Multi-angle Illumination Research Articles

3D measurements of submillimeter-scale micro-holes with light-field image fusion under multi-angle illumination

3D measurements of submillimeter-scale micro-holes with light-field image fusion under multi-angle illumination

Single-exposure multi-wavelength optical diffraction tomography based on space-angle dual multiplexing holography.

We present a space-angle dual multiplexing holographic recording system for realizing single-exposure multi-wavelength optical diffraction tomographic (ODT) imaging. This system is achieved by combining the principle of single-exposure multi-wavelength holographic imaging technique based on angle-division multiplexing with the principle of single-exposure ODT imaging technique based on microlens array multi-angle illuminations and space-division multiplexing. Compared with the existing multi-wavelength ODT imaging methods, it enables the holographic recording of all the diffraction tomography information of a measured specimen at multiple illumination wavelengths in a single camera exposure without any scan mechanism. Using our proposed data processing method, the multi-wavelength three-dimensional (3D) refractive index tomograms of a specimen can be eventually reconstructed from single recorded multiplexing hologram. Experimental results of a static polystyrene bead and a living C. elegans worm demonstrate the feasibility of this system.

Fast Fourier ptychographic quantitative phase microscopy for in vitro label-free imaging.

Quantitative phase microscopy (QPM) is indispensable in biomedical research due to its advantages in unlabeled transparent sample thickness quantification and obtaining refractive index information. Fourier ptychographic microscopy (FPM) is among the most promising QPM methods, incorporating multi-angle illumination and iterative phase recovery for high-resolution quantitative phase imaging (QPI) of large cell populations over a wide field of-view (FOV) in a single pass. However, FPM is limited by data redundancy and sequential acquisition strategies, resulting in low imaging efficiency, which in turn limits its real-time application in in vitro label-free imaging. Here, we report a fast QPM based on Fourier ptychography (FQP-FPM), which uses an optimized annular downsampling and parallel acquisition strategy to minimize the amount of data required in the front end and reduce the iteration time of the back-end algorithm (3.3% and 4.4% of conventional FPM, respectively). Theoretical and data redundancy analyses show that FQP-FPM can realize high-throughput quantitative phase reconstruction at thrice the resolution of the coherent diffraction limit by acquiring only ten raw images, providing a precondition for in vitro label-free real-time imaging. The FQP-FPM application was validated for various in vitro label-free live-cell imaging. Cell morphology and subcellular phenomena in different periods were observed with a synthetic aperture of 0.75 NA at a 10× FOV, demonstrating its advantages and application potential for fast high-throughput QPI.

Fast and robust Fourier ptychographic microscopy with position misalignment correction.

Fourier ptychographic microscopy (FPM) is a new, developing computational imaging technology. It can realize the quantitative phase imaging of a wide field of view and high-resolution (HR) simultaneously by means of multi-angle illumination via a light emitting diode (LED) array, combined with a phase recovery algorithm and the synthetic aperture principle. However, in the FPM reconstruction process, LED position misalignment affects the quality of the reconstructed image, and the reconstruction efficiency of the existing LED position correction algorithms needs to be improved. This study aims to improve the FPM correction method based on simulated annealing (SA) and proposes a position misalignment correction method (AA-C algorithm) using an improved phase recovery strategy. The spectrum function update strategy was optimized by adding an adaptive control factor, and the reconstruction efficiency of the algorithm was improved. The experimental results show that the proposed method is effective and robust for position misalignment correction of LED arrays in FPM, and the convergence speed can be improved by 21.2% and 54.9% compared with SC-FPM and PC-FPM, respectively. These results can reduce the requirement of the FPM system for LED array accuracy and improve robustness.

Large field‐of‐view holographic imager with ultra‐high phase sensitivity using multi‐angle illumination

Quantitative phase imaging and holography allow highly sensitive detection of phase changes, for example of surface non-uniformities or refractive index (RI) structures in the volume of a sample. For many applications, wide field-of-view (FoV) and high phase sensitivity are required. Lens-free interferometric microscopy can operate over a wide FoV and volume, but with the drawbacks of low axial resolution and poor optical sectioning, which comes with increased noise from out-of-focus planes. In this work, we propose and demonstrate a novel implementation of lens-free phase imaging with phase-shifting interferometry and multi-angle illumination that enhances axial resolution and image quality, enabling ultra-high phase sensitivity with optical path difference (OPD) background root-mean-square (RMS) noise of 0.2 nm, while operating over a wide FoV (>10 mm²). As a prototypical application, we demonstrate imaging of 10 nm thin transparent glass structures, and of 3D laser-written structures of RI modifications in glass over a large volume (>10 mm³), where the location of features along the z-axis can be determined with 3x higher accuracy compared to an on-axis illumination. The technology is particularly suitable for large-scale analysis and characterization of structures on the surface or embedded in transparent materials.

Enhanced Detection of Surface Plasmon Resonance Microscope via Multiangle Illumination and Pretrained Deep Neural Network

Surface plasmon resonance microscopy (SPRM) has proved to be a powerful tool for label‐free imaging, detection, and mass/size measurement of single isolated virus, cells, and nanoparticles. Herein, SPRM that can be extended to detect the dense nanoparticles, such as counting the number density of nanoparticles, is demonstrated. To improve the counting accuracy, multiangle illumination is adopted in a conventional SPRM, then a deep learning method based on pretrained neural network approach is proposed to deal with the large number of SPRM images. Numerical simulations show that the new approach can significantly enhance the accuracy in detecting the number densities of nanoparticles. The ability to precisely count the number of particles is important for the monitor and detection of environmental pollution because that the number density of the particulate matter is strongly related to the air quality and atmospheric visibility.

Multi-illumination 3D holographic display using a binary mask

We introduce a novel, to the best of our knowledge, method to increase the bandwidth in holographic displays. Here, multi-angle illumination using multiple laser diodes (LDs) is adopted to expand the limited diffraction angle of the spatial light modulator (SLM). To solve the problem of signal repetitions caused by sharing the same SLM pattern, we use a random binary mask (BM). We demonstrate via simulations and experiments that our method effectively increases the bandwidth with sufficient image quality. Furthermore, the speckle noise, a critical issue of the holographic display that decreases the contrast and is potentially harmful to eyes, is reduced by the advantage of incoherent summation in the reconstruction plane. We believe that this method is a practical approach that can expand the bandwidth of the holographic display by alleviating the bottleneck of hardware limitations.

Randomized multi-angle illumination for improved linear array photoacoustic computed tomography in brain.

One of the key challenges in linear array transducer-based photoacoustic computed tomography is to image structures embedded deep within the biological tissue with limited optical energy. Here, we utilized a manually controlled multi-angle illumination technique to allow the incident photons to interact with the imaging targets for longer periods of time and diffuse further in all directions. We have developed and optimized a compact probe that enables manual changes to the angle of illumination while acquiring photoacoustic signals. The performance has been demonstrated and evaluated by imaging complex blood vessel mimicking phantoms in-vitro and sheep brain samples ex-vivo. For effective image reconstruction from the data acquired by multi-angle illumination method, we have utilized a method based on the extraction of maximum intensity. In both cases, multi-angle illumination has out-performed the conventional fixed angle illumination technique to improve the overall image quality. Specifically, extraction of the imaging targets located at greater axial depths was possible using this multi-angle illumination technique.

Robust multi-angle structured illumination lensless microscopy via illumination angle calibration.

Multi-angle structured illumination lensless (MASIL) microscopy enables high-resolution image recovery over a large field of view. Successful image recovery of MASIL microscopy, however, relies on an accurate knowledge of the multi-angle illumination. System misalignments and slight deviations from the true illumination angle may result in image artifacts in reconstruction. Here we report a MASIL microscopy system that is robust against illumination misalignment. To calibrate the illumination angles, we design and use a double-sided mask, which is a glass wafer fabricated with a ring-array pattern on the upper surface and a disk-array pattern on the lower surface. As such, the illumination angles can be decoded from the captured images by estimating the relative displacement of the two patterns. We experimentally demonstrate that this system can achieve successful image recovery without any prior knowledge of the illumination angles. The reported approach provides a simple yet robust resolution for wide-field lensless microscopy. It can solve the LED array misalignment problem and calibrate angle-varied illumination for a variety of applications.

Ultra-high speed holographic shape and displacement measurements in the hearing sciences

The auditory system of mammals enables the perception of sound from our surrounding world. Containing some of the smallest bones in the body, the ear transduces complex acoustic signals with high-temporal sensitivity to complex mechanical vibrations with magnitudes as small as tens of picometers. Measurements of the shape and acoustically induced motions of different components of the ear are essential if we are to expand our understanding of hearing mechanisms, and also provide quantitative information for the development of numerical ear models that can be used to improve hearing protection, clinical diagnosis, and repair of damaged or diseased ears.We are developing digital holographic methods and instrumentation using an ultra-high speed camera to measure shape and acoustically-induced motions in the middle ear. Specifically we study the eardrum, the first structure of the middle ear which initializes the acoustic-mechanical transduction of sound for hearing. Our measurement system is capable of performing holographic measurement at rates up to 2.1 M frames per second. Two shape measurement modalities had previously been implemented into our holographic systems: (1) a multi-wavelength method with a wavelength tunable laser; and (2) a multi-angle illumination method with a single wavelength laser. In this paper, we present a third method using a miniaturized fringe projection system with a microelectromechanical system (MEMS) mirror. Further, we optimize the processing of large data sets of holographic displacement measurements using a vectorized Pearson's correlation algorithm. We validate and compare the shape and displacement measurements of our methodologies using a National Institute of Standards and Technology (NIST) traceable gauge and sound-activated latex membranes and human eardrums.

Single-Shot Through-Focus Image Acquisition and Phase Retrieval From Chromatic Aberration and Multi-Angle Illumination

Phase recovery from a stack of through-focus intensity images is an effective non-interference quantitative phase imaging strategy. Nevertheless, the implementations of these methods are expensive and time-consuming because the distance between each through-focus plane has to be guaranteed by precision mechanical moving devices, and the multiple images must be acquired sequentially. In this article, we propose a single-shot through-focus intensity image stack acquisition strategy without any precision movement. Isolated LED units are used to illuminate the sample in different colors from different angles. Due to the chromatic aberration characteristics of the objective, the color-channel defocus images on the theoretical imaging plane are mutually laterally shifted. By calculating the shift amount of each sub-image area in each color channel, the distances between each through-focus image can be obtained, which is a critical parameter in transport of intensity equation (TIE) and alternating projection (AP). Lastly, AP is used to recover the phase distribution and realize the 3D localization of different defocus distances of the sample under test as an example. Both simulation and experiments are conducted to verify the feasibility of the proposed method.

Surface defect detection of machined parts based on machining texture direction

Compared with detecting the regular texture of fabrics and prints, the detection of the processed texture on the surface of mechanical parts is more difficult. To quickly and accurately detect defects caused by abnormal machining of the surface of metal parts, a one-shot machine-vision method based on a texture orientation histogram is proposed. An improved Mean-C local threshold method is proposed to solve the problem of difficulty in extracting surface texture. Using the minimum enclosing rectangle, the skeleton texture is extracted from the enhanced image obtained by the improved Mean-C local threshold. The statistical information from the histogram is used to pre-process the texture direction, and then a novel angle region growth method proposed in this paper is used to search the main texture cluster and the abnormal texture cluster of the part, so as to realize the product quality detection. Experimental results show that this method is highly targeted for the detection of surface texture defects caused by abnormal processing, which is equivalent to the average performance of a multi-angle illumination detection system, but much faster. This detection method has high detection efficiency, high accuracy, and strong robustness, and can meet the requirements of industrial detection.

Lensless on-chip LED array microscope using amplitude and phase masks

A cost-effective, compact, and portable optical system using multi-angle illumination for retrieval of the amplitude and phase of a sample is implemented in this paper. The system consists of a matrix of LED followed by a mask, a USAF resolution chart, and a detector in a column. The sample information is collected by the detector directly without any objective lens. The sample information is encoded by the mask at a sensor plane during the reconstruction process. To retrieve the sample information from captured raw images corresponding to each LED illumination, the backward propagation model is applied. Three types of masks are used: a phase mask having pixel values from 0 to 2 π , an amplitude mask, and a combination of a phase and an amplitude mask with random distribution. The applications of such type of technique are most common in biomedical engineering for infection diagnostic purposes. In the future, the commercial demand of such lensless microscope would be high due to its simplicity and fast image processing capacity.

Phase errors reduction in multi-angle illumination digital holographic microscopy

Synthetic aperture has been a common method in digital holographic microscopy resolution enhancement over the years. Multiangle illumination is one type of synthetic aperture methods. Reconstructed phase errors appeared when synthesizing aperture not considering the difference between different inclined incident beams' reconstruction distances. To reduce phase errors, this paper proposes a method using different reconstruction distances in inclined incident beams to achieve synthetic aperture resolution enhancement. A multiangle illumination reflectance digital holographic microscopy system is built to prove this method. The reconstruction distances can be calculated theoretically to compute the phase image. The phase image errors can be remarkably reduced by using the proposed method by analyzing the experimental result.

Fast compressive lens-free tomography for 3D biological cell culture imaging.

We present a compressive lens-free technique that performs tomographic imaging across a cubic millimeter-scale volume from highly sparse data. Compared with existing lens-free 3D microscopy systems, our method requires an order of magnitude fewer multi-angle illuminations for tomographic reconstruction, leading to a compact, cost-effective and scanning-free setup with a reduced data acquisition time to enable high-throughput 3D imaging of dynamic biological processes. We apply a fast proximal gradient algorithm with composite regularization to address the ill-posed tomographic inverse problem. Using simulated data, we show that the proposed method can achieve a reconstruction speed ∼10× faster than the state-of-the-art inverse problem approach in 3D lens-free microscopy. We experimentally validate the effectiveness of our method by imaging a resolution test chart and polystyrene beads, demonstrating its capability to resolve micron-size features in both lateral and axial directions. Furthermore, tomographic reconstruction results of neuronspheres and intestinal organoids reveal the potential of this 3D imaging technique for high-resolution and high-throughput biological applications.

Surface defect detection of aluminum alloy welds with 3D depth image and 2D gray image

Towards the trend and demand of automatic welding inspection in industry, a composite vision system enabling simultaneous 3D-depth and 2D-gray imaging of the bead surface is constructed to detect typical surface defects of aluminum alloy weld beads. In this vision system, the structured laser light is responsible for obtaining 3D-depth image of the bead surface; meanwhile, the multi-angle illuminations are used to capture gray images. Then, four methods are proposed to extract the weld bead boundaries according to its different characters shown in the 3D depth images and 2D gray images. In the 3D depth image, the extraction algorithms of defects such as collapse, undercut, burning-through, excessive reinforcement, surface porosity, spatter, and poor forming are studied. In multi-angle gray images, the extraction algorithm of defects such as cracks and surface blackening is also proposed and studied.

Surface Defects Detection of Stamping and Grinding Flat Parts Based on Machine Vision.

Currently, surface defect detection of stamping grinding flat parts is mainly undertaken through observation by the naked eye. In order to improve the automatic degree of surface defects detection in stamping grinding flat parts, a real-time detection system based on machine vision is designed. Under plane illumination mode, the whole region of the parts is clear and the outline is obvious, but the tiny defects are difficult to find; Under multi-angle illumination mode, the tiny defects of the parts can be highlighted. In view of the above situation, a lighting method combining plane illumination mode with multi-angle illumination mode is designed, and five kinds of defects are automatically detected by different detection methods. Firstly, the parts are located and segmented according to the plane light source image, and the defects are detected according to the gray anomaly. Secondly, according to the surface of the parts reflective characteristics, the influence of the reflection on the image is minimized by adjusting the exposure time of the camera, and the position and direction of the edge line of the gray anomaly region of the multi-angle light source image are used to determine whether the anomaly region is a defect. The experimental results demonstrate that the system has a high detection success rate, which can meet the real-time detection rEquation uirements of a factory.

Wirtinger gradient descent optimization for reducing Gaussian noise in lensless microscopy

Lensless microscopy is an imaging technique that records diffraction measurements without using any lens and recovers the complex object profile via phase retrieval. The successful reconstruction of lensless imaging experiments often requires the captured images to have a high signal-to-noise ratio (SNR), but high SNR is hard to guarantee in practice. Here we report a novel iterative algorithm for reducing Gaussian noise in lensless microscopy. Our method incorporates the Gaussian noise model in the Wirtinger gradient descent optimization process to perform phase retrieval. The reported algorithm is tested on both the multi-angle illumination and multi-height detection schemes of lensless microscopy for diversity measurements. As demonstrated by simulations and experiments, the method is able to obtain better reconstructions under low SNR conditions, in comparison with the traditional Gerchberg Saxton method. We have provided open-source implementation code for non-commercial use.

An automatic system for bearing surface tiny defect detection based on multi-angle illuminations

Bearings are very common mechanical components that are widely used in all kinds of machines. Currently, bearing surface defect detection is mainly completed by naked-eye observation. This manual detection mode has low reliability and is time consuming (especially for miniature bearings). In this paper, we present an automatic synthetic tiny defect (small defects that are difficult to find at vertical angles) detection system for bearing surfaces with self-developed software and hardware. Under two illumination modes in this system, thresholding segmentation, contour extraction, contour filtering, center location, region zoning and text recognition are successively implemented. Finally, four common defects (gap, stain, shrunken lid and scratch defects) are automatically detected. The experimental results show that this system has a high detection success rate and short time consumption. This method can provide technical support for real engineering applications involving bearing manufacturing and screening.

Method for Measuring the Main Parameters of Digital Security Holograms for Expert Analysis and Real-Time Control of Their Quality

A method and a device for measuring the main parameters of synthesized security holograms using the capabilities of micron resolution microscopy and diffractometry with a multi-angle illumination system are developed. The information surface map which contains data about the main parameters of holopixels, which make up the structure of a security hologram (diffraction grating period, their angular orientation, and normalized intensity of reflected light), and the image of the hologram surface in diffuse light combined with this map are generated under sequential illumination of the hologram surface with collimated monochromatic light sources at different angles in the field of view of a special digital microscope and upon processing of the obtained images. The information map of the entire hologram surface is constructed with micron resolution by moving the investigated hologram in the focal plane of a digital microscope at a distance equal to the field of view of the microscope with the subsequent matching of the obtained data.