theoretical methods to estimate the ultimate bearing capacity of the soil-pipe system are introduced. Detailed discussions are presented regarding the evolution of pipe strain measurement under different ground settlements. Finally, a method is proposed to calculate the radial displacements based on strain measurements for a shallowly buried pipe and a relative error below ±10% is obtained in this study.

Quality of view through extended depth of focus intraocular lens in a model eye.

To compare the quality of images viewed through an extended depth of focus (EDF) intraocular lens (IOL) to that through multifocal or monofocal IOL. Experimental study METHODS: EDF IOL (Symfony®, ZXR00V, Johnson & Johnson Vision), diffractive multifocal (ZLB00, + 3.25D or ZMB00, + 4.0D), or monofocal (ZCB00V) IOL with a spherical power of + 20.0 diopter (D) was placed in a fluid-filled model eye with corneal aberrations similar to those of human eyes. A United States Air Force Resolution Grating Target was glued to the posterior surface of the model eye and viewed through a flat contact lens, a 60D or 128D wide-angle non-contact lens (Resight®) or wide-angle contact lens (MiniQuad®). The contrast of the grating images recorded with the EDF and multifocal IOLs were compared to those through the monofocal IOL. The grating images viewed through the flat contact lens were slightly blurred when viewed through the EDF IOL but clearer than those through the multifocal IOLs with very blurred images in the periphery. The contrast of the images viewed through the EDF and multifocal IOLs through the flat contact lens was significantly lower than through the monofocal IOL (P < 0.02). The contrast of the images viewed through the EDF IOL with 60D or 128D wide-angle non-contact lens was significantly lower than through the monofocal IOL (P < 0.05) but not with wide-angle contact lens. Our results suggest that vitreous surgeons can accomplish a clearer view during vitrectomy in EDF IOL-implanted eyes with a wide-angle viewing contact lens and a flat contact lens than in multifocal IOL-implanted eyes.

Analysis of Phase Grating Imaging Based on Principal Diffraction Orders Algorithm

Analysis of Phase Grating Imaging Based on Principal Diffraction Orders Algorithm

3D-design-model-assisted shiny surface shape measurement in fringe projection profilometry

In fringe projection three-dimensional (3D) shape measurement, image saturation is an important influencing factor for shiny surfaces, for example, an aero-engine blade. This paper proposes a novel method assisted by the 3D design model to resolve image saturation. The proposed algorithm has the following characteristics: (a) Based on the high matching between the measured finished object and its 3D design model, the design model is transformed into the discrete point cloud with sufficient accuracy. (b) Based on the spatial coordinate transformation principle, a multi-coordinate system transformation method is proposed to convert discrete point cloud data from the design coordinate system to the world coordinate system. (c) Considering the influence of the actual measurement conditions on the measured object, the existing illumination model is improved by integrating the system’s greyscale nonlinear response, camera exposure time and sensitivity. (d) Combined with the captured grating images and the 3D design model, the parameters are accurately calibrated by the least-squares algorithm. (e) Using the calibrated reflection model, the optimal rotation angle is determined to compensate for the spatial points corresponding to the image saturation area, and finally, the entire measured surface is registered with point clouds in the calculated optimal rotation locations. Our method is used in the actual pressure surface measurement experiment. The experimental results show that our method can effectively measure the space points corresponding to the image saturation area in the optimal rotation angle and improve the measurement accuracy of shiny surfaces.

Snapshot hyperspectral imaging using wide dilation networks

Hyperspectral (HS) cameras record the spectrum at multiple wavelengths for each pixel in an image, and are used, e.g., for quality control and agricultural remote sensing. We introduce a fast, cost-efficient and mobile method of taking HS images using a regular digital camera equipped with a passive diffraction grating filter, using machine learning for constructing the HS image. The grating distorts the image by effectively mapping the spectral information into spatial dislocations, which we convert into a HS image by a convolutional neural network utilizing novel wide dilation convolutions that accurately model optical properties of diffraction. We demonstrate high-quality HS reconstruction using a model trained on only 271 pairs of diffraction grating and ground truth HS images.

Some features of the self-imaging effect in holography

This article depicts a general case of recording a grating hologram, with the distance between the object and the hologram being arbitrarily chosen. The universality of this approach does not exclude the possibility of restoring the imaginary and real images of the grating and their self-image sequences. In the case under consideration, restoring the hologram of the grating results in the reconstruction of both imaginary and real grating images and the sequences of their self-images in ± 1st diffraction orders. An important feature of reconstructing a grating hologram is that the self-image sequences for both imaginary and real images extend to the real image area. The ratios derived in the article also show that the restoration of the grating self-image sequences occurs in the 0-th diffraction order.