Wavefront Coding Imaging System Research Articles

Convolutional Neural Networks to Decode Images in a Wavefront Coding Imaging System

Convolutional Neural Networks to Decode Images in a Wavefront Coding Imaging System

Learned phase mask to protect camera under laser irradiation

The electro-optical imaging system works under focus conditions for clear imaging. However, under unexpected laser irradiation, the focused light with extremely high intensity can easily damage the imaging sensor, resulting in permanent degradation of its perceptual capabilities. With the escalating prevalence of compact high-performance lasers, safeguarding cameras from laser damage presents a formidable challenge. Here, we report an end-to-end method to construct the wavefront coding (WFC) imaging systems with simultaneous superior laser protection and imaging performance. In the optical coding part, we employ four types of phase mask parameterization methods: pixel-wise, concentric rings, linear combinations of Zernike bases, and odd-order polynomial bases, with parameters that are learnable. In the algorithm decoding part, a method combined of deconvolution module and residual-Unet is proposed to furthest restore the phase-mask-induced image blurring. The optical and algorithm parts are jointly optimized within the end-to-end framework to determine the performance boundary. The governing rule of the laser protection capability versus imaging quality is revealed by tuning the optimization loss function, and the system database is established for various working conditions. Numerical simulations and experimental validations both demonstrate that the proposed laser-protection WFC imaging system can reduce the peak single-pixel laser power by 99.4% while maintaining high-quality imaging with peak signal-to-noise ratio more than 22 dB. This work pioneers what we believe to be a new path for the design of laser protection imaging systems, with promising applications in security and autonomous driving.

Super-Resolution Image Reconstruction of Wavefront Coding Imaging System Based on Deep Learning Network

Wavefront Coding (WFC) is an innovative technique aimed at extending the depth of focus (DOF) of optics imaging systems. In digital imaging systems, super-resolution digital reconstruction close to the diffraction limit of optical systems has always been a hot research topic. With the design of a point spread function (PSF) generated by a suitably phase mask, WFC could also be used in super-resolution image reconstruction. In this paper, we use a deep learning network combined with WFC as a general framework for images reconstruction, and verify its possibility and effectiveness. Considering the blur and additive noise simultaneously, we proposed three super-resolution image reconstruction procedures utilizing convolutional neural networks (CNN) based on mean square error (MSE) loss, conditional Generative Adversarial Networks (CGAN), and Swin Transformer Networks (SwinIR) based on mean absolute error (MAE) loss. We verified their effectiveness by simulation experiments. A comparison of experimental results shows that the SwinIR deep residual network structure based on MAE loss optimization criteria can generate more realistic super-resolution images with more details. In addition, we used a WFC camera to obtain a resolution test target and real scene images for experiments. Using the resolution test target, we demonstrated that the spatial resolution could be improved from 55.6 lp/mm to 124 lp/mm by the proposed super-resolution reconstruction procedure. The reconstruction results show that the proposed deep learning network model is superior to the traditional method in reconstructing high-frequency details and effectively suppressing noise, with the resolution approaching the diffraction limit.

Laser protection by using vortex wavefront coding imaging system

Laser blindness can reduce or disable the information acquisition ability of photoelectric imaging systems. In this paper, numerical simulation and experimental verification are both performed to systematically study the laser protection performance of the vortex phase mask. First, the imaging model and laser transmission model of the vortex wavefront coding imaging system are introduced in detail. Then, the experimental setup of the imaging system is built, and the imaging result of the imaging system is obtained. Finally, the influence of propagation distance on the maximum single-pixel receiving power and suppression ratio of the imaging system is measured experimentally. The simulation and experimental results both show that the energy suppression ratio of this method can reach more than two orders of magnitude compared with the conventional imaging system, and the probability of laser blindness can be effectively reduced.

Experimental damage thresholds of a laser suppression imaging system using a cubic phase plate

The development of laser systems leads to an increasing threat to photoelectric imaging sensors. A cubic phase plate wavefront coding imaging system is proposed to reduce the risk of damage owing to intense laser radiation. Based on the wavefront coding imaging model, the diffracted spot profile and the light intensity distribution on the observation plane are simulated. An experimental device is set up to measure the laser-induced damage thresholds and investigate the morphology of laser-induced damage patterns of the conventional and the wavefront encoding imaging system. Simulations and experimental results manifest the superior laser suppression performance of the proposed method, which can help diminish the undesirable effects of laser irradiation on an imaging sensor.

A Retroreflection Reduction Technique Based on the Wavefront Coded Imaging System

A novel anti-cat-eye effect imaging technique based on wavefront coding is proposed as a solution to the problem of previous anti-cat-eye effect imaging techniques where imaging quality was sacrificed to reduce the retroreflection from the photoelectric imaging equipment. With the application of the Fresnel–Kirchhoff diffraction theory, and the definition of generalized pupil function combining both phase modulation and defocus factors, the cat-eye echo formation of the wavefront coded imaging system is theoretically modeled. Based on the physical model, the diffracted spot profile distribution and the light intensity distribution on the observation plane are further simulated with the changes in the defocus parameter and the phase modulation coefficient. A verification test on the cat-eye laser echo power of the wavefront coded imaging system and that of the conventional imaging system at a 20 m distance are conducted, respectively. Simulations and experiment results show that compared with conventional imaging systems, the wavefront coding imaging system can reduce the retroreflection echo by two orders of magnitude while maintaining better imaging quality through defocusing.

Analog optical deconvolution computing for wavefront coding based on nanoantennas metasurfaces.

Analog optical computing based on metasurfaces has attracted much attention for achieving high-speed calculating without the electronic processing unit. Wavefront coding imaging systems involve the joint design of an encoded image-capturing module and decoding postprocessing algorithms to obtain a required final image. The decoding postprocessing algorithms, as a typical deconvolution computation, require an additional electronic processing unit to yield a high-quality decoded image. We demonstrate an analog optical deconvolution computing kernel based on nanoantennas metasurfaces for the postprocessing calculation of wavefront coding systems. Numerical simulations are presented to prove that the encoded point spread function can be refocused through a designed optical computing metasurface. The proposed approach opens an opportunity for real-time recovering images in wavefront coding optical systems.

Extended-depth-of-field object detection with wavefront coding imaging system

As an important issue in the field of computer vision, object detection has broad application prospects. Recent researches using convolutional neural networks (CNN) have shown the state-of-the-art results in the challenge competition. Most of them focused on improving the precision under ideal imaging conditions. However, it is hard to ensure that the optical imaging system works in the focused state in practice. In this study, we examine the impact of defocus on detection accuracy. The results show that even the state-of-the-art network is sensitive to a different defocus situation. Thus we put forward wavefront coding (WFC) technique for improving the performance over a large range of depth of field (DOF). Simulation results indicate the improvement on average precision of detection results by applying WFC. In addition, we propose a novel WFC method for overcoming the defects of the traditional one. Then the optical imaging system is designed under the guidance of the proposed theory. Experiments are conducted to suggest that the detection accuracy rate can be enhanced considerably with WFC.

Optimization of wavefront coding imaging system based on the phase transfer function

The variation of the phase transfer function with respect to defocus causes linear image shift and image artifacts on the restored images in wavefront coding imaging system with asymmetrical phase masks. To reduce the effect of the phase transfer function on the restored image according to the signal to noise ratio, in this paper, an optimization function based on the phase transfer function is proposed. By using this function with the signal to noise ratio equal to 30dB, optimization and simulation result for the cubic phase mask and the tangent phase mask are presented. The simulation result shows that the tangent phase mask produces more benefit to reduce image artifacts on the restored image.

Odd symmetrical square-root phase mask to extend the depth of field in wavefront-coded imaging systems

Extend-the-depth-of-field imaging can be achieved by wavefront coding technique, which inserts an odd symmetrical phase mask into the pupil plane to generate the invariant imaging property over a wide range of defocus. Obtaining the low sensibility to defocus for wavefront coding systems depends on designing a proper phase profile. Here we proposed a phase mask combining the power function with the square-root function to maximize the invariant imaging property to defocus. Judging by defocused modulation transfer function (MTF) and Hilbert space angle, numerical comparisons with the cubic mask, exponential mask and improved logarithmic mask prove the superiority of proposed square-root phase mask in extending the depth of field.

To extend the depth of field by using the asymmetrical phase mask and its conjugation phase mask in wavefront coding imaging systems

In this paper, we present a method for combining an asymmetrical phase mask and its conjugation phase mask to produce the point spread function of a wavefront coding imaging system, which has the defocus invariant characteristic of the asymmetrical phase mask and symmetric characteristic through an orthogonal axis pair of the symmetrical phase mask. Simulation results for comparing the wavefront coding imaging system with combining the sinusoidal phase mask and its conjugation phase mask and a traditional imaging system for a clear aperture without any phase mask are presented. Comparative results indicate that the wavefront coding imaging system combined with the sinusoidal phase mask and its conjugation phase mask has an imaging performance invariant over a wide range of defocus.

Tunable wavefront coded imaging system based on detachable phase mask: Mathematical analysis, optimization and underlying applications

The key to the concept of tunable wavefront coding lies in detachable phase masks. Ojeda-Castaneda et al. (Progress in Electronics Research Symposium Proceedings, Cambridge, USA, July 5–8, 2010) described a typical design in which two components with cosinusoidal phase variation operate together to make defocus sensitivity tunable. The present study proposes an improved design and makes three contributions: (1) A mathematical derivation based on the stationary phase method explains why the detachable phase mask of Ojeda-Castaneda et al. tunes the defocus sensitivity. (2) The mathematical derivations show that the effective bandwidth wavefront coded imaging system is also tunable by making each component of the detachable phase mask move asymmetrically. An improved Fisher information-based optimization procedure was also designed to ascertain the optimal mask parameters corresponding to specific bandwidth. (3) Possible applications of the tunable bandwidth are demonstrated by simulated imaging.

Analysis of the cubic-phase wavefront-coding function: Physical insight and selection of optimal coding strength

A simple graphical interpretation at the pupil of an optical system is proposed to analyse the properties of the cubic-phase wavefront-coding function. The approach explains the invariance achieved against optical defocus of wavefront coding imaging systems with simplicity and clarity. Next, an analytical approximation of the modulation-transfer function of general wavefront coding systems is derived and used, in combination with the graphical interpretation, for selecting the optimal strength of the cubic-phase encoding function.

Experimental evidence of the theoretical spatial frequency response of cubic phase mask wavefront coding imaging systems

The optical transfer function of a cubic phase mask wavefront coding imaging system is experimentally measured across the entire range of defocus values encompassing the system's functional limits. The results are compared against mathematical expressions describing the spatial frequency response of these computational imagers. Experimental data shows that the observed modulation and phase transfer functions, available spatial frequency bandwidth and design range of this imaging system strongly agree with previously published mathematical analyses. An imaging system characterization application is also presented wherein it is shown that the phase transfer function is more robust than the modulation transfer function in estimating the strength of the cubic phase mask.

Variations in the point spread function characteristics of wavelengths for a wavefront coding imaging system

The point spread function (PSF) characteristics of wavelengths for a wavefront coding imaging system are investigated. Although the phase delayed by the mask changes with wavelength, the shape of the PSF is nearly invariant with respect to wavelength. However, the position of the PSF shifts with axial chromatic aberration. If the wideband light is separated into several channels and separated channel images are restored using the same filter, then color traces may be observed in the recombined color images.

Design and implementation of a scene-dependent dynamically selfadaptable wavefront coding imaging system

A computational imaging system based on wavefront coding is presented. Wavefront coding provides an extension of the depth-of-field at the expense of a slight reduction of image quality. This trade-off results from the amount of coding used. By using spatial light modulators, a flexible coding is achieved which permits it to be increased or decreased as needed. In this paper a computational method is proposed for evaluating the output of a wavefront coding imaging system equipped with a spatial light modulator, with the aim of thus making it possible to implement the most suitable coding strength for a given scene. This is achieved in an unsupervised manner, thus the whole system acts as a dynamically selfadaptable imaging system. The program presented here controls the spatial light modulator and the camera, and also processes the images in a synchronised way in order to implement the dynamic system in real time. A prototype of the system was implemented in the laboratory and illustrative examples of the performance are reported in this paper. Program summary Program title: DynWFC (Dynamic WaveFront Coding) Catalogue identifier: AEKC_v1_0 Program summary URL: http://cpc.cs.qub.ac.uk/summaries/AEKC_v1_0.html Program obtainable from: CPC Program Library, Queenʼs University, Belfast, N. Ireland Licensing provisions: Standard CPC licence, http://cpc.cs.qub.ac.uk/licence/licence.html No. of lines in distributed program, including test data, etc.: 10 483 No. of bytes in distributed program, including test data, etc.: 2 437 713 Distribution format: tar.gz Programming language: Labview 8.5 and NI Vision and MinGW C Compiler Computer: Tested on PC Intel ® Pentium ® Operating system: Tested on Windows XP Classification: 18 Nature of problem: The program implements an enhanced wavefront coding imaging system able to adapt the degree of coding to the requirements of a specific scene. The program controls the acquisition by a camera, the display of a spatial light modulator and the image processing operations synchronously. The spatial light modulator is used to implement the phase mask with flexibility given the trade-off between depth-of-field extension and image quality achieved. The action of the program is to evaluate the depth-of-field requirements of the specific scene and subsequently control the coding established by the spatial light modulator, in real time.

Approximate analytical optical transfer function for a wavefront-coded imaging system with two-dimensional rectangularly separable phase masks

Wavefront coding can be used to extend the depth of field of incoherent imaging systems and is a powerful system-level technique. In order to assess the performance of a wavefront-coded imaging system, defocused optical transfer function (OTF) is the metric frequently used. Unfortunately, to the best of our knowledge, among all types of phase masks, it is usually difficult to obtain the analytical OTF except the cubic one. Although numerical computation seems good enough for performance evaluation, the approximate analytical OTF is still indispensable because it can reflect the relationship between mask parameters and system frequency response in a clearer way. Thus, a method is proposed to derive the approximate analytical OTF for two-dimensional rectangularly separable phase masks. The analytical results are well consistent with the direct numerical computations, but the proposed method can be accepted only from engineering point of view and needs rigorous proof in future.

Use of a spatial light modulator as an adaptable phase mask for wavefront coding

A wavefront-coded imaging system employing a spatial light modulator (SLM) for the agile implementation of phase masks is presented. The SLM is a liquid crystal display that can be modulated to implement cubic phase masks of variable coding strength. These phase masks produce broad point spread functions insensitive to defocus aberration and are used in combination with post-detection digital image processing to extend the depth-of-field of an imaging system. A detailed description of the calibration process of the SLM in phase mode is presented together with experimental results which include wavefront-coded modulation transfer functions and increased depth-of-field images. The most interesting feature is the versatility provided by the imaging system, as compared to conventional (fixed phase mask) designs, and the agility in which it is done.

Phase mask selection in wavefront coding systems: A design approach

Abstract A method for optimizing the strength of a parametric phase mask for a wavefront coding imaging system is presented. The method is based on an optimization process that minimizes a proposed merit function. The goal is to achieve modulation transfer function invariance while quantitatively maintaining final image fidelity. A parametric filter that copes with the noise present in the captured images is used to obtain the final images, and this filter is optimized. The whole process results in optimum phase mask strength and optimal parameters for the restoration filter. The results for a particular optical system are presented and tested experimentally in the laboratory. The experimental results show good agreement with the simulations, indicating that the procedure is useful.

大视场波前编码成像系统中的图像复原

大视场波前编码成像系统中的图像复原