Optical performance evaluation of an infrared system of a hypersonic vehicle in an aero-thermal environment.

At hypersonic velocities, the turbulent flow field generated by an aircraft, along with its temperature distribution, leads to significant aerodynamic optical effects that severely impede the performance of internal optical systems. This study proposes a method for analyzing the temporal characteristics of imaging degradation in a detector window infrared imaging system under different field angles of hypersonic velocity. Based on heat transfer theory, a method for solving the transient temperature field in the optical window of a high-speed aircraft is derived and established, considering unsteady thermal conduction-radiation coupling. Additionally, an optical window radiation tracing method is introduced, which directly determines the initial direction vector of light reaching the detector. This method reduces the workload of radiation transmission, significantly enhancing the efficiency of radiation calculations. The time characteristics of image degradation caused by aero-optical effects in high-speed aircraft are analyzed using metrics such as peak signal-to-noise ratio, wave aberration, and point diffusion function. The results demonstrate that as working time increases and the viewing angle widens, the impact of aero-optics on the aircraft imaging system becomes more severe. Moreover, compared to the aerodynamic light transmission effect, the aerodynamic thermal radiation effect has a more detrimental influence on imaging quality.

Hierarchical deconvolution dehazing method based on transmission map segmentation.

Images captured in fog are often affected by scattering. Due to the absorption and scattering of light by aerosols and water droplets, the image quality will be seriously degraded. The specific manifests are brightness decrease, contrast decrease, image blur, and noise increase. In the single-image dehazing method, the image degradation model is essential. In this paper, an effective image degradation model is proposed, in which the hierarchical deconvolution strategy based on transmission map segmentation can effectively improve the accuracy of image restoration. Specifically, the transmission map is obtained by using the dark channel prior (DCP) method, then the transmission histogram is fitted. The next step is to divide the image region according to the fitting results. Furthermore, to more accurately recover images of complex objects with a large depth of field, different levels of inverse convolution are adopted for different regions. Finally, the sub-images of different regions are fused to get the dehazing image. We tested the proposed method using synthetic fog images and natural fog images respectively. The proposed method is compared with eight advanced image dehazing methods on quantitative rating indexes such as peak signal-to-noise ratio (PSNR), structural similarity (SSIM), image entropy, natural image quality evaluator (NIQE), and blind/referenceless image spatial quality evaluator (BRISQUE). Both subjective and objective evaluations show that the proposed method achieves competitive results.

Generation of Non-Rayleigh Nondiffracting Speckles.

Optical speckle fields with both non-Rayleigh statistics and nondiffracting characteristics in propagation are an important light source for many applications. However, tailoring either non-Rayleigh statistical speckles or nondiffracting speckles are only investigated independently in previous studies. Here, we report the first observation of optical speckles that remain diffraction-free over a long axial distance while keeping non-Rayleigh statistics simultaneously. We further show the enhancement of Anderson localization of light with the non-Rayleigh nondiffracting speckles. The work presented here provides a versatile framework for customizing optical fields with desired speckle patterns for applications in the fields of solid-state physics, cold atoms, and optical imaging.

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.

Transmissive mid-infrared achromatic bifocal metalens with polarization sensitivity.

Metasurfaces have shown great potential in versatile areas such as vortex-beam generators, metalenses, holograms and so on. However, chromatic error hinders metasurfaces, especially metalenses, from wider applications. In this paper, we demonstrate a novel design for a transmissive mid-infrared achromatic bifocal metalens with polarization sensitivity. The compensation phase is used to eliminate the chromatic aberration. Simulation results show that, over a continuous waveband from 3.9 to 4.6µm, the focal length only changes by 2.26% with an average focusing efficiency of about 18%. This work can push the practical application of mid-infrared metasurfaces.

Measuring the complex spectrum of orbital angular momentum and radial index with a single-pixel detector.

Typical methods to decode a complex orbital-angular-momentum (OAM) spectrum suffer from issues such as a narrow OAM range, unstable interferometer, and long measuring time. In this Letter, we use a single-beam interferometer to measure the complex OAM spectrum with a single-pixel detector. The complex OAM spectrum ranging from -10 to 10 can be measured in 11 ms with the fidelity approach of 97.0%, experimentally. Our approach allows one to characterize an unknown coherent field with any complex basis, e.g., the Laguerre-Gaussian (LG) basis is used for radial index spectrum measurement. Furthermore, single-pixel complex amplitude imaging based on the LG spectrum acquisition is presented, and the advantages in resolution and flexibility are demonstrated.

Superb Monocular Depth Estimation Based on Transfer Learning and Surface Normal Guidance.

Accurately sensing the surrounding 3D scene is indispensable for drones or robots to execute path planning and navigation. In this paper, a novel monocular depth estimation method was proposed that primarily utilizes a lighter-weight Convolutional Neural Network (CNN) structure for coarse depth prediction and then refines the coarse depth images by combining surface normal guidance. Specifically, the coarse depth prediction network is designed as pre-trained encoder-decoder architecture for describing the 3D structure. When it comes to surface normal estimation, the deep learning network was designed as a two-stream encoder-decoder structure, which hierarchically merges red-green-blue-depth (RGB-D) images for capturing more accurate geometric boundaries. Relying on fewer network parameters and simpler learning structure, better detailed depth maps are produced than the existing states. Moreover, 3D point cloud maps reconstructed from depth prediction images confirm that our framework can be conveniently adopted as components of a monocular simultaneous localization and mapping (SLAM) paradigm.

Evaluating imaging quality of optical dome affected by aero-optical transmission effect and aero-thermal radiation effect.

The imaging quality of the aerodynamically heated optical dome was evaluated under the comprehensive influence of aero-optical transmission effect and aero-thermal radiation effect. The ray propagating algorithm based on the fourth order Runge-Kutta method was used to trace the target ray and the thermal radiation ray of the optical dome. Three imaging quality evaluation parameters were proposed to evaluate aero-optical effect: Modulation transfer function (MTF), irradiance, peak signal-to-noise ratio (PSNR) of distorted images. The results show that: as the flight speed increased, the MTF decreased observably compared with the diffraction-limit MTF, the irradiance on the photosensitive surface of the detector increased gradually, and the distorted imaging quality under the influence of the comprehensive aero-optical effect gradually deteriorated. However, as the thickness of the optical dome increased, the MTF decreased sharply and the irradiance decreased gradually, that indicated the aero-optical transmission effect was reinforced and the aero-thermal radiation effect was weakened. The imaging quality improved with thickness increasing. The influence of aero-thermal radiation effect on the PSNR of the image was more serious than that of the aero-optical transmission effect.

Influence of altitude on aero-optic imaging quality degradation of the hemispherical optical dome.

We investigated the influence of altitude on aero-optic imaging quality degradation of the hemispherical optical dome. Boundary conditions for the aerodynamic heating effect of the optical dome were calculated by solving the Reynolds-averaged Navier-Stokes equations provided by FLUENT. The finite element model and the thermal-structure simulation results of the optical dome were obtained using ANSYS. The 3D nonuniform refractive index field of the optical dome was obtained according to the thermal-optical effect. The optical tracking method based on the fourth-order Runge-Kutta algorithm was adopted to simulate the optical transmission through the optical dome. The Strehl ratio (SR), encircled energy, distorted target images, and peak signal-to-noise ratio were presented for imaging quality evaluation. The variation rules of these imaging quality evaluation parameters were obtained in the altitude range of 0-45 km. The results showed that, in the same flight conditions, with the increase of altitude, peak signal-to-noise ratio (PSNR) of the distorted image, and SR result were increased, and radiuses of dispersion spots, including 80% energy, were decreased; therefore, the influence of aero-optics effect on imaging quality degradation was gradually weakened.

Optimized design method for the inner surface of a conformal dome based on the ray tracing approach.

Traditional optical domes are spherical, which introduces constant aberrations with look angle. However, spherical domes are not optimum for reducing aerodynamic drag. Conformal domes deviate from spherical to reduce drag but they generate dynamic aberrations varying significantly with look angle in the field of regard. Thus, conformal domes require unique challenges for aberration correction. This paper presents a method to reduce the dynamic aberrations through designing the inner surface of conformal domes. This method follows the principle that the optical axis ray of the imaging system maintains the same direction after refraction through the conformal dome for different look angles. Based on this principle, equations that the inner surface should satisfy are established and a numerical solution method is introduced. Eventually, Zernike polynomial coefficients of Z4, Z5, Z8, and Z9, which represent defocus, astigmatism, coma, and spherical aberration, respectively, are analyzed for quadratic domes with different inner surfaces. Compared with domes with traditional inner surfaces, quadratic domes with the inner surfaces calculated by this method have smaller Zernike aberrations. In conclusion, this design method for the inner surface can effectively reduce dynamic aberrations.

Basic geometry and aberration characteristics of conicoidal conformal domes.

This paper investigated the geometry and aberration characteristics of conicoidal conformal domes. First, on the basis of previous research, we got the expression that was suitable for describing the external surface of the conicoidal conformal dome. Based on the theory of differential geometry, this paper first proved that the Dupin index line of a quadric surface was an ellipsoid and the radius of curvature had extreme values in the meridian plane and sagittal plane. Then the uniform formulas of curvature which were suitable for ellipsoid, paraboloid, and hyperboloid were deduced in the meridian plane and sagittal plane, respectively. Meanwhile, the angle between the axis of imaging systems and the surface normal was calculated. With the help of computers, the plots of curvature differences and the angle in the case of different edge slopes, fineness ratios, and the locations of the rotational center were obtained. Finally, we analyzed the Zernike polynomial coefficients of Z4, Z5, and Z8, which represent defocus, astigmatism and coma, respectively for the model established in CODE V. The research indicates that the dynamic ranges of defocus, astigmatism, and coma increase with the growing of edge slopes and fineness ratios, but have little change with the variation of the rotational center positions. Moreover, the curves of Z5 and Z8 have turning points, and the curves of curvature differences and angle difference are only similar to the curves of Z5 and Z8 when the look angle changes after the turning point. For the look angle changing from zero to the turning point, the curves of Z5 and Z8 change rapidly. This is mainly caused by the significant variations of the symmetry of the conformal dome participating in imaging. Therefore, the aberrations with small scanning angles should be given more attention when designing the conformal systems.

Extending depth of field for hybrid imaging systems via the use of both dark and dot point spread functions.

In this paper, we propose one method based on the use of both dark and dot point spread functions (PSFs) to extend depth of field in hybrid imaging systems. Two different phase modulations of two phase masks are used to generate both dark and dot PSFs. The quartic phase mask (QPM) is used to generate the dot PSF. A combined phase mask between the QPM and the angle for generating the dark PSF is investigated. The simulation images show that the proposed method can produce superior imaging performance of hybrid imaging systems in extending the depth of field.

Multi-aperture foveated imaging.

Foveated imaging, such as that evolved by biological systems to provide high angular resolution with a reduced space-bandwidth product, also offers advantages for man-made task-specific imaging. Foveated imaging systems using exclusively optical distortion are complex, bulky, and high cost, however. We demonstrate foveated imaging using a planar array of identical cameras combined with a prism array and superresolution reconstruction of a mosaicked image with a foveal variation in angular resolution of 5.9:1 and a quadrupling of the field of view. The combination of low-cost, mass-produced cameras and optics with computational image recovery offers enhanced capability of achieving large foveal ratios from compact, low-cost imaging systems.

To enhance imaging performance of hybrid imaging systems by using two asymmetrical phase masks.

We propose the use of two asymmetrical phase masks combined with the subtracted imaging method to enhance the signal-to-noise ratio in wavefront coding systems. This subtracted imaging technique is similar to the variable pinhole diameter in confocal microscopy. Two different phase modulations of same phase masks are employed to promote the magnitude of the optical transfer function (OTF). The ratio factor is used to control the phase variation between two phase masks. The noise of decoded images is suppressed owing to the higher magnitude of the OTF than the wavefront coding systems with a phase mask. A tangent phase mask as an example is used to demonstrate our concept. Simulated results show that the performance promotion controls noise amplification of decoded images while maintaining a depth-of-field extension.

Three new compounds isolated from the seeds of Vaccaria segetalis.

Two new capsaicin analogs, N-(3-methoxy-4-hydroxyphenethyl)-tetracosanamide (1) and N-(3,4-dihydroxyphenethyl)-tetracosanamide (2), along with one new flavonoidal glycoside pinnatifin E (3) were isolated from the ethanolic extract of the seeds of Vaccaria segetalis. Their structures were elucidated on the basis of spectroscopic methods including 1D, 2D NMR, MS, and other spectroscopic techniques, as well as by comparison with the relevant literatures. All compounds were evaluated for their coagulation Factor Xa inhibition activities.

Optimized asymmetrical tangent phase mask to obtain defocus invariant modulation transfer function in incoherent imaging systems.

Wavefront coding as an optical-digital hybrid imaging technique can be used to extend the depth of field. The key to wavefront coding lies in the design of suitable phase masks to achieve the invariant imaging properties over a wide range of defocus. In this Letter, we propose another phase mask with a tangent function to enrich the odd symmetrical kind of phase masks. The performance of the tangent phase mask is evaluated by comparison with a cubic mask, improved-1 logarithmic mask, improved-2 logarithmic mask, and sinusoidal mask. The results demonstrate that the tangent phase mask has superior performance in extending the depth of field.

Conformal dome aberration correction with gradient index optical elements.

A conformal dome optical system was established and aberration characteristics of the dome were investigated using Zernike aberration theory. The conformal dome was designed with gradient index element. The designing method was introduced and the optimizing results were analyzed in detail. The results show that the Zernike aberrations produced by the conformal dome decreased dramatically. Also, a complete conformal optical system was designed to further illustrate the aberration correction effect of gradient index elements. The results show that the utilization of gradient index optical elements not only improves the imaging quality, but also simplifies the structure of the conformal optical system.

Design of fixed correctors used in conformal optical system based on diffractive optical elements.

A conformal dome was designed and the aberration characteristics of the dome were analyzed using Zernike aberration theory. By deriving the equation used to correct Zernike aberrations, the phase coefficients and the phase orders of diffractive optical elements (DOEs) used to correct primary Zernike aberrations were obtained. DOEs were simulated to correct the aberrations of the conformal dome by using optical design software, and the aberrations of the conformal dome decreased dramatically. Finally, a complete cooled conformal optical system was designed. The results show that the number of the fixed corrector's elements decreases by using DOEs, and the optical system has better imaging quality.

Comprehensive analysis of imaging quality degradation of an airborne optical system for aerodynamic flow field around the optical window.

We investigated the influences exerted by the nonuniform aerodynamic flow field surrounding the optical window on the imaging quality degradation of an airborne optical system. The density distribution of flow fields around three typical optical windows, including a spherical window, an ellipsoidal window, and a paraboloidal window, were calculated by adopting the Reynolds-averaged Navier-Stokes equations with the Spalart-Allmaras model provided by FLUENT. The fourth-order Runge-Kutta algorithm based ray-tracing program was used to simulate the optical transmission through the aerodynamic flow field. Four kinds of imaging quality evaluation parameters were presented: wave aberration of the entrance pupil, point spread function, encircled energy, and modulation transfer function. The results show that the imaging quality of the airborne optical system was affected by the shape of the optical window and angle of attack of the aircraft.

Wavefront coding technique for controlling thermal defocus aberration in an infrared imaging system.

We use a wavefront coding approach to control thermal defocus aberration in an IR imaging system. The design method of athermalized system using a wavefront coding technique is discussed. An athermalized long wave IR optical system, which works at temperatures ranging from -40 °C to 60 °C, is designed by employing a cubic phase mask. Computer simulations and the first experimental demonstration are executed to verify the performance of this wavefront coded athermalized system and to clarify the issues related to its implementation.