Long-range polarimetric imaging through fog

Time-gated imaging through dense fog via physics-driven Swin transformer

Imaging through the fog is valuable for many areas, such as autonomous driving and cosmic exploration. However, due to the influence of strong backscattering and diffuse reflection generated by the dense fog on the temporal-spatial correlations of photons returning from the target object, the reconstruction quality of most existing methods is significantly reduced under dense fog conditions. In this study, we describe the optical scatter imaging process and propose a physics-driven Swin Transformer method utilizing Time-of-Flight (ToF) and Deep Learning principles to mitigate scattering effects and reconstruct targets in conditions of heterogeneous dense fog. The results suggest that, despite the exponential decrease in the number of ballistic photons as the optical thickness of fog increases, the Physics-Driven Swin Transformer method demonstrates satisfactory performance in imaging targets obscured by dense fog. Importantly, this article highlights that even in dense fog imaging experiments with optical thickness reaching up to 3.0, which exceeds previous studies, commonly utilized quantitative evaluation metrics like PSNR and SSIM indicate that our method is cutting-edge in imaging through dense fog.

A New Method for Ground-Based Optical Polarization Observation of the Moon

As a natural satellite of the Earth, the moon is a prime target for planetary remote sensing exploration. However, lunar polarization studies are not popular in the planetary science community. Polarimetry of the lunar surface had not been carried out from a spacecraft until the Korean lunar exploration program was initiated. In previous polarization observations of the moon, images of different polarization states were obtained by a rotating linear polarizer. This method is not well suited for future polarization observations from space-based spacecraft. To this end, we present a new kind of polarized observation of the moon using a division of a focal-plane polarization camera and propose a pipeline on the processing method of the polarization observation of the moon. We obtain a map of the degree of white-light polarization on the nearside of the moon through polarization observation, data processing, and correction. The observation and data processing methods presented in this study have the potential to serve as a reference for analyzing polarization observation data from future orbiting spacecraft. These are expected to lead to new discoveries in the fields of astronomy and planetary science.

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.

Analysis of the effect of optical thickness on polarization in a sea fog stratified environment

Since there are usually multiple layers present in a real-world sea fog environment, and because previous studies have tended to analyze sea fog as a single layer rather than as refined layered sea fog, this paper splits sea fog into two categories: water fog and salt fog double-layer environments. By adjusting the optical thickness of the two layers of media, we may investigate the issue of the law governing the transmission of polarized light. In this paper, the analysis is mainly carried out through a simulation and experimental tests. The simulation portion is based mostly on the improved layered Monte Carlo approach, which builds a simulation model more appropriate for multilayer non-spherical media by using the accumulation principle to determine the scattering and transmission properties between layers. The tests are conducted by altering the double-layer medium's optical thickness, incoming wavelength, and polarization state, and then getting the polarization information of visible light after transmission through the complicated environment. The findings demonstrate that the optical thickness of the sea fog double-layer media affects polarized light transmission in a non-negligible way. Longer wavelength polarized light may keep polarization information better as the optical thickness increases, and circularly polarized light has polarization-preserving properties that are superior to linearly polarized light. By contrasting the simulation findings with the experimental data, the consistency of the two conclusions is confirmed, and the study offers a helpful resource for the transmission of polarized light in the sea fog environment.

Acousto-optic modulator-based improvement in imaging through scattering media

Reduced visibility is a common problem when light traverses through a scattering medium, and it becomes difficult to identify an object in such scenarios. What we believe to be a novel proof-of-principle technique for improving image visibility based on the quadrature lock-in discrimination algorithm in which the demodulation is performed using an acousto-optic modulator is presented here. A significant improvement in image visibility is achieved using a series of frames. We have also performed systematic imaging by varying the camera parameters, such as exposure time, frame rate, and series length, to investigate their effect on enhancing image visibility.

Active imaging through dense fog by utilizing the joint polarization defogging and denoising optimization based on range-gated detection

Imaging through the scattering medium, such as fog, is important for military and civilian applications. However, the fog concentration restricts the current defogging methods; the image will be seriously degraded in dense fog scenes. Here, an imaging technique by developing joint active polarization defogging and denoising optimization methods based on range-gated detection is proposed for the target in fog conditions. The range-gated imaging method shields the scattering light from outside the selected region to improve the signal intensity. The properties of signal light, backscattering light, and forward scattering light in the range-gated imaging way are analyzed experimentally and theoretically. Thus the elimination method of backscattering light is developed in terms of polarization differences in the degree of polarization and angle of polarization, and the block-matching with 3D transform-domain collaborative filtering (BM3D) algorithm is developed to remove the effect of the forward scattering light on the image. By adopting the proposed defogging method, the clear imaging of the target under fog with an optical thickness of up to 5 is realized, and the target contour and detail information are successfully recovered. Compared with the complete failure of the current defogging method, this method can recover targets with high contrast and signal-to-noise ratio in dense fog scenes, which exhibits widespread application potential for target detection and recognition in severe weather and turbid underwater environment.

Polarized light transmission characteristics in a smoky ellipsoidal particle medium

True natural environments are more complex, and light travels through non-spherical particle media, which can affect the transmission of light. The medium environment of non-spherical particles is more common than that of spherical particles, and some studies have shown that there are differences between spherical and non-spherical particles in polarized light transmission. Therefore, the use of spherical particles instead of non-spherical particles will result in great error. In view of this feature, this paper samples the scattering angle based on the Monte Carlo method, and then constructs a simulation model of a random sampling fitting phase function suitable for ellipsoidal particles. In this study, yeast spheroids and Ganoderma lucidum spores were prepared. The effects of different polarization states and optical thicknesses on the transmission of polarized light at three wavelengths were investigated using ellipsoidal particles with a ratio of 1.5 transverse to vertical axes. The results show that when the concentration of the medium environment increases, the polarized lights of different states all show obvious depolarization, but circularly polarized light has better polarization-preserving characteristics than linearly polarized light, and polarized light with larger wavelengths also shows more stable optical properties. When yeast and Ganoderma lucidum spores were used as the transport medium, the degree of polarization of polarized light had the same trend. However, the equal volume radius of yeast particles is smaller than that of Ganoderma lucidum spores, so when the laser is in the yeast particle medium, the polarization-maintaining property of polarized light is superior. This study provides an effective reference for the variation of polarized light transmission in an atmospheric transmission environment with heavy smoke.

Increased range and contrast in fog with circularly polarized imaging

Fogs, low lying clouds, and other highly scattering environments pose a challenge for many commercial and national security sensing systems. Current autonomous systems rely on optical sensors for navigation whose performance is degraded by highly scattering environments. In our previous simulation work, we have shown that polarized light can penetrate through a scattering environment such as fog. We have demonstrated that circularly polarized light maintains its initial polarization state better than linearly polarized light, even through large numbers of scattering events and thus ranges. This has recently been experimentally verified by other researchers. In this work, we present the design, construction, and testing of active polarization imagers at short-wave infrared and visible wavelengths. We explore multiple polarimetric configurations for the imagers, focusing on linear and circular polarization states. The polarized imagers were tested at the Sandia National Laboratories Fog Chamber under realistic fog conditions. We show that active circular polarization imagers can increase range and contrast in fog better than linear polarization imagers. We show that when imaging typical road sign and safety retro-reflective films, circularly polarized imaging has enhanced contrast throughout most fog densities/ranges compared to linearly polarized imaging and can penetrate over 15 to 25 m into the fog beyond the range limit of linearly polarized imaging, with a strong dependence on the interaction of the polarization state with the target materials.

Hyperspectral full polarization imaging system based on spatial modulation

The hyperspectral full polarization imaging system (HFPIS) based on spatial modulation and liquid crystal tunable filter (LCTF) can modulate the polarization information of the target into the interferogram by means of interference imaging. It has the advantage of rapid imaging of the hyperspectral full polarization information from the target, and has good real time imaging and practicality. Through the spectroscopic imaging mode of a Savart prism, the corresponding interference optical path and imaging system are designed, including a beam expander, spatial modulator, LCTF, focusing system, and imaging sensor. This system can extract the different information from the target and demodulate it so as to obtain the hyperspectral polarization image. The experiment shows that the HFPIS can reveal the texture, contour, and other details of the target in the fog, and has obvious advantages over the traditional intensity imaging methods.

Polarization dehazing method based on separating and iterative optimizing airlight from the frequency domain for different concentrations of haze

Polarization technology has been widely used in imaging through a scattering medium. However, the existing polarization dehazing methods are unstable because they require manual selections of polarization correction parameters. In addition, most of them only focus on the spatial domain without utilizing the frequency domain features, so their dehazing performances are insufficient. In this paper, we propose a polarization dehazing method based on separating and iterative optimizing airlight from the frequency domain. By separating the low-frequency sub-bands of polarization images and refining them as the airlight at three states, we calculated the Stokes parameters of airlight and obtained the preliminary dehazed image. We also propose an iterative optimization approach between the high-frequency sub-band of the dehazed image and airlight to effectively improve the dehazing performance. As a by-product, we introduce our real-world polarization datasets collected in different concentrations of haze. Both the qualitative and quantitative experiments show that our method is effective and robust in different concentrations of haze.

Non-line-of-sight imaging in the presence of scattering media using phasor fields

Non-line-of-sight (NLOS) imaging aims to reconstruct partially or completely occluded scenes. Recent approaches have demonstrated high-quality reconstructions of complex scenes with arbitrary reflectance, occlusions, and significant multi-path effects. However, previous works focused on surface scattering only, which reduces the generality in more challenging scenarios such as scenes submerged in scattering media. In this work, we investigate current state-of-the-art NLOS imaging methods based on phasor fields to reconstruct scenes submerged in scattering media. We empirically analyze the capability of phasor fields in reconstructing complex synthetic scenes submerged in thick scattering media. We also apply the method to real scenes, showing that it performs similarly to recent diffuse optical tomography methods.

A polarization-based image restoration method for both haze and underwater scattering environment

Existing polarization-based defogging algorithms rely on the polarization degree or polarization angle and are not effective enough in scenes with little polarized light. In this article, a method of image restoration for both haze and underwater scattering environment is proposed. It bases on the general assumption that gray variance and average gradient of a clear image are larger than those of an image in a scattering medium. Firstly, based on the assumption, polarimetric images with the maximum variance (Ibest) and minimum variance (Iworst) are calculated from the captured four polarization images. Secondly, the transmittance is estimated and used to remove the scattering light from background medium of Ibest and Iworst. Thirdly, two images are fused to form a clear image and the color is also restored. Experimental results show that the proposed method obtains clear restored images both in haze and underwater scattering media. Because it does not rely on the polarization degree or polarization angle, it is more universal and suitable for scenes with little polarized light.

Quantitative demonstration of the superiority of circularly polarized light in fog environments

The polarization behavior of light transmitted through scattering media is studied quantitatively. A division of focal plane (DOFP) imaging polarimeter modified with a wideband quarter-wave plate (QWP) is used to evaluate the linear and circular depolarization signals. This system allows the measurement of the linear and circular co-polarization and cross-polarization channels simultaneously. The experiments are carried out at CEREMA's 30 m fog chamber under controlled fog density conditions. The polarization memory effect with circularly polarized light is demonstrated to be superior in forward transmission compared to the same phenomena with linearly polarized light when imaging inside a scattering medium. This paves the way for its use in imaging through scattering media for hazard detection in different applications.

Polarization dehazing method based on spatial frequency division and fusion for a far-field and dense hazy image

Polarization dehazing technology is effective in imaging through scattering media because of additional information different from the light intensity and spectrum. However, the existing methods relying on the manual choice of bias factor are non-universal in different imaging conditions. In addition, these methods are not suitable for dense scenes with long distances. Aiming at the dehazing application requirements in far-field and dense hazy weather, a polarization dehazing method based on spatial frequency division and fusion (SFDF) is proposed in this paper. In addition, we optimize the interpolation process before dehazing so that the spatial resolution can be maintained without the noise influence. The experimental results indicate that the proposed method outperforms the existing schemes in dense hazy weather more than kilometer distances. Furthermore, we discuss that the effects of bias factors only act on the low-frequency parts of the polarization images, and their influence is greatly weakened after being fused with the high-frequency parts. This robust advantage without manual intervention causes the proposed SFDF method to have a broader prospect in practical application scenarios.

Polarization enhancement of linearly polarized light through foggy environments at UV-NIR wavelengths

Wavelength is an essential factor affecting polarization propagation. We investigate the polarization persistence of linearly polarized light from ultraviolet to near-IR in foggy environments. Certain spectral bands, from ultraviolet to IR wavelengths that exhibit lower path loss, were initially selected. Using polarization-tracking Monte Carlo simulations for varying particle size, wavelength, refractive index and detection range, it is shown that linear polarization exhibits different persistence performance at different wavelengths in various foggy environments. For wet haze of 0.6 µm or 1 µm droplets, parallel polarization increases persistently as the wavelength increases, and has superior persistence in the near-IR region. For radiation fog of 5 µm or 7.5 µm droplets, parallel polarization shows superior persistence in the ultraviolet region. For advection fog of 15 µm or 45 µm droplets, parallel polarization shows a superior persistence in the ultraviolet region. It is therefore shown that changing the wavelength can improve linear polarization persistence in foggy environments.

Low-pass filtering based polarimetric dehazing method for dense haze removal

Polarimetric dehazing method is very promising in enhancing the quality of images captured in the scattering media. However, it is found that the dehazing results calculated by hazy images are very sensitive to the noise, which may cause the method unstable or even invalid. To overcome this drawback and enhance the capability and stability of the polarimetric dehazing method, digital image processing algorithms or bias parameters need to be added into the method, however, they will make the algorithm complex and time consuming. In this paper, using low pass filter to suppress the noise of the hazy images, a novel polarimetric dehazing method is proposed to enhance the visibility of hazy images, especially for dense haze removal. Experimental results demonstrate that this method is totally automatic and very effective in dense haze processing. This method may have great potential usage in many applications, such as optical surveillance, underwater imaging, and bio-tissue imaging, etc.

Invasive and Non-Invasive Observation of Occluded Fast Transient Events: Computational Tools

Industrial processes involving thermal plasma such as cutting, welding, laser machining with ultra-short laser pulses (nonequilibrium conditions), high temperature melting using electrical discharge or ion-beams, etc., generate non-repeatable fast transient events which can reveal valuable information about the processes. In such industrial environments containing high temperature and radiation, it is often difficult to install conventional lens-based imaging windows and components to observe such events. In this study, we compare imaging requirements and performances with invasive and non-invasive modes when a fast transient event is occluded by a metal window consisting of numerous holes punched through it. Simulation studies were carried out for metal windows with different types of patterns, reconstructed for both invasive and non-invasive modes and compared. Sparks were generated by rapid electrical discharge behind a metal window consisting of thousands of punched through-holes and the time sequence was recorded using a high-speed camera. The time sequence was reconstructed with and without the spatio-spectral point spread functions and compared. Commented MATLAB codes are provided for both invasive and non-invasive modes of reconstruction.

Cholesteric liquid crystal mirror-based imaging Stokes polarimeter

We have developed a cholesteric liquid crystal (CLC) mirror-based innovative model for detection and visualization of images in turbid media. Due to its unique optical-polarization properties, the CLC mirror is suggested as the basic element of the imaging Stokes polarimeter. The particular design of the proposed polarimeter, coupled with its distinguished operational simplicity, reliability, and real-time operational facilities, promises to fabricate a new generation of the imaging Stokes polarimeter, which can find applications in areas such as diagnostics, biology, astronomy, and remote sensing.

Visibility Restoration: A Systematic Review and Meta-Analysis

Image acquisition is a complex process that is affected by a wide variety of internal and environmental factors. Hence, visibility restoration is crucial for many high-level applications in photography and computer vision. This paper provides a systematic review and meta-analysis of visibility restoration algorithms with a focus on those that are pertinent to poor weather conditions. This paper starts with an introduction to optical image formation and then provides a comprehensive description of existing algorithms as well as a comparative evaluation. Subsequently, there is a thorough discussion on current difficulties that are worthy of a scientific effort. Moreover, this paper proposes a general framework for visibility restoration in hazy weather conditions while using haze-relevant features and maximum likelihood estimates. Finally, a discussion on the findings and future developments concludes this paper.

Polarization resolving and imaging with a single-photon sensitive superconducting nanowire array

Superconducting nanowire single-photon detectors (SNSPDs) have attracted remarkable interest for visible and near-infrared single-photon detection due to their outstanding performance. However, conventional SNSPDs are generally used as binary photon-counting detectors. Another important characteristic of light, i.e., polarization, which can provide additional information of the object, has not been resolved using the standalone SNSPD. In this work, we present a first prototype of the polarimeter based on a four-pixel superconducting nanowire array, capable of resolving the polarization state of linearly-polarized light at the single-photon level. The detector array design is based on a division of focal plane configuration in which the orientation of each nanowire division (pixel) is offset by 45°. Each single nanowire pixel operates as a combination of a photon detector and almost linear polarization filter, with an average polarization extinction ratio of ∼10. The total system detection efficiency of the array is ∼1% at a total dark count rate of 680 cps, with a timing jitter of 126 ps, when the detector array is free-space coupled and illuminated with 1550-nm photons. The mean errors of the measured angle of polarization and degree of linear polarization were about -3° and 0.12, respectively. Furthermore, we successfully demonstrated polarization imaging at low-light level using the proposed detector. Our results pave the way for the development of a single-photon sensitive, fast, and large-scale integrated polarization polarimeter or imager. Such detector may find promising application in photon-starved polarization resolving and imaging with high spatial and temporal resolution.

Review on Complete Mueller Matrix Optical Scanning Microscopy Imaging

Optical scanning microscopy techniques based on the polarization control of the light have the capability of providing non invasive label-free contrast. By comparing the polarization states of the excitation light with its transformation after interaction with the sample, the full optical properties can be summarized in a single 4×4 Mueller matrix. The main challenge of such a technique is to encode and decode the polarized light in an optimal way pixel-by-pixel and take into account the polarimetric artifacts from the optical devices composing the instrument in a rigorous calibration step. In this review, we describe the different approaches for implementing such a technique into an optical scanning microscope, that requires a high speed rate polarization control. Thus, we explore the recent advances in term of technology from the industrial to the medical applications.

Scatter-plate microscopy with spatially coherent illumination and temporal scatter modulation

Scatter-plate microscopy (SPM) is a lensless imaging technique for high-resolution imaging through scattering media. So far, the method was demonstrated for spatially incoherent illumination and static scattering media. In this publication, we demonstrate that these restrictions are not necessary. We realized imaging with spatially coherent and spatially incoherent illumination. We further demonstrate that SPM is still a valid imaging method for scatter-plates, which change their scattering behaviour (i.e. the phase-shift) at each position on the plate continuously but independently from other positions. Especially we realized imaging through rotating ground glass diffusers.

Circular Intensity Differential Scattering for Label-Free Chromatin Characterization: A Review for Optical Microscopy

Circular Intensity Differential Scattering (CIDS) provides a differential measurement of the circular right and left polarized light and has been proven to be a gold standard label-free technique to study the molecular conformation of complex biopolymers, such as chromatin. In early works, it has been shown that the scattering component of the CIDS signal gives information from the long-range chiral organization on a scale down to 1/10th-1/20th of the excitation wavelength, leading to information related to the structure and orientation of biopolymers in situ at the nanoscale. In this paper, we review the typical methods and technologies employed for measuring this signal coming from complex macro-molecules ordering. Additionally, we include a general description of the experimental architectures employed for spectroscopic CIDS measurements, angular or spectral, and of the most recent advances in the field of optical imaging microscopy, allowing a visualization of the chromatin organization in situ.

Study of polarization memory's impact on detection range in natural water fogs

The influence of the initial polarization state of a source on the detection range of a system probing through natural dense water fog is analyzed. Information about the source is conveyed by ballistic, snake, and highly scattered photons. During propagation, the polarization state of ballistic and snake photons is not altered. It is shown that though circular polarization is not altered by simple direction changes during scattering, and has thus a tendency to be preserved longer in the highly scattered photons, it does not necessarily convey more useful information about the source than linear polarization or even an unpolarized beam. It is also shown that in any forward propagating system that can be described by the small-angle approximation the impact of polarization memory can be neglected.

An all-optical technique enables instantaneous single-shot demodulation of images at high frequency

High-frequency demodulation of wide area optical signals in a snapshot manner remains a technological challenge. If solved, it could open tremendous perspectives in 3D imaging, vibrometry, free-space communications, automated vision, or ballistic photon imaging in scattering media with numerous applications in smart autonomous vehicles and medical diagnosis. We present here a snapshot quadrature demodulation imaging technique, capable of estimating the amplitude and phase from a single acquisition, without synchronization of emitter and receiver, and with the added capability of continuous frequency tuning. This all-optical optimized setup comprises an electro-optic crystal acting as a fast sinusoidal optical transmission gate, and allows four quadrature image channels to be recorded simultaneously with any conventional camera. We report the design, experimental validation and examples of applications of such wide-field quadrature demodulating system that allowed snapshot demodulation of images with good spatial resolution and continuous frequency selectivity up to a few 100s of kilohertz.

Traditional lock-in detection methods have been limited for wide-field imaging. Here, the authors present an all-optical design which enables four quadrature image channels to be recorded simultaneously, and show demodulation of wide-field images based on a single frame acquisition.

Off-axis spatiotemporally gated multimode detection toward deep fog imaging

Towards better imaging through fog for vehicles, we developed an off-axis spatiotemporally gated multimode laser scanning imaging system. Utilizing fog mimicking liquid suspension, we tested the imaging system with different levels of scattering. The experimental results suggest that the system can yield high quality images at seven scattering path lengths, which far exceeds the capability of conventional imaging systems. We also found that the multimode detection can not only make the system robust in the presence of aberration but also help to reduce the speckles in images for the case of coherent illumination.

Image reconstruction and enhancement by deconvolution in scatter-plate microscopy

We investigated the capabilities of deconvolution for image enhancement in scatter-plate microscopy. This lensless imaging technique enables the investigation of microstructures through scattering media by cross-correlating the scattered light intensity with a previously recorded point spread function (PSF) of the scattering medium. The autocorrelation function of the PSF appears as the transfer function of the imaging process. Deconvolution methods use the knowledge of this transfer function to enhance the image quality by reducing the blur and strengthening the contrast with the objective to achieve diffraction-limited resolution. We obtained significant image enhancement both with means of inverse filtering and by applying iterative deconvolution algorithms.

Propagation of linear and circular polarization in a settling smoke environment: theory and experiment

We present both simulated and experimental methods to show the propagation of linear and circular polarization under a settling smoke environment. Simulations and experiments were performed for artificial smoke with typical visible wavelengths under light and a moderate settling smoke environment. Using the Monte Carlo method, we simulate polarization propagation with the particle size, refractive index, wavelength, and the optical length under the settling smoke. Through the real-time measurement of optical length during the settling, the relationship of the settling environment parameters in the experiment and the simulation is established to improve the delay problem of the traditional parameters' measurement. Both the experimental and the simulated results show that with the increase of the concentration the degree of polarization of the four types of polarization is constantly decreased during the settling. Circular polarization persists better than linear polarization with the higher concentration, and the longer the wavelength, the better the persistence of the circular polarization. This study expands the application range of polarization under a settling smoke environment.

Contrast optimization in broadband passive polarimetric imaging based on color camera

Broadband polarimetric imaging consists of forming an image under spectrally wide illumination after having optimized the polarization state analyzer (PSA) to maximize the target/background discriminability. In previous works, the image sensor was monochrome, and only the intensity contrast was optimized. However, due to its spectrally varying response, the PSA not only changes the light's intensity, but also its color. This color information can serve as a further parameter to improve discrimination. In this paper, we employ a color camera in a broadband Stokes (passive) polarimetric imaging system and take into color difference's contribution to discrimination ability in optimizing the PSA setting. We show through experiments that a significant improvement of discrimination ability over monochrome imaging is obtained, especially when there are multiple objects in the scene.

Polarimetric image recovery in turbid media employing circularly polarized light

Circular polarization memory is a well-known phenomenon indicating that the circular polarization light can persist better its polarization property during propagating through turbid media compared with the linear polarization light. Therefore, in principle, using circularly polarized light can probably improve the quality of image recovery in dense turbid media than using the linearly polarized light. In this paper, we propose a new polarimetric image recovery method in dense turbid media with the illumination light of circular polarization, and we realize the image recovery combining the circular polarization information and linearly polarization information. The real-world experiment results demonstrate that the proposed method is more effective than previous methods, including the traditional polarimetric image recovery method by Schechner's [Appl. Opt.42, 511 (2003)] based on linear polarization.

Polarimetric image recovery method combining histogram stretching for underwater imaging

The underwater imaging could be severely degraded by the scattering media because of the backscattered light and signal attenuation, especially in the case of strong scattering for dense turbid medium. In this paper, we propose an improved method for recovering the underwater image combining the histogram stretching and polarimetric recovery in a proper way. In this method, we stretch the histograms of the orthogonal polarization images while maintaining the polarization relation between them, and then, based on the processed orthogonal polarization images, the recovered image with higher quality can be obtained by the traditional polarimetric recovery method. Several groups of experimental results demonstrate that the quality of underwater images can be effectively enhanced by our method, and its performance is better than that of the traditional polarimetric recovery method. In particular, the proposed method is also quite effective in the condition of dense turbid medium.

Visible-IR transmission enhancement through fog using circularly polarized light

Detection range is an important factor affecting the transmission characteristics of polarized light through fog. We first selected certain spectral bands from visible to IR wavelengths that exhibit lower path loss. For both radiation fog and advection fog, these optimized wavelength ranges include 0.4-1.1 μm, 1.48-1.56 μm, 1.63-1.86 μm, 2.03-2.18 μm, and 2.39-2.45 μm, and radiation fog in particular contains 3.5-4.3 μm. The long-wave IR wavelengths were excluded due to higher absorption losses. We further investigated the transmission performance of circular and linear polarization in variable foggy environments, exploring the impact of the detection range in particular. Using polarization-tracking Monte Carlo simulations for varying particle size, wavelength, refractive index, and detection range, we show that circular polarization outperforms linear polarization when transmitting in both radiation and advection fog. For radiation fog, circular polarization persists longer than linear polarization for 5 μm and 9 μm particles over the entire optimized wavelength range from the visible to mid-wave IR (MWIR). However, linear polarization outperforms circular polarization for 1 μm particles over the entire MWIR and a part of the short-wave IR (SWIR). For advection fog, circular polarization persists longer than linear polarization for all three particle sizes (10, 20, and 40 μm) over the entire optimized wavelength range from the visible to SWIR. We show that circular polarization retains a higher degree of polarization and has better enhancement in some detection ranges.

Multiple-view polarimetric camera

A multiple-view polarimetric camera is developed. The system uses four separate action cameras, and software is employed to map the images onto each other in order to generate the Stokes vectors, the degree of linear polarization, and the angle images. To ensure robustness, an automated calibration system has been developed that ensures the pixels are correctly mapped. Video frame synchronization is also developed.

High-efficiency chiral meta-lens

We present here a compact metasurface lens element that enables simultaneous and spatially separated imaging of light of opposite circular polarization states. The design overcomes a limitation of previous chiral lenses reliant on the traditional geometric phase approach by allowing for independent focusing of both circular polarizations without a 50% efficiency trade-off. We demonstrate circular polarization-dependent imaging at visible wavelengths with polarization contrast greater than 20dB and efficiencies as high as 70%.

Airlight-imposed errors for space-object polarimetric observations from the ground

We discuss and characterize how polarimetric sensing is contaminated by various "airlight" phenomena, as well as unpolarized light from the target, when space objects are observed with a ground-based telescope. Estimates of the polarization state are limited by unpolarized target light regardless of sensor technology or estimator algorithm, and increased target brightness actually degrades estimation of the S₁, S₂, and S₃ Stokes parameters if the added light is unpolarized. Unpolarized airlight in the field of view has an identical degrading effect. Atmospheric scattering can significantly polarize airlight, so airlight polarization must be calibrated and subtracted from the estimated target polarization. We derive an expression for the mean-square Stokes estimation error when noisy, biased estimates for the airlight polarization state are subtracted from noisy, biased estimates of the target polarization state; this expression shows that target and airlight Stokes estimation noise and bias generally sum in the ms estimation error for airlight-calibrated target Stokes. While SNR for the estimate of a given Stokes parameter increases with the magnitude of that parameter, estimation bias also appears to be correlated with magnitude. We note that when the linear Stokes reference is not arbitrary, requiring a rotational transformation of the estimated Stokes vector, the SNRs of the S₁ and S₂ estimates vary with the rotation angle. Finally, we show that measured data can be used in numerical calculations described here to approximate the errors associated with Stokes estimation, with or without airlight calibration.

Fundamental limits of target detection performance in passive polarization imaging

We quantitatively determine the target detection performance of different passive polarization imaging architectures perturbed by signal-independent detection noise or signal-dependent Poisson shot noise. We compare the fully adaptive polarimetric imager and the best channel of a static polarimetric imager, and in each case, we compare the use of a polarizer and a polarizing beam splitter as the polarization analyzing device. For all these configurations, we derive a closed-form expression of the target/background separability and quantify the performance gain brought by polarization imaging compared to standard intensity imaging. We show in particular that all the considered polarimetric imaging configurations but one require a minimum value of the polarimetric contrast in order to outperform intensity imaging. The only configuration that always performs better than intensity imaging uses a polarizing beam splitter in the presence of background shot noise. These results are useful in evaluating the fundamental limits of the gain brought by polarization imaging and determining, in practice, which type of imaging architecture is preferable for a given application.

Polarimetric dehazing method for visibility improvement based on visible and infrared image fusion

Polarimetric dehazing methods have proven effective in enhancing the quality of chromatic hazy images. Considering that the infrared radiance has a better capacity for traveling through the haze, in this paper we propose a polarimetric dehazing method based on visible and infrared image fusion to improve the visibility of hazy images, especially for dense haze conditions. Experimental results demonstrate that the visibility of hazy images can be effectively enhanced, and the color information can be finely maintained. The visibility of dehazed images can be promoted at least 100%. This kind of dehazing method can be used widely in many dehazing applications.

Scanning Mueller polarimetric microscopy

A full Mueller polarimeter was implemented on a commercial laser-scanning microscope. The new polarimetric microscope is based on high-speed polarization modulation by spectral coding using a wavelength-swept laser as a source. Calibration as well as estimation of the measurement errors of the device are reported. The acquisition of Mueller images at the speed of a scanning microscope is demonstrated for the first time. Mueller images of mineral and biological samples illustrate this new polarimetric microscopy.

Long-working-distance microscopic imaging in a turbid medium by use of an ultrafast optical Kerr gate

We demonstrate a long-working-distance microscopic imaging of hidden objects in a turbid medium by use of an ultrafast optical Kerr gate (OKG). The results show that the working distance and the spatial resolution of the long-working-distance microscopic imaging system have been increased simultaneously compared with those of the conventional 4f OKG imaging systems. A compound lens consisting of a long-focus achromatic doublet and a microscope objective is used to increase the long working distance and ensure the sufficient spatial resolution. The microscopic OKG imaging system with a working distance of 245 mm and a maximal spatial resolution of approximately 7 μm has been performed.

Real-time imaging through strongly scattering media: seeing through turbid media, instantly

Numerous everyday situations like navigation, medical imaging and rescue operations require viewing through optically inhomogeneous media. This is a challenging task as photons propagate predominantly diffusively (rather than ballistically) due to random multiple scattering off the inhomogenieties. Real-time imaging with ballistic light under continuous-wave illumination is even more challenging due to the extremely weak signal, necessitating voluminous data-processing. Here we report imaging through strongly scattering media in real-time and at rates several times the critical flicker frequency of the eye, so that motion is perceived as continuous. Two factors contributed to the speedup of more than three orders of magnitude over conventional techniques - the use of a simplified algorithm enabling processing of data on the fly, and the utilisation of task and data parallelization capabilities of typical desktop computers. The extreme simplicity of the technique, and its implementation with present day low-cost technology promises its utility in a variety of devices in maritime, aerospace, rail and road transport, in medical imaging and defence. It is of equal interest to the common man and adventure sportsperson like hikers, divers, mountaineers, who frequently encounter situations requiring realtime imaging through obscuring media. As a specific example, navigation under poor visibility is examined.

Orthogonality breaking through few-mode optical fiber

Polarization sensing and imaging through optical fibers is a technological challenge motivated by promising applications for in vivo, in situ polarimetric endoscopy for biomedical diagnosis. Among the recent approaches proposed to solve this issue, the depolarization/dichroism sensing by polarization orthogonality breaking (DSOB) technique was shown to perform remotely through single-mode optical fibers for depolarization/diattenuation measurements. In this article, we investigate the applicability of such a technique in slightly multimode waveguides. Through theoretical modeling and numerical simulations, we evidence the conditions required for the polarization orthogonality to be preserved after propagation in a few-mode fiber, notably in terms of detection geometry of the spatial modes. Original experiments realized in few-mode fibers both in transmission and reflection configurations are also reported and validate the theoretical predictions. These results allow us to analyze the influence of the experimental parameters, such as detection geometry, sample tilt, or fiber length, on orthogonality preservation and on the measurement dynamics of the DSOB technique in slightly multimode waveguides.

Orthogonality-breaking sensing model based on the instantaneous Stokes vector and the Mueller calculus

Polarimetric sensing by orthogonality breaking has been recently proposed as an alternative technique for performing direct and fast polarimetric measurements using a specific dual-frequency-dual-polarization (DFDP) source. Based on the instantaneous Stokes-Mueller formalism to describe the high-frequency evolution of the DFDP beam intensity, we thoroughly analyze the interaction of such a beam with birefringent, dichroic, and depolarizing samples. This allows us to confirm that orthogonality breaking is produced by the sample diattenuation, whereas this technique is immune to both birefringence and diagonal depolarization. We further analyze the robustness of this technique when polarimetric sensing is performed through a birefringent waveguide, and the optimal DFDP source configuration for fiber-based endoscopic measurements is subsequently identified. Finally, we consider a stochastic depolarization model based on an ensemble of random linear diattenuators, which makes it possible to understand the progressive vanishing of the detected orthogonality-breaking signal as the spatial heterogeneity of the sample increases, thus confirming the insensitivity of this method to diagonal depolarization. The fact that the orthogonality-breaking signal is exclusively due to the sample dichroism is an advantageous feature for the precise decoupled characterization of such an anisotropic parameter in samples showing several simultaneous effects.

Polarimetric imaging and retrieval of target polarization characteristics in underwater environment

Polarized light fields contain more information than simple irradiance and such capabilities provide an advanced tool for underwater imaging. The concept of the beam spread function (BSF) for analysis of scalar underwater imaging was extended to a polarized BSF which considers polarization. The following studies of the polarized BSF in an underwater environment through Monte Carlo simulations and experiments led to a simplified underwater polarimetric imaging model. With the knowledge acquired in the analysis of the polarimetric imaging formation process of a manmade underwater target with known polarization properties, a method to extract the inherent optical properties of the water and to retrieve polarization characteristics of the target was explored. The proposed method for retrieval of underwater target polarization characteristics should contribute to future efforts to reveal the underlying mechanism of polarization camouflage possessed by marine animals and finally to generalize guidelines for creating engineered surfaces capable of similar polarization camouflage abilities in an underwater environment.

Improving target discrimination ability of active polarization imagers by spectral broadening

Active polarization imagers using liquid crystal variable retarders (LCVR) usually operate at one given wavelength for the sake of polarimetric accuracy. However, this often requires to use narrowband filters which reduces the amount of light entering the system and thus the signal-to-noise ratio. For applications where good target/background discriminability (contrast) is required rather than polarimetric accuracy, this may not be the best choice. In this Article, we address contrast optimization in the case of broadband active polarimetric imaging for target detection applications. Through numerical and experimental studies, we show that broadening the spectrum of the light entering the system can increase the contrast between two regions of a scene. Furthermore, we show that this contrast can be further increased by taking into account the spectral dependence of the scene and of the polarimetric properties of the imaging system in the optimization of the measurement procedure.

Polarimetric dehazing method for dense haze removal based on distribution analysis of angle of polarization

Many dehazing methods have proven to be effective in removing haze out of the hazy image, but few of them are adaptive in handling the dense haze. In this paper, based on the angle of polarization (AOP) distribution analysis we propose a kind of polarimetric dehazing method, which is verified to be capable of enhancing the contrast and the range of visibility of images taken in dense haze substantially. It is found that the estimating precision of the intensity of airlight is a key factor which determines the dehazing quality, and fortunately our method involves a high precision estimation inherently. In the experiments a good dehazing performance is demonstrated, especially for dense haze removal. We find that the visibility can be enhanced at least 74%. Besides, the method can be used not only in dense haze but also in severe sea fog.

Resolution enhancement in active underwater polarization imaging with modulation transfer function analysis

Active polarization imaging technology is a convenient and promising method for imaging in a scattering medium such as fog and turbid water. However, few studies have investigated the influence of polarization on the resolution in underwater imaging. This paper reports on the effects of polarization detection on the resolution of underwater imaging by using active polarization imaging technology. An experimental system is designed to determine the influence under various polarization and water conditions. The modulation transfer function is introduced to estimate the resolution variations at different spatial frequencies. Results show that orthogonal detection supplies the best resolution compared with other polarization directions in the turbid water. The performance of the circular polarization method is better than the linear process. However, if the light propagates under low scattering conditions, such as imaging in clean water or at small optical thickness, the resolution enhancement is not sensitive to the polarization angles.

Detection range enhancement using circularly polarized light in scattering environments for infrared wavelengths

We find for infrared wavelengths that there are broad ranges of particle sizes and refractive indices that represent fog and rain, where circular polarization can persist to longer ranges than linear polarization. Using polarization tracking Monte Carlo simulations for varying particle size, wavelength, and refractive index, we show that, for specific scene parameters, circular polarization outperforms linear polarization in maintaining the illuminating polarization state for large optical depths. This enhancement with circular polarization can be exploited to improve range and target detection in obscurant environments that are important in many critical sensing applications. Initially, researchers employed polarization-discriminating schemes, often using linearly polarized active illumination, to further distinguish target signals from the background noise. More recently, researchers have investigated circular polarization as a means to separate signal from noise even more. Specifically, we quantify both linearly and circularly polarized active illumination and show here that circular polarization persists better than linear for radiation fog in the short-wave infrared, for advection fog in the short-wave and long-wave infrared, and large particle sizes of Sahara dust around the 4 μm wavelength. Conversely, we quantify where linear polarization persists better than circular polarization for some limited particle sizes of radiation fog in the long-wave infrared, small particle sizes of Sahara dust for wavelengths of 9-10.5 μm, and large particle sizes of Sahara dust through the 8-11 μm wavelength range in the long-wave infrared.

100 kHz Mueller polarimeter in reflection configuration

A new setup is proposed to perform high-speed Mueller polarimetry by spectral coding of polarization in a reflection configuration. The system uses a swept laser source and a photodiode, which results in a simple optical setup that allows measurement of Mueller matrices at 100 kHz repetition rate. A special focus is made on the influence of the cube beam splitter polarimetric response, which is essential to measurements in a reflection configuration. The instrument is first validated on reference samples for single-point measurements, and the effect of a proper system calibration is also demonstrated on polarimetric images. The device is intended to be implemented within a laser scanning microscope to perform multimodal imaging (confocal/multiphoton and Mueller polarimetry).

Contrast optimization in broadband passive polarimetric imaging

Polarimetric imaging is often performed using light with a narrow spectrum for the sake of polarization measurement accuracy. However, due to the use of narrowband filters, this reduces the amount of light entering the system and thus the signal-to-noise ratio. This may not be the best choice for target detection applications, where a high target contrast is required rather than polarimetric accuracy. We address contrast optimization for broadband passive polarimetric imaging. We show through simulation and experiments that polarimetric contrast can be significantly increased by broadening the spectrum of analyzed light. In addition, we show that the contrast can be optimized by taking into account the spectral dependence of the scene and of the polarization analysis devices.