基于快速蒙特卡罗的散射介质光学参量干涉测量方法研究

提出了一种散射介质光学参量的干涉测量方法.在蒙特卡罗数值模拟的基础上引入聚焦高斯光源模型和基于比例缩放的压缩算法,实现了对光子后向散射分布和干涉特性的快速模拟.利用计算数据训练了前向人工神经网络进行了介质光学参量的反向求解.实验中以脂肪乳注射剂和生物染色剂印度墨水组成的悬混液为样本,通过光学干涉系统测量得到了后向散射光依赖于深度的干涉光强分布,并对其中散射和吸收系数进行了反向计算.所得结果能够正确反映散射和吸收系数与散射和吸收物质浓度之间的线性关系,验证了该方法的可行性.     

基于蒙特卡罗模拟的散射介质中后向光散射模型及分析应用

构建了内光源模型探讨散射介质中的光散射现象,利用蒙特卡罗方法研究了逃逸出组织的后向散射光子数随光子在组织内部发生的散射次数的分布关系,探讨了光源照明方式、辐射强度、接收方式、调制等参数的变化对后向散射的影响,结果表明后向散射光子的数量随散射次数的分布并非简单的单调递增或递减,而是一条先增大后减小出现峰值的曲线. 峰值位置、峰值大小及曲线形状与光源、探测方式、组织光学特性参数等有关. 关键词： 医用光学与生物技术 散射介质 后向散射 蒙特卡罗模拟     

由吸收系数和粒度分布计算浮游植物的散射光谱特征

散射特性在水色卫星遥感模式的开发、水下光辐射传输、优势藻光谱特征的提取以及赤潮监测等方面有着重要的应用价值.在假定藻细胞为均匀球形结构的条件下,建立了浮游植物的散射正演模型.模型输入参量为实际测量的藻细胞吸收系数和粒度分布,输出参量为浮游植物的散射效率和吸收效率光谱.对短伪海链藻和威氏海链藻进行实验,模拟结果与实验结果的比较发现:该模型估箅得到的散射光谱能与实验的散射光谱得到很好的吻合,两种藻的模拟散射效率相对实验散射效率的误差分别为7%和10.6%,吸收效率的误差分别为7.4%和13.4%.模拟与实验结果的吻合表明,由吸收系数和粒度分布可以模拟出藻类的散射光谱特性.单细胞近似球形藻类散射正演模型可行.     

基于光子散射的功能成像技术

阐述了基于光子散射模型下的功能成像技术.运用蒙特卡罗方法对光子在介质中的散射路径进行了模拟和分析,从而引入光子散射的概念.通过对光子散射方程的解释引入了散射光子密度波的概念.通过无限大模型下光子密度波的波方程,引出光子密度波的异质模型.建立了基于消元的奇异值弱化重建算法,完成了对病态方程的求解,实现了对内嵌于均匀介质中异质的吸收系数分布图像的重建.在基于光散射成像技术所开发的成像系统平台下实现了异质小球的成像实验,对多个不同浓度的物体完成了成像,探测到介质中由于微小浓度改变而引起的吸收系数变化.     

吸收介质中微椭球体颗粒光学参数的研究

本文应用Eikonal近似将微椭球用其等效的球来近似, 结合Mie理论对吸收介质中微椭球体颗粒光学参数进行了数值计算。 结果表明, 椭球位置变化时, 散射和吸收性能发生变化。离心率增大时, 散射和吸收系数都增大, 离心率越大增大的越明显。波长增大时, 在紫光波长为0.4 μm和近红外区波长为1.58 μm处散射系数出现了峰值, 而吸收系数单调增大。相对折射率实部以及虚部变化对光学参数均有影响, 颗粒的吸收性越强, 散射相应地减弱。结果表明这种数值解析方法能有效地计算椭球体颗粒的光学参数。     

高质量光学涡旋阵列的实验研究

将光学涡旋与计算全息技术相结合,提出一种高质量光学涡旋阵列的产生方法.从理论上研究了光学涡旋阵列的形成和分布特征,并模拟仿真产生涡旋光束阵列.基于面向目标的共轭对称延拓傅里叶计算全息方法编码生成光学涡旋阵列的全息图,利用单个反射式空间光调制器光电再现了与理论一致的光学涡旋阵列,并通过马赫-增德尔干涉法对生成的光学涡旋阵列进行验证.产生的高质量光学涡旋阵列提供了更复杂的结构分布和更多的可控参量,且实验光路易实现.研究结果在光学微操控、光通信等领域具有潜在应用价值.     

手性介质中后向散射米勒矩阵特性及其在血糖无创检测中的应用初探

利用斯托克斯-米勒矩阵表述分析偏振光在手性介质中的传输规律,利用瑞利近似下的单次散射模型和基于米氏散射理论的多次散射的蒙特卡罗方法,探讨葡萄糖对后向散射偏振光传输特性的影响,计算了不同葡萄糖浓度下的后向散射米勒矩阵. 结果表明在含有葡萄糖的手性介质中后向散射米勒矩阵的若干矩阵元表现出微弱的旋光效应,旋光度随葡萄糖浓度增大而增大. 为了检测低浓度下微小的旋光变化,定义了函数对含-不含葡萄糖的米勒矩阵元图像进行处理,提取由葡萄糖引入的图像差异. 结合无创血糖检测应用要求,分析了葡萄糖生理浓度下的矩阵元图像,获 关键词： 医用光学与生物技术 偏振光 斯托克斯-米勒表述 蒙特卡罗模拟     

大空间生物组织透射的Monte Carlo模拟方法的加速

乳腺等类型生物组织的透射Monte Carlo模拟(简称MC模拟)耗时很长,这主要是由于被模拟的组织较厚和Monte Carlo模拟自身统计特性引起的所需光子数量多造成的。针对光源与接收面在同一直线上,采用光纤接收透射光的模型进行分析,通过考虑生物组织自身的光学特性,进行了边界约束和有效后向散射次数的限制来缩短仿真时间,经过多次实验验证,以两次散射所到位置为分界,并充分利用组织光学参数计算约束边界,计算相对比较简单,同时考虑到入射与出射的实际情况进行微扩,从而得到的约束边界。对光学参数相同但厚度不同的组织进行仿真,分析进入同一位置光纤的光子数所发生后向散射次数,发现后向散射次数随着组织厚度与散射系数的增大而增大,随着吸收系数与各向异性因子的增大而减小,通过限定后向散射次数来节省时间。仿真结果表明,在传统计算机上,该方法能够显著缩短MC模拟的运算时间50%左右,尤其适用于较厚且边界较为复杂的高散射物质的MC模拟。该提速方法主要应用于光源与接收面处于同一直线,且生物组织较厚,散射系数较大的情况,能够有效节约仿真所需时间,有利于分析组织透射成像。     

圆柱坐标系下生物组织中激光传输的七流模拟

提出了圆柱坐标系下生物组织中激光传输的七流模型,根据此模型进行的计算结果与直角坐标系下七流模型的计算结果比较,二者吻合较好.在此基础上,研究了组织光学性质对激光在组织内传输过程的影响,结果表明:散射效应使入射激光分布到更大的组织范围内,并使激光的穿透率降低,但其影响是有限的,即当散射系数增大到一定程度后,组织内的激光能量分布就不再有明显的变化.相对而言,吸收系数对组织内激光能量分布的影响比散射系数要大,各向异性系数增大使组织内的散射趋于前向散射,激光能量分布也更深入组织内部.     

激光气象雷达多次后向散射信号特性分析

利用小角近似方法，分别给出了单次和多次后向散射激光雷达方程.通过引入的多次散射评价参量，分析了接收视场角和光学厚度等因素对多次后向散射信号的影响.理论研究表明:多次后向散射主要与接收视场角和光学厚度密切相关，当接收视场角比发射视场角大10倍，光学厚度超过3时, 多次后向散射信号逐渐增大并占主要优势，当接收视场角比发射视场角大100倍时，光学厚度超过1时，多次后向散射信号开始明显增强.     

Accelerated Monte Carlo method to reconstruct absorption coefficient from low coherent signal

A fast and effective method for reconstruction of optical absorption property of scattering object is developed. A revised Monte Carlo forward model is set up with new detection criteria for simulation of low coherence interferometer (LCI) system. Calculation of coherent signal could be accelerated by a scaling method based on baseline data simulated by Monte Carlo method. The inverse procedure of scaling method is utilized to extract absorption coefficient by dichotomy algorithm. Fat emulsion and stain mixture of Intralipid? and India ink are taken as scattering and absorbing sample. The results of reconstruction have shown the linear relationship between absorption and concentration of India ink. The maximum absolute relative deviation is 6.21%.     

Radiative properties of densely packed spheres in semitransparent media: A new geometric optics approach

This contribution presents a new Ray-tracing method for calculating effective radiative properties of densely packed spheres in non-absorbing or semitransparent host medium. The method is restricted to the geometric optic objects and neglects the wave effects. The effective radiative properties such as the absorption and scattering coefficients, and phase function are retrieved from the calculation of mean-free paths of scattering and absorption, and the angular scattering probability of radiation propagating in the dispersed medium. The model accounts for the two geometric effects called here as non-point scattering and ray transportation effects. The successful comparison of the current model with data of radiative properties and transmittances of particle beds in a non-absorbing medium reported in the literature confirm its suitability. It is shown that: (i) for opaque or absorbing particles (not systematically opaque), the non-point scattering is the dominant geometric effects whereas both non-point scattering and ray transportation effects occur for weakly absorbing and transparent particles. In the later cases, these two geometric effects oppose and may cancel out. This may explain why the Independent scattering theory works well for packed of quasi-transparent particles; (ii) the non-point scattering and ray transportation effects can be captured through the scattering and absorption coefficients while using the classical form of phase function. This enables using the standard radiative transfer equation (RTE); (iii) the surrounding medium absorption can be accounted for without any homogenization rule. It contributes to increasing the effective absorption coefficient of the composite medium as expected but, at the same time, it reduces the particle extinction; and (iv) the current transfer calculation predicts remarkably the results of direct Monte Carlo (MC) simulation. This study tends therefore to confirm that the RTE can be applied to densely packed media by using effective radiative properties.     

Properties of a surface contaminated by gaseous products of polymer composition materials under vacuum conditions

Optical properties and chemical composition of a surface that is contaminated by molecular flows consisting of gaseous products of polymer composition materials (e.g., EKOM-2 enamel) are experimentally studied. The polymer materials are irradiated using a short-wavelength source with a luminous intensity of 6 × 10^?4 W/cm² sr in the wavelength interval 90–320 nm in a vacuum chamber that is evacuated to a pressure of 10^?4–10^?3 Pa. An analytical solution is obtained for the problem of radiation fluxes in scattering and absorbing medium of precipitate on a mirror surface, and a relation of model absorption and scattering coefficients for optical radiation in such medium and transport coefficients of the Gurevich theory of turbid medium is demonstrated.     

A neutron diffraction study of liquid chloroform

Neutron diffraction studies of the isotopes of chloroform CD³⁵Cl₃ and CD³⁷Cl₃ in liquid phase were carried out at 20°C and a wavelength of 0·7 Å. The data were corrected for background, absorption, multiple scattering and inelastic effects. The coherent distinct differential cross section was separated into intramolecular and intermolecular contributions. The latter, together with intermolecular contributions from neutron scattering data on chloroform of natural isotopic chlorine composition and from X-ray data, was employed to determine four expansion coefficients of the molecular pair correlation function.     

Sum rules including first order quantum corrections for neutron scattering experiments on isotopic mixtures of monatomic fluids

The sum rules for neutron scattering experiments on monatomic isotopic mixtures are studied. The scattering is separated into a self part and a distinct part rather than into an incoherent part and a coherent part. Exact expressions for the moments are derived in terms of polynomials in ? ². The coefficients in these polynomials are sums of averages of hermitean operators that have a classical analogue. For the interpretation, the coefficients are approximated including first order quantum corrections.

It is argued that the separation into coherent and incoherent scattering is not justified in the presence of ‘isotopic incoherence’. Approximate expressions for the sum rules are proposed, in which combined averages (over the isotopic composition) appear of the scattering amplitudes and masses of the scattering atoms. These expressions, including first order quantum corrections to the sum rules, are not available in the literature.

The importance of correct averaging over the isotopic composition of expressions that involve scattering amplitudes and masses is discussed for the case of a maximum incoherent mixture of ³⁶Ar and ⁴⁰Ar.     

Frequency domain optical tomography using a conjugate gradient method without line search

A conjugate gradient method without line search (CGMWLS) is presented. This method is used to retrieve the local maps of absorption and scattering coefficients inside the tissue-like test medium, with the synthetic data. The forward problem is solved with a discrete-ordinates finite-difference method based on the frequency domain formulation of radiative transfer equation. The inversion results demonstrate that the CGMWLS can retrieve simultaneously the spatial distributions of optical properties inside the medium within a reasonable accuracy, by reducing cross-talk between absorption and scattering coefficients.     

Backscattering of linearly and circularly polarized light in randomly inhomogeneous media

The scattering of linearly or circularly polarized light from a semibounded randomly inhomogeneous medium is considered. A new technique for simulating the electromagnetic radiation transport using the Monte Carlo method is proposed, which makes it possible to avoid cumbersome calculation of Muller matrices. Expressions are obtained for the co- and cross-polarized components of backscattered light for incident light of arbitrary polarization. The coherent and incoherent backscattering components are calculated for arbitrary combinations of incident and scattered light polarizations. It is shown that the main contribution to coherent backscattering is from the co- and cross-polarized components for linearly and circularly polarized light, respectively. The backscattering from an optically active random medium is calculated.     

Nonstationary two-flux model of radiation transport for optical tomography of scattering media

A nonstationary two-flux model is formulated for the transport of radiation in an inhomogeneous scattering medium and is applied to the situation where such a medium is irradiated by the narrow beam of a pulsed laser. It is shown that when the time distribution of the transmitted photons is measured it is possible simultaneously to reconstruct the two spatial functions (the coefficients of absorption coefficient and of scattering of the radiation by the medium) by means of an inverse Radon transformation and the solution of a system of nonlinear differential equations on the projection lines. An analytic solution is obtained in quadratures for these differential equations. The results constitute a method of solving problems in optical tomography in an inhomogeneous scattering medium Zh. Tekh. Fiz. 67, 61–65 (May 1997)     

A sensitivity function-based conjugate gradient method for optical tomography with the frequency-domain equation of radiative transfer

The Sensitivity Function-based Conjugate Gradient Method (SFCGM) is described. This method is used to solve the inverse problems of function estimation, such as the local maps of absorption and scattering coefficients, as applied to optical tomography for biomedical imaging. A highly scattering, absorbing, non-reflecting, non-emitting medium is considered here and simultaneous reconstructions of absorption and scattering coefficients inside the test medium are achieved with the proposed optimization technique, by using the exit intensity measured at boundary surfaces. The forward problem is solved with a discrete-ordinates finite-difference method on the framework of the frequency-domain full equation of radiative transfer. The modulation frequency is set to 600 MHz and the frequency data, obtained with the source modulation, is used as the input data. The inversion results demonstrate that the SFCGM can retrieve simultaneously the spatial distributions of optical properties inside the medium within a reasonable accuracy, by significantly reducing a cross-talk between inter-parameters. It is also observed that the closer-to-detector objects are better retrieved.