Backscattering target detection in a turbid medium by polarization discrimination

Spatial helicity response metric to quantify particle size and turbidity of heterogeneous media through circular polarization imaging

Backscattered circularly polarized light from turbid media consists of helicity-flipped and helicity-preserved photon sub-populations (i.e., photons of perpendicular and parallel circular handedness). Their intensities and spatial distributions are found to be acutely sensitive to average scatterer size and modestly sensitive to the scattering coefficient (medium turbidity) through an interplay of single and multiple scattering effects. Using a highly sensitive intensified-CCD camera, helicity-based images of backscattered light are captured, which, with the aid of corroborating Monte Carlo simulation images and statistics, enable (1) investigation of subsurface photonic pathways and (2) development of the novel 'spatial helicity response' metric to quantify average scatterer size and turbidity of tissue-like samples. An exciting potential application of this work is noninvasive early cancer detection since malignant tissues exhibit alterations in scatterer size (larger nuclei) and turbidity (increased cell density).

Radiofrequency modulator for marine lidar radar systems featuring compact and agile extra-cavity architecture using a polarimetric effect

This paper proposes a new, to the best of our knowledge, modulator architecture for a microwave-modulated lidar for marine applications. The principle is based on the use of an infrared picosecond laser source, coupled to an external cavity, ensuring wavelength conversion in the visible range as well as radiofrequency modulation. Wavelength conversion is performed by a nonlinear crystal associated with adapted mirrors, while multiple round trips and polarization control in the cavity ensure microwave modulation. This paper presents both modeling of the emitted signal and the practical realization of this device.

Experimental Study on Bottom-Up Detection of Underwater Targets Based on Polarization Imaging

Previous studies on the polarization imaging of underwater targets mainly focused on top-down detection; however, the capacities of bottom-up detection were poorly known. Based on in situ experiments, the capability of bottom-up detection of underwater targets using polarization imaging was investigated. First, to realize the objective of bottom-up polarization imaging, a SALSA polarization camera was integrated into our Underwater Polarization Imaging System (UPIS), which was integrated with an attitude sensor. At Qiandao Lake, where the water is relatively clear, experiments were conducted to examine the capacity of the UPIS to detect objects from the bottom up. Simultaneously, entropy, clarity, and contrast were adopted to compare the imaging performance with different radiation parameters. The results show that among all the used imaging parameters, the angle of polarization is the optimal parameter for bottom-up detection of underwater targets based on polarization imaging, which may result from the different diffused reflectance of the target surface to the linear polarization components of the Stokes vector.

Polarization memory rate as a metric to differentiate benign and malignant tissues

Non-invasive optical methods for cancer diagnostics, such as microscopy, spectroscopy, and polarimetry, are rapidly advancing. In this respect, finding new and powerful optical metrics is an indispensable task. Here we introduce polarization memory rate (PMR) as a sensitive metric for optical cancer diagnostics. PMR characterizes the preservation of circularly polarized light relative to linearly polarized light as light propagates in a medium. We hypothesize that because of well-known indicators associated with the morphological changes of cancer cells, like an enlarged nucleus size and higher chromatin density, PMR should be greater for cancerous than for the non-cancerous tissues. A thorough literature review reveals how this difference arises from the anomalous depolarization behaviour of many biological tissues. In physical terms, though most biological tissue primarily exhibits Mie scattering, it typically exhibits Rayleigh depolarization. However, in cancerous tissue the Mie depolarization regime becomes more prominent than Rayleigh. Experimental evidence of this metric is found in a preliminary clinical study using a novel Stokes polarimetry probe. We conducted in vivo measurements of 20 benign, 28 malignant and 59 normal skin sites with a 660 nm laser diode. The median PMR values for cancer vs non-cancer are significantly higher for cancer which supports our hypothesis. The reported fundamental differences in depolarization may persist for other types of cancer and create a conceptual basis for further developments in polarimetry applications for cancer detection.

Polarization-Sensitive and Wide Incidence Angle-Insensitive Fabry-Perot Optical Cavity Bounded by Two Metal Grating Layers

Infrared (IR) polarimetric imaging has attracted attention as a promising technology in many fields. Generally, superpixels consisting of linear polarizer elements at different angles plus IR imaging array are used to obtain the polarized target signature by using the detected polarization-sensitive intensities. However, the spatial arrangement of superpixels across the imaging array may lead to an incorrect polarimetric signature of a target, due to the range of angles from which the incident radiation can be collected by the detector. In this article, we demonstrate the effect of the incident angle on the polarization performance of an alternative structure where a dielectric layer is inserted between the nanoimprinted subwavelength grating layers. The well-designed spacer creates the Fabry–Perot cavity resonance, and thereby, the intensity of transverse-magnetic I-polarized light transmitted through two metal grating layers is increased as compared with a single-layer metal grating, whereas transverse-electric (TE)-transmitted light intensity is decreased. TM-transmittance and polarization extinction ratio (PER) of normally incident light of wavelength 4.5 μm are obtained with 0.49 and 132, respectively, as the performance of the stacked subwavelength gratings. The relative change of the PERs for nanoimprint-lithographically fabricated double-layer grating samples that are less than 6% at an angle of incidence up to 25°, as compared to the normal incidence. Our work can pave the way for practical and efficient polarization-sensitive elements, which are useful for many IR polarimetric imaging applications.

Microsphere-assisted super-resolved Mueller matrix microscopy

Mueller matrix microscopy (MMM) is a powerful approach to probe microstructural and optical information of many important specimens (e.g., tissue and bacteria), which otherwise cannot be obtained directly from intensity or spectral images. Achieving high lateral resolution in MMM, similar to other microscopy approaches, remains a challenge. Here, we extend the idea of microsphere (MS) -assisted microscopy into MMM toward resolution-enhanced polarimetric imaging. The goal is achieved by insertion of a transparent MS in the working distance of the imaging microscope objective in the optical train of an MMM system. We experimentally show that an MS close to the sample in MMM may increase the resolution beyond the intrinsic diffraction limit of the system by redirecting the higher spatial frequencies of the sample into the acceptance cone. In order to be a case in point, the experiment is conducted on a standard holographic diffraction grating with 1 µm line-width, which is beyond the diffraction limit of a 10× objective. Two-dimensional images of the Mueller matrix and some of the widely used quantitative polarimetric parameters of the sample are calculated and compared in the two cases before and after insertion of MS. The proposed arrangement is easy to implement and has the potential to serve as a high-resolution polarimetric microscope for visualizing the polarization characteristics of the microscopic objects.

Different orders of scattering through time-resolved Mueller matrix imaging estimates of pre-malignancy in human cervical tissues

Time-resolved Mueller matrix (MM) imaging polarimetry in transmission mode has been implemented in both epithelium and stromal regions of cervical tissues to explore the various polarization dynamics in connection with the diagnosis of cervical precancer. The picosecond-resolved intensity patterns of various MM elements, resulting from the various orders of scattering, at different time delays provide clear demarcation between the epithelium and stroma of cervical tissue. The time dependent depolarization and retardance maps are seen to differentiate the epithelium from stroma. The average values of time dependent linear, linear-45, and circular depolarization and linear, circular, and scalar retardance parameters in different regimes of scattering from the optically anisotropic stromal region identify the pre-malignancy in cervical tissue. As the disease evolves, time dependent linear depolarization varies to larger values as compared to time dependent circular depolarization. Interestingly, the chirality of the collagen network that rotates the plane of polarized light in either direction in normal samples is limited to only the clockwise direction during the progression of the disease. These results show potential in the early detection and understanding of the mechanisms of morphological changes in cervical cancer development.

Assessment of anisotropy of collagen structures through spatial frequencies of Mueller matrix images for cervical pre-cancer detection

Analysis of spatial frequency of Mueller matrix (MM) images in the Fourier domain yields quantifying parameters of anisotropy in the stromal region in normal and precancerous tissue sections of human uterine cervix. The spatial frequencies of MM elements reveal reliable information of microscopic structural organization arising from the different orientations of collagen fibers in the connective tissue and their randomization with disease progression. Specifically, the local disorder generated in the normal periodic and regular structure of collagen during the growth of the cervical cancer finds characteristic manifestation in the Fourier spectrum of the selected Mueller matrix elements encoding the anisotropy effects through retardance and birefringence. In contrast, Fourier spectra of differential polarization gated images are limited to only one orientation of collagen. Fourier spectra of first row elements M11, M12, M13, and M14 and first column elements M11, M21, M31, and M41 discriminates cervical inter-epithelial neoplasia (CIN)-I from normal cervical tissue samples with 95%-100% sensitivity and specificity. FFT spectra of first and fourth row elements classify CIN-I and CIN-II grades of cervical cancerous tissues with 90%-100% sensitivity and 87%-100% specificity. Normal and CIN-II grade samples are successfully discriminated through Fourier spectra of every MM element while that of M31 element arises as the key classifier among normal, CIN-I, and CIN-II grades of cervical cancer with 100% sensitivity and specificity. These results demonstrate the promise of spatial frequency analysis of Mueller matrix images as a novel, to the best of our knowledge, approach for cancer/precancer detection.

Broadband Linear-to-Circular Polarization Conversion Enabled by Birefringent Off-Resonance Reflective Metasurfaces

Due to the scarcity of circular polarization light sources, linear-to-circular polarization conversion is required to generate circularly polarized light for a variety of applications. Despite significant past efforts, broadband linear-to-circular polarization conversion remains elusive particularly in the terahertz and midinfrared frequency ranges. Here we propose a novel mechanism based on coupled mode theory, and experimentally demonstrate at terahertz frequencies that highly efficient (power conversion efficiency approaching unity) and ultrabroadband (fractional bandwidth up to 80%) linear-to-circular polarization conversion can be accomplished by the judicious design of birefringent metasurfaces. The underlying mechanism operates in the frequency range between well separated resonances, and relies upon the phase response of these resonances away from the resonant frequencies, as well as the balance of the resonant and nonresonant channels. This mechanism is applicable for any operating frequencies from microwave to visible. The present Letter potentially opens a wide range of opportunities in wireless communications, spectroscopy, and emergent quantum materials research where circularly polarized light is desired.

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.

Mueller matrix polarimetry with 3D integral imaging

In this paper, we introduce the Mueller matrix imaging concepts for 3D Integral Imaging Polarimetry. The Mueller matrix of a complex scene is measured and estimated with 3D integral imaging. This information can be used to analyze the complex polarimetric behavior of any 3D scene. In particular, we show that the degree of polarization can be estimated at any selected plane for any arbitrary synthetic illumination source which may be difficult to produce in practice. This tool might open new perspectives for polarimetric analysis in the 3D domain. Also, we illustrate that 2D polarimetric images are noisier than 3D reconstructed polarimetric integral imaging. To the best of our knowledge, this is the first report on Mueller matrix polarimetry in 3D Integral Imaging.

Effect of imaging geometry and noise model on polarimetric contrast optimization

We study the effect of the noise model and imaging geometry on contrast optimization with an active polarimetric imager. Considering scenes with various polarization contrast sources and fluctuation statistics of polarimetric properties, we show that to maximize the range of scene orientation angles for which contrast is sufficient, one has to take into account not only the polarization difference between the target and background but also the variance of fluctuations in the image. In particular, for scenes exhibiting depolarization contrast, the best strategy is often to set the illumination and analysis polarization states orthogonal to each other in order to reduce the variance of the image. These results are important for designing and optimizing remote sensing or robotics-oriented polarization imagers.

Enhancing underwater optical imaging by using a low-pass polarization filter

Object identification in highly turbid optical media depends mainly on the quality of collected images. Underwater images acquired in a turbid environment are generally of very poor quality. Attenuation and backscattering of light by water, by materials dissolved in the water, and by particulate material are the main causes of the degradation of underwater images. It is therefore essential to improve the quality of such images to facilitate object identification. The focus of this paper is to report the principle and validation of a fast and effective method of improving the quality of underwater images. On the one hand, this method uses a polarimetric imaging optical system to reduce the effect of diffusion on the image acquisition. On the other hand, it is based on an optimized version of the dark channel prior (DCP) method that has received a great deal of attention for image dehazing. Results derived from images obtained in a controlled laboratory water tank environment with different turbidity conditions and images from tests using the proposed method at sea demonstrate an ability to significantly improve visibility and reduce runtime by a factor of about 50 for a 4K image when compared to conventional DCP methods.

Extra-cavity radiofrequency modulator for a lidar radar designed for underwater target detection

The hybrid lidar-radar technique is commonly used for shallow underwater target detection. This technique requires a powerful blue-green RF-modulated laser with stable microwave frequency. In this paper, a novel modulator design perfectly suited for this application is proposed. It consists of an extra-cavity modulator composed of two plane mirrors and a second-harmonic generator stage within the cavity to maximize the energy efficiency. This modulator is coupled with a picosecond Nd:YAG infrared laser source to insure the emission of a powerful pulse (up to 5 mJ). Results considering this architecture show stable modulation frequency and a powerful modulated beam. As the emitted signal lasts only a few nanoseconds, a range-gating method can be used for target localization. This emitted signal has experimentally proven to preserve input beam polarization, which also can be employed to enhance the target return.

Active underwater descattering and image recovery

Underwater imaging is a promising but challenging topic due to the scattering particles in water, which result in serious light attenuation. Therefore, underwater images suffer from low-contrast and low-resolution issues. In this study, in order to recover high-quality underwater images, the point spread functions (PSFs) are estimated by a slant-edge method. The experiment modulates the illumination source to deal with backscattering and the imager to take two images in orthogonally polarized states. This imaging method benefits the satisfactory edge extraction. The PSF estimation is performed based on the extracted slant edge to enable recovery of the image. In addition, the modulation transfer function (MTF) is introduced to evaluate the resolution variation with the spatial frequencies. It manifests considerable resolution enhancement in the recovered images. Moreover, the proposed underwater image recovery method also reduces the effect from the scattering as an effective compensation to the polarization imaging approach.

Numerical investigation of polarization filtering for direct optical imaging within scattering media

We investigate the ability of polarization filtering to improve direct imaging of absorbing objects which are buried within scattering environments. We extend on previous empirical investigations by exploiting an efficient perturbation-based formalism, which is applicable to arbitrarily arranged sources and detectors with arbitrary polarizations. From this approach, we are able in some cases to find certain non-trivial linear combinations of polarization measurement channels that maximize the object resolution and visibility.

Novel Descattering Approach for Stereo Vision in Dense Suspended Scatterer Environments

In this paper, we propose a model-based scattering removal method for stereo vision for robot manipulation in indoor scattering media where the commonly used ranging sensors are unable to work. Stereo vision is an inherently ill-posed and challenging problem. It is even more difficult in the case of images of dense fog or dense steam scenes illuminated by active light sources. Images taken in such environments suffer attenuation of object radiance and scattering of the active light sources. To solve this problem, we first derive the imaging model for images taken in a dense scattering medium with a single active illumination close to the cameras. Based on this physical model, the non-uniform backscattering signal is efficiently removed. The descattered images are then utilized as the input images of stereo vision. The performance of the method is evaluated based on the quality of the depth map from stereo vision. We also demonstrate the effectiveness of the proposed method by carrying out the real robot manipulation task.

Evolution of circular and linear polarization in scattering environments

This work quantifies the polarization persistence and memory of circularly polarized light in forward-scattering and isotropic (Rayleigh regime) environments; and for the first time, details the evolution of both circularly and linearly polarized states through scattering environments. Circularly polarized light persists through a larger number of scattering events longer than linearly polarized light for all forward-scattering environments; but not for scattering in the Rayleigh regime. Circular polarization's increased persistence occurs for both forward and backscattered light. The simulated environments model polystyrene microspheres in water with particle diameters of 0.1 μm, 2.0 μm, and 3.0 μm. The evolution of the polarization states as they scatter throughout the various environments are illustrated on the Poincaré sphere after one, two, and ten scattering events.

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.

Optimal estimation in polarimetric imaging in the presence of correlated noise fluctuations

We quantitatively analyze how a polarization-sensitive imager can overcome the precision of a standard intensity camera when estimating a parameter on a polarized source over an intense background. We show that the gain is maximized when the two polarimetric channels are perturbed with significantly correlated noise fluctuations. An optimal estimator is derived and compared to standard intensity and polarimetric estimators.

Target detection in turbid medium using polarization-based range-gated technology

Range-gated technology is well known for its good reliability, large field of view (FOV) and low cost in target detection through scattering or turbid medium. However, the tail-gating technology suffers from low signal-to-noise ratio in high turbidity levels due to superposition of photons multiply scattered from the medium and that reflected from the target. In this paper, polarization properties of multiply scattered photons emerging from the turbid medium are studied. Results demonstrate that diffusive photons are almost completely depolarized with no diattenuation and retardance. We combined the tail-gated technology with polarization detection method to effectively image in high level of turbidity. This approach showed about two times enhancement in image contrast as compared with the conventional range-gated technology.

Exploring underwater target detection by imaging polarimetry and correlation techniques

Underwater target detection is investigated by combining active polarization imaging and optical correlation-based approaches. Experiments were conducted in a glass tank filled with tap water with diluted milk or seawater and containing targets of arbitrary polarimetric responses. We found that target estimation obtained by imaging with two orthogonal polarization states always improves detection performances when correlation is used as detection criterion. This experimental study illustrates the potential of polarization imaging for underwater target detection and opens interesting perspectives for the development of underwater imaging systems.

Selective polarization imager for contrast enhancements in remote scattering media

Conventional intensity imaging through turbid media suffers from rapid loss of image contrast due to light scattering from particles or random variations of refractive index. This paper features the development of an active imaging, snapshot, system design and postprocessing algorithms that differentiate between radiation that scatters or reflects from remote, obscured objects and the radiation from the scattering media itself through a combination of polarization difference imaging, channel blurring, and Fourier spatial filtering. The produced sensor acquires and processes image data in real time, yielding improved image contrasts by factors of 10 or greater for dense water vapor obscurants.

Adaptive target detection with a polarization-sensitive optical system

We developed an adaptive polarimetric target detector (APTD) to determine the optimum combination strategy for a multichannel polarization-sensitive optical system. The proposed algorithm is based on scene-derived polarization properties of the target and background, and it seeks to find an optimum multichannel combination of linear polarizing filters that maximizes the signal-to-clutter ratio (SCR) in intensity and Stokes parameter images. The algorithm is validated by performing RX anomaly detection and a generalized likelihood ratio test on both synthetic and real imagery. The experimental results are analyzed through calculated SCR and receiver operating characteristic curves. Compared with several conventional operation methods, we find that better target detection performance is achieved with the APTD algorithm.

Sources of possible artefacts in the contrast evaluation for the backscattering polarimetric images of different targets in turbid medium

It is known that polarization-sensitive backscattering images of different objects in turbid media may show better contrasts than usual intensity images. Polarimetric image contrast depends on both target and background polarization properties and typically involves averaging over groups of pixels, corresponding to given areas of the image. By means of numerical modelling we show that the experimental arrangement, namely, the shape of turbid medium container, the optical properties of the container walls, the relative positioning of the absorbing, scattering and reflecting targets with respect to each other and to the container walls, as well as the choice of the image areas for the contrast calculations, can strongly affect the final results for both linearly and circularly polarized light.

Experimental and theoretical evaluation of rotating orthogonal polarization imaging

Rotating orthogonal polarization imaging is a new technique that provides quantitative measurements of the polarization properties of scattering media, such as tissue, which are free from surface reflections. The technique is investigated using both experiments and Monte Carlo simulations of a polarizing target embedded within a scattering medium. The technique is sensitive to the polarization properties of the target up to a depth of 17 mean free paths. Preliminary images of bovine tendon, lamb tendon, chicken breast, and human skin are also demonstrated.

Bidirectional reflectance and polarization measurements on packed surfaces of benthic sediments and spherical particles

Laboratory bidirectional reflectance and polarization measurements were carried out on packed layers of both natural sediments and manufactured spherical particles. The results indicate that among the natural sediments showing a strong backscattering peak ("hotspot"), the rough platelets are the only sediments with a negative polarization effect. Measurements of circular and linear polarization ratios indicate that both smooth ooids and rough platelets are strongly depolarizing. Measurements of perfect spherical grains show both negative polarization and strong backscattering as a remnant of the single scattering process.

Active polarization descattering

Vision in scattering media is important but challenging. Images suffer from poor visibility due to backscattering and attenuation. Most prior methods for scene recovery use active illumination scanners (structured and gated), which can be slow and cumbersome, while natural illumination is inapplicable to dark environments. The current paper addresses the need for a non-scanning recovery method, that uses active scene irradiance. We study the formation of images under widefield artificial illumination. Based on the formation model, the paper presents an approach for recovering the object signal. It also yields rough information about the 3D scene structure. The approach can work with compact, simple hardware, having active widefield, polychromatic polarized illumination. The camera is fitted with a polarization analyzer. Two frames of the scene are taken, with different states of the analyzer or polarizer. A recovery algorithm follows the acquisition. It allows both the backscatter and the object reflection to be partially polarized. It thus unifies and generalizes prior polarization-based methods, which had assumed exclusive polarization of either of these components. The approach is limited to an effective range, due to image noise and illumination falloff. Thus, the limits and noise sensitivity are analyzed. We demonstrate the approach in underwater field experiments.

Backscatter suppression for underwater modulating retroreflector links using polarization discrimination

Free space optical links underwater have the potential to enable short range (<100 m) high-bandwidth (megabits per second) data links that have a low probability of detection and interception. The use of a retroreflecting free space optical link in water has the added advantage of allowing much of the weight and power burden of the link to remain at one end. While modulating retroreflectors have been successfully implemented in above-water links, the underwater environment introduces new challenges. The focus of this paper is to address these challenges and to investigate techniques for minimizing their effect on the link performance.

Rotating orthogonal polarization imaging

Surface reflections often present a problem in the polarization difference imaging of tissue. The technique described involves illumination in a single polarization state and detection in the orthogonal polarization state. Synchronously rotating both the illumination and orthogonal detection states provides an image free from surface reflections that is sensitive to the polarization properties of the underlying tissue. Results on tissue phantoms demonstrate that polarization sensitive measurements of a test target can be made up to a depth of 12 mean free paths within a scattering medium. Preliminary images of bovine tendon are also demonstrated.

Analytical cumulant solution of the vector radiative transfer equation investigates backscattering of circularly polarized light from turbid media

The backscattering of circularly polarized light pulses from an infinite uniform scattering medium is studied as a function of helicity of the incident light and size of scatterers in the medium. The approach considers a polarized short pulse of light incident on the scattering medium, and uses an analytical cumulant solution of the vector radiative transfer equation with the phase matrix obtained from the Mie theory to calculate the temporal profile of scattered polarized photons for any position and any angle of detection. The general expression for the scattered photon distribution function is an expansion in spatial cumulants up to an arbitrary high order. Truncating the expansion at the second-order cumulant, a Gaussian analytical approximate expression for the temporal profile of scattered polarized photons is obtained, whose average center position and half width are always exact. The components of scattered light copolarized and cross polarized with that of the incident light can be calculated and used for determining the degree of polarization of the scattered light. The results show that circularly polarized light of the same helicity dominates the backscattered signal when scatterer size is larger than the wavelength of light. For the scatterers smaller than the wavelength, the light of opposite helicity makes the dominant contribution to the backscattered signal. The theoretical estimates are in good agreement with our experimental results.

Effects of turbid media optical properties on object visibility in subsurface polarization imaging

We studied the effectiveness of using polarized illumination and detection to enhance the visibility of targets buried in highly scattering media. The effects of background optical properties including scattering coefficient, absorption coefficient, and anisotropy on image visibility were examined. Both linearly and circularly polarized light were used in the imaging. Three different types of target were investigated: scattering, absorption, and reflection. The experimental results indicate that target visibility improvement achieved by a specific polarization method depends on both the background optical properties and the target type. By analyzing all polarization images, it is possible to reveal certain information about target or the scattering background.

Review of passive imaging polarimetry for remote sensing applications

Imaging polarimetry has emerged over the past three decades as a powerful tool to enhance the information available in a variety of remote sensing applications. We discuss the foundations of passive imaging polarimetry, the phenomenological reasons for designing a polarimetric sensor, and the primary architectures that have been exploited for developing imaging polarimeters. Considerations on imaging polarimeters such as calibration, optimization, and error performance are also discussed. We review many important sources and examples from the scientific literature.

Stokes polarimetry imaging of rat tail tissue in a turbid medium: degree of linear polarization image maps using incident linearly polarized light

Illumination with incident linearly polarized light on tissue and polarization state measurements of the remitted light provide a means by which various tissue structures can be differentiated. A rat tail is embedded within a turbid gelatin such that there is a variable depth of medium above it. By varying the incident polarization angle (IPA) of the illuminating linearly polarized light, the geometry, and the orientation angle of the tissue, a series of 2-D degree of linear polarization image maps are created using our Stokes polarimetry imaging technique. The image maps show locations of the polarization-sensitive structures in the rat tail, including soft tissue, intervertebral disks, and tendons. The observed morphologies in the image maps indicate locations where the depolarization of light differs according to the tissue type and underlying layers. The data indicate the importance of varying the IPA, and that tissue dichroism and birefringence affect the degree of linear polarization image maps. Diagnostic information regarding subsurface tissue structures is obtained.

Backscattering target detection in a turbid medium by use of circularly and linearly polarized light

The polarization properties of the backscattered light from a turbid medium containing large-diameter (10.143-microm) and small-diameter (0.202-microm) spherical polystyrene particles are studied. It is shown that the difference in the polarization properties of the emerging light that originates at the target and that is backscattered from the medium allows for improvement of image contrast by use of polarized light. Based on the images obtained by the CCD camera, the polarization memory effect with circularly polarized light is demonstrated to have an advantage over the linear polarization technique in imaging a highly reflective target inside a turbid medium containing large particles.

Polarization visualization and selection of biotissue image two-layer scattering medium

We analyze and experimentally test the concept of laser polarization biotissue probing. The methods of increasing the SNR in coherent images of the optically anisotropic architectonics of the morphological biotissue structure are considered. The possibilities of polarization selection and contrasting of such images screened by other biotissues are examined. The influence of the depolarization degree of the scattered background on the SNR is investigated. The possibilities of polarization correction of the probing beam for contrasting biotissue images are analyzed.

Time-resolved backscattering of circularly and linearly polarized light in a turbid medium

Time-resolved backscattering profiles of circularly and linearly polarized light were measured from a turbid medium composed of small and large polystyrene sphere particles in water. It is shown that, based on the measurements of the time-resolved backscattered copolarized and cross-polarized components of the incident polarized light, either linearly or circularly polarized light can be used to effectively image an object that is deep inside a turbid medium composed of small particles, depending on the depolarization properties of the object itself. For large particles such as in tissue, fog, and clouds, the experimentally observed polarization memory effect on the backscattering temporal profiles suggests that a significant improvement in the image contrast can be achieved by use of circularly polarized light.

Clustering of Mueller matrix images for skeletonized structure detection

This paper extends and refines previous work on clustering of polarization-encoded images. The polarization-encoded images used in this work are considered as multidimensional parametric images where a clustering scheme based on Markovian Bayesian inference is applied. Hidden Markov Chains Model (HMCM) and Hidden Hierarchical Markovian Model (HHMM) show to handle effectively Mueller images and give very good results for biological tissues (vegetal leaves). Pretreatments attempting to reduce the image dimensionality based on the Principal Component Analysis (PCA) turns out to be useless for Mueller matrix images.

Clustering of polarization-encoded images

Polarization-encoded imaging consists of the distributed measurements of polarization parameters for each pixel of an image. We address clustering of multidimensional polarization-encoded images. The spatial coherence of polarization information is considered. Two methods of analysis are proposed: polarization contrast enhancement and a more-sophisticated image-processing algorithm based on a Markovian model. The proposed algorithms are applied and validated with two different Mueller images acquired by a fully polarimetric imaging system.

Characterization of layered scattering media using polarized light measurements and neural networks

Measurements of the spatial distributions of polarized light backscattered from a two-layer scattering medium are used to train a neural network. We investigated whether the absorption coefficients and thickness of the layer can be determined when the scattering properties are known. When determining the absorption of the upper layer or the layer's thickness, polarized light measurements provide better performance than unpolarized measurements, demonstrating the sensitivity of polarized light to superficial tissue. Determination of the lower layer's absorption coefficient is not improved by polarized light measurements. Prior knowledge of the tissue under investigation is also beneficial because errors are reduced if the range of absorption or thickness is restricted.

Analysis of the spatial distribution of polarized light backscattered from layered scattering media

The scattering of polarized light from a two layer scattering medium is investigated using Monte Carlo simulations. First order and normalized second order moments are used to analyze the spatial properties of the emerging light in different polarization states. Linearly and circularly polarized illumination is used to probe different depths. Absorption and layer thickness are varied and it is demonstrated that the determination of these values is aided by the inclusion of polarization information. The lateral and depth localization of light by polarization subtraction is also quantified. Potential applications of these techniques are burn depth and melanoma thickness measurements.

Off-diagonal Mueller matrix elements in backscattering from highly diffusive media

Measurements of a reduced Mueller matrix in backscattering from highly diffusive, dielectric samples are reported as a function of the angle of incidence. It was found that the off-diagonal terms depend greatly on the angle of incidence, increasing to a maximum near grazing incidence. We show that, despite a significant scattering originating in the bulk of such diffusive media, the nontrivial behavior of the off-diagonal Muller matrix is primarily due to surface scattering phenomena. The experimental data can be simply explained by assuming a random orientation of small particles and considering only double scattering in the plane of the surface.

Polarized pulse waves in random discrete scatterers

In recent years there has been increasing interest in the use of polarization for imaging objects in a cluttered environment. Examples are optical imaging through clouds, optical detection of objects in a biological medium, and microwave detection of objects in clutter. We extend previous studies of continuous-wave scattering to pulse-polarization scattering in discrete scatterers. We solve the time-dependent vector radiative transfer equation for a plane-parallel medium by using Mie scattering and the discrete ordinates method. The time-dependent degree of polarization and cross-polarization discrimination are calculated and verify the advantages of circular over linear polarization in maintaining greater copolarized components rather than cross-polarized components.

Visibility depth improvement in active polarization imaging in scattering media

A simple image-subtraction technique for further enhancement of the visibility depth in polarized imaging of surfaces immersed in scattering media is proposed and assessed. The technique is based on active illumination with circular or linear polarization states and image detection in the original and the opposite, or orthogonal, states. Contrast enhancement is achieved by subtraction of a fraction of the image recorded in the original state from that recorded in the opposite state. Results demonstrating the effectiveness of this method, obtained with Monte Carlo techniques, show that the visibility depth can be increased by as much as a mean free path. The results obtained are compared with those obtained by use of two alternative methods.

Excitation-and-collection geometry insensitive fluorescence imaging of tissue-simulating turbid media

We present an imaging technique for the correction of geometrical effects in fluorescence measurement of optically thick, turbid media such as human tissue. Specifically, we use the cross-polarization method to reject specular reflection and enhance the diffusive backscattering of polarized fluorescence excitation light from the turbid media. We correct the nonuniformity of the image field caused by the excitation-and-collection geometry of a fluorescence imaging system by normalizing the fluorescence image to the cross-polarized reflection image. The ratio image provides a map of relative fluorescence yield, defined as the ratio of emerging fluorescence power to incident excitation, over the surface of an imaged homogeneous turbid medium when fluorescence excitation-and-collection geometries vary in a wide range. We investigate the mechanism of ratio imaging by using Monte Carlo modeling. Our findings show that this technique could have a potential use in the detection of early cancer, which usually starts from a superficial layer of tissue, based on the contrast in the tissue fluorescence of an early lesion and of the surrounding normal tissue.

Virtues of mueller matrix imaging for underwater target detection

We present a theoretical analysis on use of polarized light in the detection of a model target in a scattering and absorbing medium similar to seawater. Monte Carlo numerical simulations are used in the calculation of the effective Mueller matrix which describes the scattering process. A target in the shape of a disk is divided into three regions, each of which has the same albedo but different reduced Mueller matrices. Contrast between various parts of the target and background is analyzed in the images created by ordinary radiance, by various elements of the Mueller matrix, and by certain suitable combinations of these elements. It is shown that the application of polarized light has distinct advantages in target detection and characterization when compared with use of unpolarized light.