各向异性介质辐射特性参数联合反演

本文基于BP神经网络方法结合蒙特卡洛和BEER定律辐射传输模拟方法建立了联合反演各向异性散射介质的辐射特性参数模型。首先采用半球透射率结合半球反射率反演模型反演了各向同性介质的吸收系数和散射系数,在此基础上增加准直透射率,建立了联合反演各向异性介质的吸收系数、散射系数和散射不对称因子三参数联合反演模型。反演结果表明该模型能准确反演出介质辐射特性参数,具有实用意义。此外,为了检验测量误差对模型的反演准确性的影响,分别在不同程度测量误差情况下进行反演,结果显示测量误差对散射不对称因子反演值影响较大。     

Radiative Transfer in a Layer of Oriented Spheroidal Particles with Oblique Incidence of Light

The results of numerical modeling of diffuse reflection and transmission by layers of oriented spheroidal particles (whose rotation axes are normal to the layer surface) with oblique incidence of a parallel beam of radiation on the layer are presented. To describe the properties of particles in the elementary act of scattering, the Rayleigh-Gans approximation is used. To describe the propagation of radiation in the layer, the radiative transfer equation in which the characteristics of the medium depend on the direction of radiation propagation is used. The influence of Fresnel reflection at the layer boundaries and the influence of the substrate are taken into account. Calculations of the luminance factors at the layer boundaries over a wide range of characteristics of the medium and layer thicknesses have been made.     

Transient radiative transfer in participating media with pulse-laser irradiation—an approximate Galerkin solution

An assessment is made of the Galerkin technique as an effective method of solution for transient radiative transfer problems in participating media. A one-dimensional absorbing and isotropically scattering plane-parallel gray medium irradiated with a short-pulse laser on one of its boundaries is considered for the application of the method. The medium is non-emitting and the boundaries are non-reflecting and non-refracting. In the integral formulation of the problem for the source function, the time-wise variation of the radiation intensity at any point and in any direction in the medium is assumed to be the same as the time-wise variation of the average intensity at the same point as an approximation for the application of the method. The transient transmittance and reflectance of the medium are evaluated for various values of the optical thickness, scattering albedo and pulse duration. The results are in agreement with those available in the literature. It is demonstrated that the method is relatively simple to implement and yields accurate results.     

基于MDA法计算水雾粒子红外隐身粒径

水雾隐身技术是一种较为理想且经济适用的目标特征信号控制技术。本文从辐射传输方程出发,考虑水雾粒子的本身辐射和散射,提出以红外表观透射率评价水雾粒子的红外隐身性能。应用Modified Differential Approximation(MDA)法结合MIE理论,计算了水雾粒子红外表观透射率随水雾粒子浓度和粒径的变化关系,得到水雾粒子最佳红外隐身粒径。结果表明:水雾粒子的红外表观透射率随粒子浓度的增加而减小,但当粒子的浓度增加到一定值时,红外表观透射率基本保持不变;水雾粒子的红外表观透射率随粒径的增加先减小后增大,当探测波段为3～5μm时,水雾粒子的最佳红外隐身粒径为6μm;而探测波段为8～12μm时,水雾粒子的最佳红外隐身粒径为12μm。     

偏振光在多层散射介质中传输的蒙特卡罗模拟研究

基于米氏散射理论,运用斯托克斯-穆勒形式,结合子午面法和拒绝法,提出了偏振光在多层散射介质中传输的蒙特卡罗模型,并首次分析了当菲涅耳公式失效时,多层介质界面处的全反射行为.同时用该模型研究了微粒直径和介质折射率对漫反射和漫透射的影响.结果表明:无论是折射率匹配介质还是非匹配介质,当粒子浓度一致时,漫反射率都随微粒直径的增加而增加,随介质折射率的增加而减小,而漫透射率则相反.研究结果为生物组织的偏振光散射研究提供了理论验证模型.     

偏振光在多层散射介质中传输的蒙特卡罗模拟研究*

基于米氏散射理论,运用斯托克斯-穆勒形式,结合子午面法和拒绝法,提出了偏振光在多层散射介质中传输的蒙特卡罗模型,并首次分析了当菲涅耳公式失效时,多层介质界面处的全反射行为.同时用该模型研究了微粒直径和介质折射率对漫反射和漫透射的影响.结果表明：无论是折射率匹配介质还是非匹配介质,当粒子浓度一致时,漫反射率都随微粒直径的增加而增加,随介质折射率的增加而减小,而漫透射率则相反.研究结果为生物组织的偏振光散射研究提供了理论验证模型.     

Maximum time-resolved hemispherical reflectance of absorbing and isotropically scattering media

This paper presents a parametric study of the time-resolved hemispherical reflectance of a plane-parallel slab of homogeneous, cold, absorbing, and isotropically scattering medium exposed to a collimated Gaussian pulse. The front surface of the slab is transparent while the back surface is assumed to be cold and black. The 1-D time-dependent radiation transfer equation is solved using the modified method of characteristics. The parameters explored include (1) the optical thickness, (2) the single scattering albedo of the medium, and (3) the incident pulse width. The study pays particular attention to the maximum transient hemispherical reflectance and identifies optically thin and thick regimes. It shows that the maximum reflectance is independent of the optical thickness in the optically thick regime. In the optically thin regime, however, the maximum hemispherical reflectance depends on all three parameters explored. The transition between the optically thick and thin regimes occurs when the optical thickness is approximately equal to the dimensionless pulse width. Finally, correlations relating the maximum of the hemispherical reflectance as a function of the optical thickness, the single scattering albedo of the materials, and the incident pulse width have been developed. These correlations could be used to retrieve radiation characteristics or serve as initial guesses for more complex inversion schemes accounting for anisotropic scattering.     

一维梯度折射率介质光学特性的反演

在单个方波脉冲入射情况下,利用共轭梯度法对一维梯度折射率介质的折射率、吸收系数以及散射系数进行了反演.正问题采用间断有限元法求解,反问题的解则在正问题的基础上通过共轭梯度法得到.研究结果表明,利用单个方波脉冲入射情况下的时域半球反射率以及时域半球透射率作为测试值能够有效地反演一维梯度折射率介质的光学特性及其分布情况.     

Thermal signatures of a localized inhomogeneity in a 2-D participating medium subjected to an ultra-fast step-pulse laser wave

The transport of a train of short-pulse radiation through a 2-D rectangular participating medium consisting of local inhomogeneities is investigated in this article. A collimated beam of step temporal profile is considered. The pulse wave consists of 1 and 4 pulses. The absorbing and scattering participating medium contains a square-shaped local inhomogeneity. The inhomogeneity differs from the rest of the medium by its scattering albedo. The pulse width and the period of the laser wave are of the order of a nano-second. Transmittance and reflectance signals are analyzed for the effects of the extinction coefficient and the scattering albedo. Heat flux distributions inside the medium are also studied. The finite-volume method is used to solve the problem.     

The Iterative-DRESOR method to solve radiative transfer in a plane-parallel, anisotropic scattering medium with specular-diffuse boundaries

Using the intensity with high directional resolution obtained by the Basic-DRESOR method as an initial guess, which is substituted into the integrated radiative transfer equation (IRTE), an iterative algorithm is proposed, called the Iterative-DRESOR method. This method can reduce the error levels of the intensity from several percent using the Basic-DRESOR method to a level of less than 1.0×10⁻⁶ with acceptable computation costs. The method is also validated against the exact heat flux in literature in some cases. It further clarifies some uncertain results for the reflectance in a pure, linearly anisotropic scattering medium with specular-diffuse boundaries. The directional distributions of intensity are obviously influenced by the reflecting modes of the boundary, especially in the zone near the boundary. The reflecting mode of an emitting boundary has little effect on the transmittance or reflectance. The reflecting mode of a non-emitting boundary also has little effect on the transmittance, but it obviously influences the reflectance. The difference between the reflectance for specular and diffuse boundaries increases at first, and then decreases, as the optical thickness of the medium increases. The difference will decrease as the scattering albedo of the medium increases, and it is negligible when the medium is pure scattering. The effect of the scattering phase function of the medium on the difference can also not be ignored. The Iterative-DRESOR method is expected to strengthen the capability of the Monte Carlo method to produce accurate results and to validate the results of other methods to solve RTE.     

Incoherent and coherent backscattering of light by a layer of densely packed random medium

The problem of light scattering by a layer of densely packed discrete random medium is considered. The theory of light scattering by systems of nonspherical particles is applied to derive equations corresponding to incoherent (diffuse) and interference parts of radiation reflected from the medium. A solution of the system of linear equations describing light scattering by a system of particles is represented by iteration. It is shown that the symmetry properties of the T-matrices and of the translation coefficients for the vector Helmholtz harmonics lead to the reciprocity relation for an arbitrary iteration. This relation is applied to consider the backscattering enhancement phenomenon. Equations expressing the incoherent and interference parts of reflected light from statistically homogeneous and isotropic plane-parallel layer of medium are given. In the exact backscattering direction the relation between incoherent and interference parts is identical to that of sparse media.     

地表双向反射对天基矢量辐射探测的影响分析

矢量辐射传输方程定量描述了辐射在地表-大气耦合介质中的传输过程,是定量遥感的基础.在处理辐射和离散介质相互作用时,如何处理多次散射、辐射偏振效应和耦合地表模型是研究的重点,直接影响定量化遥感反演的精度.文中基于逐次散射近似法求解了矢量辐射传输方程,求解过程中耦合典型地表的非偏双向反射（BRDF）模型和偏振双面反射（BPDF）模型.采用相对误差因子定量分析了地表双向反射效应和大气偏振效应对天基矢量辐射的影响.为进一步研究地表-大气耦合介质系统的偏振特性及地表大气参数的定量遥感反演提供理论支持. 关键词： 矢量辐射传输方程 逐次散射近似 定量遥感 偏振遥感     

Differential approximations for transient radiative transfer in refractive planar media with pulse irradiation

Transient radiative transfer in an anisotropically scattering refractive planar medium with pulse irradiation is solved by various approximation methods, such as P?1, P?1 parabolic, P_1/3 and two-flux. The time-resolved transmittance and reflectance are calculated for various radiative parameters, and are compared with those obtained by the discrete ordinate method (DOM). Among the approximation methods considered, the P_1/3 approximation is the better one, because its results are in overall good agreement with those obtained by the more rigorous DOM, except the transmittance around the peak for neither thin nor very thick slabs. It is found that the curved paths of radiation and the internal reflection of the back scattered radiation enhance the effect of scattering.     

Virtual gonio-spectrophotometer for validation of BRDF designs

Measurement of the appearance of an object consists of a group of measurements to characterize the color and surface finish of the object. This group of measurements involves the spectral energy distribution of propagated light measured in terms of reflectance and transmittance, and the spatial energy distribution of that light measured in terms of the bidirectional reflectance distribution function (BRDF). In this article we present the virtual gonio-spectrophotometer, a device that measures flux (power) as a function of illumination and observation. Virtual gonio-spectrophotometer measurements allow the determination of the scattering profile of specimens that can be used to verify the physical characteristics of the computer model used to simulate the scattering profile. Among the characteristics that we verify is the energy conservation of the computer model. A virtual gonio-spectrophotometer is utilized to find the correspondence between industrial measurements obtained from gloss meters and the parameters of a computer reflectance model.     

Calculation of radiation field in semi-transparent medium with consideration of incident collimated radiation

The mathematical model of radiation field in a semi-transparent medium formed under the action of incident collimated and diffusion radiation was developed and implemented numerically. To solve the problem, the approach on the basis of modified average flux method was developed. Method testing via comparison with results of other authors has proved its high reliability and accuracy. As an example, the effect of different factors on radiation field was analyzed: self-radiation of medium, scattering anisotropy, and bottom reflectance.     

Optical properties of mesoporous one-dimensional photonic crystals

The systematic features in the reflectance spectra of broadband optical radiation from the surface of one-dimensional photonic crystals based on mesoporous structures are presented. The dispersion dependence for electromagnetic waves in the mesoporous one-dimensional photonic crystal is established. On its basis, reflectance and transmittance spectra are calculated for particular structures, and the microscopic parameters of the samples under study are determined. The developed theory is used to explain the shape of reflectance spectra in mesoporous photonic crystals based on silicon, quartz, and aluminum oxide.     

Albedo and gain threshold of a diffusive Raman random laser

The diffuse reflectance (albedo) and transmittance of a Raman random gain medium are derived from a diffusion equation with power dependent gain. The results show good agreement with the experimental data for barium nitrate powder. Both the Raman albedo A_R and Raman transmittance T_R diverge at a critical gain γ_c, interpreted as the threshold for diffusive Raman laser generation. The parametric dependence of the albedo and threshold gain on the scattering characteristics of the random medium is analysed and the feedback effect of Fresnel reflection at the gain boundaries evaluated. The addition of external mirrors, particularly at the pumped surface, significantly reduces the generation threshold.     

Unconventional application of the two-flux approximation for the calculation of the Ambartsumyan-Chandrasekhar function and the angular spectrum of the backward-scattered radiation for a semi-infinite isotropically scattering medium

It is commonly accepted that the Schwarzschild-Schuster two-flux approximation (1905, 1914) can be employed only for the calculation of the energy characteristics of the radiation field (energy density and energy flux density) and cannot be used to characterize the angular distribution of radiation field. However, such an inference is not valid. In several cases, one can calculate the radiation intensity inside matter and the reflected radiation with the aid of this simplest approximation in the transport theory. In this work, we use the results of the simplest one-parameter variant of the two-flux approximation to calculate the angular distribution (reflection function) of the radiation reflected by a semi-infinite isotropically scattering dissipative medium when a relatively broad beam is incident on the medium at an arbitrary angle relative to the surface. We do not employ the invariance principle and demonstrate that the reflection function exhibits the multiplicative property. It can be represented as a product of three functions: the reflection function corresponding to the single scattering and two identical h functions, which have the same physical meaning as the Ambartsumyan-Chandrasekhar function (H) has. This circumstance allows a relatively easy derivation of simple analytical expressions for the H function, total reflectance, and reflection function. We can easily determine the relative contribution of the true single scattering in the photon backscattering at an arbitrary probability of photon survival Λ. We compare all of the parameters of the backscattered radiation with the data resulting from the calculations using the exact theory of Ambartsumyan, Chandrasekhar, et al., which was developed decades after the two-flux approximation. Thus, we avoid the application of fine mathematical methods (the Wiener-Hopf method, the Case method of singular functions, etc.) and obtain simple analytical expressions for the parameters of the scattered radiation. Note that the simplicity of the expressions is supplemented with unexpectedly high accuracy. The results demonstrate the unknown possibilities offered by the two-flux approximation, which is the simplest approximate method to solve the equations of transport theory. We assume that the method can be employed in the calculations of the angular characteristics of the reflected radiation for media whose single scattering is described using complicated (in comparison with isotropic) laws.