Visualization of scattering media upon backscattering of a linearly polarized nonmonochromatic light

The method of polarization visualization of multiply scattering macroinhomogeneous media, based on analysis of the spatial distributions of polarization characteristics of a linearly polarized radiation backscattered from a medium, is discussed. The effect of optical characteristics of the medium and the scattering geometry on the quality of the images obtained in the case of visualization of an absorbing heterogeneity immersed into a multiply scattering medium is considered. The comparative analysis of the quality of formed images was performed with the use of different polarization characteristics of the backscattered radiation as a visualization parameter. The theoretical interpretation of the obtained experimental results is given within the framework of the phenomenological approach based on the concept of the distribution of the effective optical paths of partial components of the scattered optical field. To calculate the probability density of the effective optical paths, the statistical simulation method was used.     

Light scattering in cholesteric liquid crystals with a large pitch

Optical properties of cholesteric liquid crystals with a pitch larger than the wavelength of light are considered. Normal waves of the medium and the Green function of the electromagnetic field are analyzed. A general algorithm based on the application of the Kirchhoff method is proposed for calculating the scattered light intensity in media with a one-dimensional periodic structure. The WKB vector method is used for calculating the spatial correlation function of thermal fluctuations of the director. It is found that the transformation of two fluctuation modes takes place is some regions. The angular and polarization dependences of the intensity of light scattered from fluctuations of the director are calculated. It is found, in particular, that the intensity of scattering is a nonmonotonic function of the size of the system.     

Polarization preservation in diffusive scattering from in vivo turbid biological media: effects of tissue optical absorption in the exact backscattering direction

There is considerable recent interest in using polarized light to investigate turbid biological media. Although tissue multiple scattering randomizes incident polarization states, there are circumstances when appreciable degree of polarization can be observed in diffusive scattering. In this study, we use polarization modulation and synchronous detection to examine in the exact backscattering direction the polarization properties of diffusely reflected visible light from hands of human volunteers of varying pigmentation levels. The surviving polarization fraction increases with increasing pigmentation, likely due to preferential loss of highly scattered, long-pathlength photons; this mechanism lowers the average pathlength traversed by the detected light and hence increases the measured polarization preservation. This behavior is contrasted with the overall diffuse reflectance intensity, whose magnitude decreases with increasing absorption. These experiments demonstrate the important influences of medium optical properties on the polarization characteristics of multiply scattered light, which must be further investigated to enable quantitative polarization evaluation of turbid media such as biological tissues.     

偏振相移与相位共轭结合的散射光聚焦技术研究

散射介质对光的随机散射作用是制约其光学聚焦和成像的重要因素，光学相位共轭技术能够通过对散射光场共轭还原实现透过散射介质的光学聚焦和成像，其中散射光场相位的获取是共轭还原的核心。阐述了偏振相移的基本原理，将偏振相移与相位共轭技术相结合，设计了新的基于偏振相移的数字光学相位共轭系统。采用633 nm的HeNe激光器作为系统光源，毛玻璃作为散射介质开展散射光聚焦实验研究。实验结果表明:该装置能够成功实现透过散射介质的光学聚焦，其中聚焦点与背景光强的比值可达约400倍。     

Scattered Light Photometer for On-Line monitoring of size distribution parameters and particle number concentration

A scattered light photometer which monitors the particle number concentration of aerosols is described. The photometer measures the scattered light from illuminated submicron particle clouds with known material properties at certain scattering angles. Intensity ratios in combination with the degree of polarization are used to determine the mean particle diameter and the geometric standard deviation of an assumed log-normal particle size distribution. The determination of the particle size distribution is based on an algorithm which compares the measured and calculated (Mie theory) relative intensity quantities described. Furthermore, the particle number concentration is monitored from a single absolute intensity measurement at one scattering angle. In order to obtain quantitative results a spherical particle shape is required.     

球形雾霾粒子的紫外光散射偏振特性研究

在我国经济社会快速发展的同时,雾霾天气成为了突出的环境问题,雾霾粒子的测量非常重要。偏振紫外光与大气雾霾粒子发生散射后,散射光偏振状态(Stokes矢量以及偏振度)的改变能反映雾霾粒子的相关物理特性(粒径、复杂折射率等)。基于Mie散射理论建立了紫外光雾霾球形粒子直视和非直视单次散射模型,研究了单个球形粒子和链状结构球形粒子物理特性的改变对散射光偏振状态的影响,并用蒙特卡洛仿真分析已知粒径分布的雾霾粒子浓度对散射光偏振状态的影响。结果表明：针对单个球形粒子,随着粒子粒径的增大Stokes矢量中散射光光强(I_s)随之增强,粒子复折射率虚部为先增大后较小,偏振度也是在不断增大,且复折射率虚部较小时,偏振度增加趋势快;对于粒径分布不变的雾霾粒子,随着粒子的浓度增加,雾霾粒子的散射系数、消光系数和吸收系数均呈线性增加,但是I_s先增大后减小。针对链状球形粒子,随着粒子个数的增加,I_s均呈现增大的趋势,且偏振度可用于区分链状球形粒子是否由相同球形粒子组成; 相同球形粒子组成链状结构中,I_s随着粒子数量的增加而线性增大,偏振度不改变;不同球形粒子组成的链状结构,I_s以及偏振度的变化趋势可以区分粒子物理特性。     

Stokes parameters of an electromagnetic light wave on scattering

Stokes parameters of an electromagnetic light wave on scattering have been discussed, which permit us to study the spectral degree of polarization of the far-zone scattered field. An example of scattering of a spatially coherent electromagnetic light wave from a quasi-homogeneous medium has been discussed to illustrate the changes of the Stokes parameters and the changes of the spectral degree of polarization of the far-zone scattered field.     

Effect of absorption of multiply scattering media on the degree of residual polarization of backscattered light

The effect of absorption in a scattering medium on the degree of residual polarization of backscattered radiation is studied in the case of probing of multiply scattering media by a linearly polarized light. An approximate expression describing the dependence of the degree of residual linear polarization of the backscattered radiation on the optical characteristics of a multiply scattering medium is derived within the framework of the phenomenological approach, based on the concept of the distribution of the optical paths of partial components of the scattered optical field under the conditions of multiple scattering, and with the use of the ideas about the similarity of statistical moments of the multiply scattered optical fields. The cut-off of the partial components, characterized by a large value of the optical path, because of their absorption, results in a substantial increase of the degree of residual polarization for the bands of the selective absorption caused by the presence of chromophores in the scattering medium. The results of experiments with model scattering media (whole milk) and biological tissues (human skin in vivo) are presented.     

Similarity effects in multiple scattering of coherent radiation: Phenomenology and experiments

Based on phenomenological concepts of statistics of effective optical paths for multiple scattering of coherent radiation, an analysis is carried out of similarity effects observed for the dependences of statistical moments of the scattered field on the relaxation parameters with a dimension of reciprocal length. Within the framework of the diffusion approximation, expressions are obtained that describe the autocorrelation function of fluctuations of the scattered-field amplitude, the degree of polarization, and the normalized intensity of scattered light for media with a finite absorption length in the case of forward scattering of coherent radiation in a plane layer of an isotropic scattering medium. The results of the analysis show the similarity of the dependences of these quantities on the corresponding spatial scales. Experiments with model scattering media (aqueous suspensions of polystyrene spherical particles) supported the existence of similarity effects in multiple scattering. An experimental study was made of the relation between the characteristic depolarization length and the transport length for multiple scattering of coherent radiation in a plane layer. The effective value of the radiation diffusion coefficient providing the best agreement between the experimental and the calculated values of parameters of the scattered field is shown to be independent of the absorption coefficient of a medium.     

Imaging and multiple scattering through media containing optically active particles

This paper examines the behaviour of polarized light scattered by a medium containing small chiral spheroidal particles. We show that for single scattering the observed phenomena of optical activity may be interpreted in terms of an averaged Mueller matrix and describe how the degree of polarization is affected by such a medium. The polarization properties of multiply scattered light by chiral particles are considered through the use of Monte Carlo simulations. It is shown that the effects of chirality under multiple scattering can be interpreted as an order-preserving influence in a disordered system and that this influence can, in principle, be exploited for the purposes of imaging.     

Imaging and multiple scattering through media containing optically active particles

Abstract

This paper examines the behaviour of polarized light scattered by a medium containing small chiral spheroidal particles. We show that for single scattering the observed phenomena of optical activity may be interpreted in terms of an averaged Mueller matrix and describe how the degree of polarization is affected by such a medium. The polarization properties of multiply scattered light by chiral particles are considered through the use of Monte Carlo simulations. It is shown that the effects of chirality under multiple scattering can be interpreted as an order-preserving influence in a disordered system and that this influence can, in principle, be exploited for the purposes of imaging.     

Interference effects in backscattering of light by a layer of a discrete random medium

Multiple backscattering of light by a layer of a discrete random medium is considered. A brief derivation of equations for describing the coherent and incoherent components of scattered light is presented. These equations are solved numerically in the approximation of doubled scattering of light by a semi-infinite medium of spherical scatterers having a size comparable with the wavelength in order to study the effect of the properties of particles on the angular dependence of interference effects. Calculations show that the half-width of the interference peak decreases upon an increase in lateral scattering by particles and that the degree of polarization has a complex angular dependence on the properties of the particles. For an optically thin layer of the medium, the relations defining the interference peak half-width and the scattering angle upon extreme linear polarization as functions of the effective refractive index are given.     

基于偏振门的动态光散射颗粒测量法的研究

为了解决动态光散射纳米颗粒测量技术无法测量高浓度颗粒粒径的难题,提出了一种基于偏振门的动态光散射测量法。从动态光散射和Mie理论出发,理论分析了在高浓度溶液下多重散射效应对散射光偏振态和颗粒粒度测量结果的影响。根据散射光偏振特点,结合偏振门检测技术,改进了传统的动态光散射光学系统。实验研究了在低浓度和高浓度溶液时,不同偏振角度下的散射光强和粒度测量值,完善了散射光的偏振理论。采用90°偏振门检偏,通过各种浓度下的实验,证明了方法的可行性。该方法较之目前同类方法具有原理和结构简单,系统易于维护的特点。     

考虑地表反射类型的气溶胶散射偏振特性

采用逐次阶散射法求解矢量辐射传输方程来研究气溶胶在不同地表反射模型下的散射偏振特性.首先,选取单一地表反射模型和耦合地表反射模型两种地表反射模型.然后,根据地表反射模型计算得到相应的地表反射率,进而采用逐次阶散射法对矢量辐射传输方程进行求解,得到散射光的Stokes矢量.最后,由Stokes矢量计算得出散射光的偏振度.仿真结果表明,两种地表反射模型下气溶胶单次散射的散射辐射强度和线偏振度均相等;耦合地表反射模型的总散射辐射强度和线偏振度总是大于单一地表反射模型;单一地表反射模型的气溶胶单次散射相对总散射的贡献总是大于耦合地表反射模型.研究结果对气溶胶光学特性的反演具有一定意义.     

Cylindrical Fibre Orientation Analysis by Light Scattering. Part 1: Numerical Aspects

The scattering of a finite beam by an ensemble of infinite cylinders, with a spatial orientation function of the depth, is simulated by using a Monte Carlo type code, including multiple scattering effects. The properties of the scattered light collected by detectors, such as the spatial distribution of intensities, are predicted, taking into account the polarization dependence.     

Analysis of non-ergodic behaviour in spatio-temporal coherence properties of speckle light

Spatio-temporal coherence properties of light scattered by rough surfaces that leads to speckle fluctuations are analysed. It is demonstrated that the scattered light is non-ergodic with the disorder due to the scattering process. Although the mutual coherence matrix vanishes with isotropic polarization fluctuations, it is shown that spatio-temporal coherence properties can be described with interference experiments that can be obtained between different speckles of the scattered light. For non-singular scattering processes, the maximal value of the modulus of the Wolf degree of coherence is analysed in the spatial time domain. This approach is also applied to totally unpolarized incident light with an isotropic and spatially independent scattering process. The mean value and the standard deviation of the Wolf degree of coherence are then determined from the coherence properties of the incident light.     

Vector statistics of multiply scattered light in a random medium with large inhomogeneities

The distribution function of the scattered radiation intensity is calculated in the approximation of the basic polarization modes. In such an approximation, the distribution function is represented in terms of the degree of polarization of the initially circularly and linearly polarized waves transmitted by the sample. The effect of the difference of the decay rates for the circular and linear polarization modes on the statistics of the light waves that are multiply scattered in the medium with large inhomogeneities is analyzed.     

沙尘气溶胶粒子群的散射和偏振特性

根据Mie散射理论,以对数正态分布函数描述沙尘气溶胶粒子群的粒径尺度分布,计算了沙尘气溶胶粒子群在0.2～40μm波段间对太阳短波辐射和地球大气长波辐射的单次散射反照率、散射相矩阵函数,揭示了不同相对湿度时,沙尘粒子群对入射辐射的散射和偏振的特征。结果表明,沙尘粒子群的单次散射反照率随着入射波长的增加有较大起伏,不同相对湿度条件下,变化趋势基本一致;在可见光、近红外波段单次散射反照率随湿度增加而变大,湿度95%时非常接近于1;大于10μm的热红外波段单次散射反照率随相对湿度增加而减小,具有较强的吸收辐射能力。散射辐射强度受湿度影响较小,随散射角的增加呈现先减小后增大的趋势,且增大的趋势随着波长的增加而减弱;不同波段上,线偏振和圆偏振随散射角和相对湿度变化存在差异;在前向和后向仅对入射辐射为圆偏振辐射产生圆偏振散射;散射光的偏振特性及其湿度差异主要表现在后向散射区,多以拱形形式体现。拱顶峰值散射角位置存在差异,且峰值散射角随相对湿度的降低向后向漂移。     

非球形粒子散射光的去偏振特性研究

目标对入射偏振光的散射特性反映了目标的属性信息。在Rayleigh散射理论的基础上，通过单层非球形粒子对入射偏振光的散射数学模型，应用矢量传输方程来计算非球形粒子散射的Mueller矩阵元，求解散射介质的偏振度，讨论在不同介质层厚度、粒子半径和探测角的条件下，非球形粒子散射光去偏振度的变化特性，给出了模拟仿真结果。该方法为研究目标的内部结构、厚度和粗糙度等特征以及目标的探测和识别提供了一种新的途径。     

Effect of electric field on light scattering by aqueous colloids of diamond and graphite

We present the results of our experimental investigation of light scattering by polydisperse colloids of diamond and graphite. The scattering is studied at a random orientation of particles and in an external radiofrequency electric field, which orients particles along the strength. The average dimensions of particles in both colloids are close to each other and comparable with the wavelength of the incident light. The shape of particles and the optical and electrooptical properties of diamond and graphite colloids are significantly different. We analyze the polarization components of scattered light energy when the light incident on the colloids is linearly polarized. We show that the quadrupole light scattering by isotropic diamond particles has the main effect on angular dependences of depolarization of scattered light. For light scattering by anisotropic graphite particles, the depolarization of scattered light is mainly determined by a particular feature of the dipole scattering of particles. It is shown that, in both colloids, the orientational order of particles considerably reduces the depolarization of light scattered by particles. We show that relative changes in the intensity and depolarization of scattered light, which depend on the scattering angle and polarization direction of light, as well as on the parameters of particles, can be used as a measure of electrooptical effects observed in colloids.