Depth Of Field Research Articles

Extended depth of field in augmented reality

The 3D display device shows an image with depth information. Conventional 3D display devices based on binocular parallax can focus accurately only on the depth of a specific screen. Because the human eye has a narrow depth of field (DOF) under normal circumstances, 3D displays that provide a relatively wide range of virtual depth areas have limitations on the DOF where clear 3D images are seen. To resolve this problem, it is necessary to find the optical conditions to extend the DOF and analyze the phenomena related to it. For this, by using the Rayleigh criterion and the Strehl ratio, a criterion for this extension of the DOF is suggested. A practical optical structure that can effectively extend the DOF is devised using a flat panel display. This optical structure could be applied to AR, VR, and MR in the field of near-eye displays. From the results of this research, the fundamental optical conditions and standards are proposed for 3D displays that will provide 3D images with extended DOF in the future. Furthermore, it is also expected that these conditions and criteria can be applied to optical designs for the required performance in the development of 3D displays in various fields.

Depth-Enhanced Holographic Super Multi-View Maxwellian Display Based on Variable Filter Aperture.

The super multi-view (SMV) near-eye display (NED) effectively provides depth cues for three-dimensional (3D) displays by projecting multiple viewpoint images or parallax images onto the retina simultaneously. Previous SMV NED suffers from a limited depth of field (DOF) due to the fixed image plane. Aperture filtering is widely used to enhance the DOF; however, an invariably sized aperture may have opposite effects on objects with different reconstruction depths. In this paper, a holographic SMV display based on the variable filter aperture is proposed to enhance the DOF. In parallax image acquisition, multiple groups of parallax images, each group recording a part of the 3D scene on a fixed depth range, are captured first. In the hologram calculation, each group of wavefronts at the image recording plane (IRP) is calculated by multiplying the parallax images with the corresponding spherical wave phase. Then, they are propagated to the pupil plane and multiplied by the corresponding aperture filter function. The size of the filter aperture is variable which is determined by the depth of the object. Finally, the complex amplitudes at the pupil plane are back-propagated to the holographic plane and added together to form the DOF-enhanced hologram. Simulation and experimental results verify the proposed method could improve the DOF of holographic SMV display, which will contribute to the application of 3D NED.

A visual guidance calibration method for out-of-focus cameras based on iterative phase target

Calibration of out-of-focus cameras is crucial for some special 3d reconstruction applications. An essential factor in improving the calibration result is avoiding the solution's singularity which tends to pose the calibration plate at a large inclination. However, the depth of field (DOF) of cameras is limited, especially in the defocused scenes, and the blurred corners will significantly reduce the calibration accuracy. A vision-guided calibration method is proposed, including an optimal pose selection algorithm and an iterative phase-target technology. Compared with other phase-target methods, this method simultaneously optimizes the mathematical properties of the solution and the angular uncertainty of the corners. Compared with the Chessboard-target or Circle-target, this method has a higher calibration accuracy at three different distances that gradually change from focusing to defocusing. In addition, experiments also prove the effectiveness of the two submodules: Gaussian iteration and vision guidance. Finally, its code is available in the URL: https://github.com/SHU-FLYMAN/Visual-Guidence-Calibration.

Multi-image-distance imaging system for extending depth-of-field

Depth of field (DoF) defines the longitudinal range of objects in sharp focus. Imaging systems for machine vision are generally limited to a small DoF by various conditions, constraining the visual inspection of objects with large depth. In this paper, we develop a multi-image-distance imaging system, which can extend the DoF by capturing multi-focus images for fusion. A three-way beam-splitter prism is designed to support independent imaging of three image sensors. Through adjusting the sensors to different back focal lengths, multiple images focusing at different depths are captured in a single exposure. Finally, the focused regions of each image are fused for extending the DoF. Analyses of the proposed DoF extension strategy illustrate excellent enhancement more than three times. Moreover, we introduce a DoF measurement method based on the estimation of the point spread function, and measurement experiments are conducted using the fabricated prototype. Imaging experiments for various scenes demonstrate great imaging quality and DoF extension performance. The proposed multi-image-distance imaging system provides an effective and practical DoF extension solution for close-range visual inspection.

Enhanced depth of field of integral imaging display using bifocal microlens array fabricated by two‐step lithography

AbstractDue to the limitation of traditional microlens arrays (MLAs) in integral imaging display, the depth of field (DOF) is restricted in space and the center depth plane is difficult to extend in a large range. Here, we propose a microfabrication method based on bifocal MLAs to improve DOF. The bifocal MLAs for extended DOF were fabricated by using two‐step photolithography and thermal reflow. This method allows diverse microlenses of high to low numerical aperture to achieve high spatial resolution as well as accurate depth estimation. Microlenses of different focal lengths were simultaneously deposited on a substrate by repeated photolithography with multiple photomasks with alignment mark to define micro‐posts of different thicknesses. Hexagonally packaged bifocal MLAs clearly show the DOF extended from 0.004 to 4.908 mm for 57.6 μm in lens diameter, and their corresponding object distance ranges from 0.125 to 0.165 mm. Based on the proposed scheme, this method provides potential applications in integral imaging 3D display or light field display.

Pupil engineering for extended depth-of-field imaging in a fluorescence miniscope.

Fluorescence head-mounted microscopes, i.e., miniscopes, have emerged as powerful tools to analyze in-vivo neural populations but exhibit a limited depth-of-field (DoF) due to the use of high numerical aperture (NA) gradient refractive index (GRIN) objective lenses. We present extended depth-of-field (EDoF) miniscope, which integrates an optimized thin and lightweight binary diffractive optical element (DOE) onto the GRIN lens of a miniscope to extend the DoF by between twin foci in fixed scattering samples. We use a genetic algorithm that considers the GRIN lens' aberration and intensity loss from scattering in a Fourier optics-forward model to optimize a DOE and manufacture the DOE through single-step photolithography. We integrate the DOE into EDoF-Miniscope with a lateral accuracy of to produce high-contrast signals without compromising the speed, spatial resolution, size, or weight. We characterize the performance of EDoF-Miniscope across 5- and fluorescent beads embedded in scattering phantoms and demonstrate that EDoF-Miniscope facilitates deeper interrogations of neuronal populations in a -thick mouse brain sample and vessels in a whole mouse brain sample. Built from off-the-shelf components and augmented by a customizable DOE, we expect that this low-cost EDoF-Miniscope may find utility in a wide range of neural recording applications.

Gear Profile Measurement Based on Thin Line Structured Light

In order to achieve the high precision 3D measurement of gear tooth profile, a telecentric optical measurement system with large FOV (field of view), WD (working distance) and measuring range was proposed, the system was applied to the gear tooth profile measurement experiment and obtained a good effect. Firstly, the composition and the principle of the system were introduced. The lights of projection and imaging with an angle of 45° from the normal line of the measured surface can reduce the requirement of DOF (depth of field) of the imaging objective lens. Secondly, in combine with the application background of small modulus gear tooth profile measurement, the magnification, NA (numerical aperture) and other optical parameters were calculated, the optical system was constructed and the system performance parameters are listed. Then, the calibration model of the system was demonstrated and the calibrations of depth pixel resolution and horizontal pixel resolution were completed through experiments. Finally, an involute artifact with a modulus of 2.9992mm and a total tooth profile deviation of 1.1μm was measured. The results show that the deviation of the total tooth profile measured is about 2μm, which is consistent with the calibration curve of the involute artifact.

Aided Imaging Phase Measuring Deflectometry Based on Concave Focusing Mirror

With the rapid development of aerospace, high-speed train, and automotive industries, the demand for the measurement of high-precision specular components is increasing. The acquisition of high-precision three-dimensional (3D) data is conducive to improving the performance of and extending the service life of these components. However, the existing 3D measurement methods of specular surfaces are affected by the inherent limitation of the depth of field (DOF) of camera lenses. Based on the principle of paraxial reflection imaging of a concave mirror, this paper introduces a concave mirror into a phase measuring deflectometry (PMD) system and proposes an aided imaging PMD (AIPMD) based on a concave focusing mirror. The proposed system realizes the clear imaging of the encoded patterns and the surface under test in the DOF of the camera lens, simultaneously. Meanwhile, the iterative coefficient specular reconstruction algorithm is studied based on this system. The feasibility and accuracy of the proposed method are verified by simulations and experiments.

Extending the Imaging Depth of Field through Scattering Media by Wavefront Shaping of Non-Diffraction Beams

Increasing the depth of field (DOF) is a crucial issue for imaging through scattering media. In this paper, an improved genetic algorithm is used to modulate the wavefront of light through scattering media, by which high-quality refocusing and imaging through scattering media are achieved. Then, the DOF of the imaging system is effectively extended by further modulating the refocused beam into a non-diffraction beam. Two kinds of non-diffraction beams, i.e., a Bessel beam and Airy beam, were produced as a demonstration. The experimental results show that compared to the Gaussian beam, the DOF of the imaging system by combining the wavefront shaping and non-diffraction Bessel beam or Airy beam can be improved by a factor of 1.1 or 1.5, respectively. The proposed method is helpful for the technical development of high-quality imaging through scattering media with a large DOF.

Large depth-of-field microscopic structured-light 3D imaging with focus stacking

State-of-the-art microscopic structured-light (SL) three-dimensional (3D) imaging systems typically use conventional lenses with a fixed focal length achieving a limited depth of field (DOF). This paper proposes to drastically increase the DOF of the microscopic 3D imaging by leveraging the focus stacking technique and developing a novel computational framework. We first capture fringe images with various camera focal lengths using an electrically tunable lens (ETL) and align the recovered phase maps using phase constraints. Then, we extract the focused pixel phase using fringe contrast and stitch them into an all-in-focus phase map via energy minimization. Finally, we reconstruct the 3D shape using the all-in-focus phase map. Experimental results demonstrate that our proposed method can achieve a large DOF of approximately 2 mm and field of view (FOV) of approximately 4 mm × 3 mm with a pixel size at the object space of approximately 2.6 μm. The achieved DOF is approximately 10 × the DOF of the system without the proposed method.

A novel calibration method for the zoom micro-vision system containing electrically tunable lens

For zoom micro-vision system containing electrically tunable lens (ETL), traditional calibration methods are usually ineffective or inefficient due to the limited field of view (FOV) and shallow depth of field (DOF). This article proposes a novel calibration method that can overcome these problems. Firstly, the geometric imaging model of the zoom micro-vision system considering the input focal power is established. Subsequently, a unique image acquisition scheme and the corresponding feature extraction algorithm developed in frequency domain are given, which can greatly reduce the calibration workload. In addition, how to calculate the initial value of the parameters in the imaging model and how to optimize these parameters globally are presented. Finally, calibration experiments including the comparisons with traditional multi-focus and fitting methods are designed and implemented. Experimental results show that the proposed method has good consistency and accuracy when calibrating at different focal intervals, and the average reprojection error is less than 0.25 pixels. Comparative experiments show that the traditional multi-focus and fitting methods not only need more calibration images and the auxiliary high-precision positioning stage, but also have poor applicability for zoom micro-vision systems in practical.

Femtosecond laser marking depth of field 3D code

In order to reduce the market in a large number of counterfeit imitation products, product traceability anti-counterfeiting by more and more attention. Therefore, it is imperative to study product traceability and anti-counterfeiting marking technology. However, for products within a limited size, the data capacity of traditional Data Matrix (DM) code is insufficient to meet the storage requirements, and it is easy to clone and copy, which limits the development of laser marking technology. In order to improve the replication difficulty and information storage capacity of the Data Matrix code, the depth-of-field (DOF) 3D code is devised by adding Z-axis depth information to the original Data Matrix code, which can accommodate a larger information storage capacity and improve the reproduction difficulty of DM code, thus meeting the current commercial demand. In this paper, cemented carbide is taken as the research object, and DOF 3D code is marked on its surface by femtosecond laser, and the optimal process parameters are determined. At the same time, the recognition and information reading of DOF 3D code is studied, and the correctness and reliability of its information acquisition are tested.

High-throughput single-molecule localization microscopy: Potential clinical applications.

High-throughput single-molecule localization microscopy: Potential clinical applications.

Transmissive multifocal laser speckle contrast imaging through thick tissue

Laser speckle contrast imaging (LSCI) is a powerful tool for monitoring blood flow changes in tissue or vessels in vivo, but its applications are limited by shallow penetration depth under reflective imaging configuration. The traditional LSCI setup has been used in transmissive imaging for depth extension up to [Formula: see text]–[Formula: see text] ([Formula: see text] is the transport mean free path), but the blood flow estimation is biased due to the depth uncertainty in large depth of field (DOF) images. In this study, we propose a transmissive multifocal LSCI for depth-resolved blood flow in thick tissue, further extending the transmissive LSCI for tissue thickness up to [Formula: see text]. The limited-DOF imaging system is applied to the multifocal acquisition, and the depth of the vessel is estimated using a robust visibility parameter [Formula: see text] in the coherent domain. The accuracy and linearity of depth estimation are tested by Monte Carlo simulations. Based on the proposed method, the model of contrast analysis resolving the depth information is established and verified in a phantom experiment. We demonstrated its effectiveness in acquiring depth-resolved vessel structures and flow dynamics in in vivo imaging of chick embryos.

High-resolution and high-speed 3D tracking of microrobots using a fluorescent light field microscope.

Imaging and tracking are crucial for microrobots which navigate through complex 3D environments. Fluorescent imaging (FI) by microscope offers a high-resolution and high-sensitive imaging method to study the property of microrobots. However, conventional microscope suffers from shallow depth of field (DOF) and lacks 3D imaging capability. We proposed a high-resolution and high-speed 3D tracking method for microrobots based on a fluorescent light field microscope (FLFM). We designed the FLFM system according to the size of a representative helical microrobot (150 μm body length, 50 μm screw diameter), and studied the system's performance. We also proposed a 3D tracking algorithm for microrobots using digital refocusing. We validated the method by simulations and built an FLFM system to perform the tracking experiments of microrobots with representative size. Our 3D tracking method achieves a 30 fps data acquisition rate, 10 μm lateral resolution and approximately 40 μm axial resolution over a volume of 1,200×1,200×326 μm3. Results indicate that the accuracy of the method can reach about 9 μm. Compared with the FI by a conventional microscope, the FLFM-based method gains wider DOF and 3D imaging capability with a single-shot image. The tracking method succeeds in providing the trajectory of the microrobot with a good lateral resolution.

Rapid all-in-focus imaging via physical neural network optical encoding

Lightfield phase modulation has become an effective implementation for extending depth-of-field (DOF) of computational imaging. However, correct reconstruction of tiny details and color configuration with a fast capture is still challenging. Here, we demonstrate a high-dynamic all-focus imaging based on a proposed paradigm of constructing diffractive network encoding and electronic network decoding (DE-ED) conformation. With an efficient collaboration of physical diffractive layers and electronic convolutional layers, this learning-based model demonstrates significantly enhanced generalization and data-fitting capabilities. In experiments, the proposed method exhibits an obvious superiority in high-dynamic adaptation, autofocusing, and denoise compared with conventional methods. Specifically, an ultrahigh capture frame rate with ST <1/3000 s can be precisely adapted with only ∼5.6 mm diameter of imaging aperture under natural illumination. Several videos for proposed high-dynamic reconstruction show this method's time-efficiency and consistency. These results highlight some unique advantages of the constructed hybrid opto-electronic network model based on data-driven end-to-end learning for next-generation innovative computational imaging.

Large depth-of-field image fusion method for complex gyration class mechanical parts

The local regions of complex gyration class mechanical parts are largely different, which results in the problem that their surface images are partially clear. This problem directly affects the effectiveness of vision methods on detecting surface defects. To tackle this problem, a large depth-of-field (DoF) surface image fusion method is proposed to obtain a full-focus image of complex mechanical parts, such as gyration class mechanical parts. We design an image acquisition platform based on the characteristics of gyration class parts to acquire their surface images accurately and clearly. We then propose a feature detection-based image registration method, by which the registered image can represent the surface information of the part completely and accurately. Additionally, we adopt a convolutional neural network-based image fusion method to achieve a fused large DoF surface image. Experimental studies were conducted to evaluate the performance of the deep-learning-based methods. The experimental results show that the proposed method can completely and clearly fuse the large DoF surface images of complex gyration class mechanical parts. The quality of the fused images has a significant improvement, and the proposed method is significantly more efficient than traditional fusion methods and other deep-learning methods.

Accurate and flexible calibration method for a 3D microscopic structured light system with telecentric imaging and Scheimpflug projection.

3D imaging and metrology of complex micro-structures is a critical task for precision manufacturing and inspection. In this paper, an accurate and flexible calibration method for 3D microscopic structured light system with telecentric imaging and Scheimpflug projector is proposed. Firstly, a fringe projection 3D microscopy (FP-3DM) system consisting of a telecentric camera and a Scheimpflug projector is developed, which can take full advantage of the depth of field (DOF) and increase the measurement depth range. Secondly, an accurate and flexible joint calibration method is proposed to calibrate the developed system, which utilizes the established pinhole imaging model and Scheimpflug distortion model to calibrate telecentric imaging, and fully considers the correction and error optimization of the Scheimpflug projection model. Meanwhile, the optimized local homography is calculated to obtain more accurate sub-pixel correspondence between the camera and the projector, and the perspective-n-point (PnP) method make the 3D coordinate estimation of the feature point more accurate. Finally, a prototype and a dedicated calibration program are developed to realize high-resolution and high-precision 3D imaging. The experimental results demonstrate that the re-projection error is less than 1µm, and the 3D repeated measurement error based on feature fitting is less than 4µm, within the calibrated volume of 10(H)mm × 50(W)mm × 40(D)mm.

Reflective axicon based energy-efficient extended depth of focus quasi-Bessel beam probe for common-path optical coherence tomography.

This work presents an optical fiber negative/reflective axicon probe that generates an energy-efficient quasi-Bessel beam (QBB) having a central spot (CS) possessing ∼20% of the QBB power. With silver coating around the axicon, the CS power has been increased by ∼45%. The QBB possesses a large depth of field, ∼400µm, with a micron order spot size as obtained experimentally. The probe has further been explored for common-path optical coherence tomography. The probe length has been optimized to minimize the path length difference between the reference and sample signal. With a divergence angle of just 0.013°, the beam provides a lateral resolution of ∼2.5 to ∼16µm for an axial distance of 0.1 to 1.0mm. The imaging results are presented for standard samples such as onion and Scotch tape.

Characterizing close-focus lenses for microendoscopy.

Microendoscopes are commonly used in small lumens in the body, for which a focus near to the distal tip and ability to operate in an aqueous environment are paramount for navigation and disease detection. Commercially available distal optic systems below 1mm in diameter are severely limited, and custom micro lenses are generally very expensive. Gradient index of refraction (GRIN) singlets are available in small diameters but have limited optical performance adjustability. Three-dimensional (3D) printed monolithic optical systems are an emerging option that may be suitable for enabling high performance, close-focus imaging. In this manuscript, we compared the optical performance of three custom distal optic systems; a custom-pitch GRIN singlet, 3D-printed monolithic doublet, and 3D-printed monolithic triplet, with a nominal working distance (WD) of 1.5mm, 0.5mm and 0.4mm in 0.9% saline. These short WDs are ideal for microendoscopy in collapsed or flushed lumens such as pancreatic duct or fallopian tube. The GRIN singlet had performance limited only by the fiber bundle relay over 0.9mm to 1.6 mm depth of field (DOF). The 3D printed doublet was able to achieve a comparable DOF of 0.71mm, while the 3D printed triplet suffered the most limited DOF of 0.55mm. 3D printing enables flexible design of monolithic multi-element systems with aspheric surfaces of very short WDs and relative ease of integration.