利用蒙特卡洛法分析红外光学系统的杂散辐射

介绍了杂散辐射的概念及其对光学系统成像质量的影响。以一透视式红外系统为例,通过对系统的光机结构的合理简化,利用杂散辐射分析软件Light tools中已有的蒙特卡洛分析方法,对已建立的红外系统光机结构的三维模型进行光线追迹分析,得到了系统像面的照度分布结果。同时,基于辐射度学相关理论对该系统中各结构表面的热辐射进行了定量计算,分析了结构中不同组成部分对系统像面的杂散辐射影响,提出本文系统中前镜框内表面为主要杂散辐射源。针对分析结果,探讨了红外系统的杂散辐射抑制方法,提出的方法可为后续设计提供参考。     

制冷型红外成像系统内部杂散辐射测量方法

杂散辐射是红外光学系统设计和检测过程中涉及的一项重要指标.为了定量测量红外成像系统内部杂散辐射, 提出一种基于辐射定标的测量方法, 并通过理论推导和实验验证以说明该方法的合理性.首先, 建立了不带光学系统的辐射定标模型, 即探测器直接接收定标源辐射能, 获得探测器内部因素对系统输出的影响; 然后将其与带有光学系统的定标结果进行比较, 得到由光学系统自身辐射对系统输出的影响, 进而计算红外成像系统内部杂散辐射; 最后通过实验证明了本文理论的正确性.该方法操作简单, 对实验条件要求低, 并可以精确地测量红外成像系统内部杂散辐射.可用于指导红外系统设计中的杂散辐射抑制, 验证系统杂散辐射分析结果是否准确以及检测系统杂散辐射指标是否合格.     

双向蒙特卡罗法模拟参与性介质中的辐射换热

双向蒙特卡罗法继承了正向蒙特卡罗法和反向蒙特卡罗法两种方法的优势,不仅可以解决各种各向异性散射问题,还能简易直接地解决辐射能量的方向性和定位置问题。为了验证双向蒙特卡罗法模拟热辐射传递,与文献中其它方法计算结果进行了比较。最后,利用双向蒙特卡罗法模拟计算圆柱形参与性介质中的辐射换热。     

大相对孔径紧凑型无热化红外光学系统设计

根据目前搜索和跟踪系统要求其红外成像光学系统具有高成像质量、超轻小型化和高温度适应性的特点。采用折反射式光学系统结构形式,基于J-T制冷型320×320凝视焦平面阵列探测器,设计了一种大相对孔径紧凑型无热化红外光学系统,光学系统远摄比达到0.6。采用光学被动消热差方法进行设计,使该系统在-40℃～60℃温度范围内实现了无热化。同时采用杂散辐射分析软件对系统进行杂散辐射分析,提出合理杂辐射抑制方案,给出了完整的光学系统设计。结果表明,光学系统在不同温度环境下所有视场的调制传递函数(MTF)(17lp/mm)均接近衍射极限,80%的能量集中在1个像元内,且具有结构紧凑、体积小等优点,可满足搜索和跟踪红外光学系统的使用要求。     

具有蜂窝内壁的遮光罩杂散光抑制特性分析

将具有蜂窝内壁结构的遮光罩内的杂散光传播分解为蜂窝结构的散射、内壁等效面传播两个层次的行为。通过建立蜂窝结构的等效面各向异性反射模型,采用蒙特卡罗法模拟获得内壁等效面的双向反射分布函数(BRDF)。以此为基础,再次采用蒙特卡罗法模拟遮光罩内壁等效面的杂散光传播过程,分析了该类遮光罩的杂散光抑制特性,并讨论了蜂窝高度和涂层反射率的影响。结果表明,遮光罩内壁蜂窝结构有很好的杂散光抑制作用;蜂窝高度与边长之比δ≥1时,遮光罩的抑制能力不再变化,降低涂层反射率能够有效地增强抑制效果。     

红外光学系统的污染镜面杂散辐射分析

以卡塞格伦结构为例,利用ASAP光学分析软件,建立了红外光学系统的三维仿真模型,在光学元件表面存在均匀分布和非均匀分布的微粒污染情况下,对红外光学系统的杂散辐射特性进行了仿真和比较分析,对影响系统杂散辐射特性的主要因素进行了讨论,并对系统中机械元件的杂散辐射路径进行了分析。在此基础上,采用有效发射率对系统的杂散辐射性能进行了评价,讨论了镜面污染程度对红外光学系统杂散辐射性能的影响。     

遮光罩内壁蜂窝结构单元的双向反射特性

内壁蜂窝结构是决定遮光罩杂散光抑制性能的主要因素,掌握其各向异性辐射传输特性对遮光罩的杂散光分析和优化设计具有重要意义.针对蜂窝结构单元,建立辐射传输模型,导出了表面双向反射分布函数的离散表达式;通过蒙特卡罗法模拟蜂窝结构单元的辐射传输过程,分析了几何参量和涂层反射率对等效面反射特性的影响;根据模拟所得双向反射分布函数数据库,建立了用于遮光罩杂散光分析的蜂窝结构等效面的反射特性概率模型.分析结果表明,蜂窝结构对杂散光呈现很强的后向散射特征,等效面的反射特性概率模型与直接模拟结果符合很好,可用于遮光罩杂散光分析和设计.     

遮光罩内壁蜂窝结构单元的双向反射特性

内壁蜂窝结构是决定遮光罩杂散光抑制性能的主要因素,掌握其各向异性辐射传输特性对遮光罩的杂散光分析和优化设计具有重要意义.针对蜂窝结构单元,建立辐射传输模型,导出了表面双向反射分布函数的离散表达式;通过蒙特卡罗法模拟蜂窝结构单元的辐射传输过程,分析了几何参量和涂层反射率对等效面反射特性的影响;根据模拟所得双向反射分布函数数据库,建立了用于遮光罩杂散光分析的蜂窝结构等效面的反射特性概率模型.分析结果表明,蜂窝结构对杂散光呈现很强的后向散射特征,等效面的反射特性概率模型与直接模拟结果符合很好,可用于遮光罩杂散光分析和设计.     

吸收-透射界面下正弦折射率介质内辐射换热

对具有吸收-透射性边界面的梯度折射率半透明介质层,建立了介质内热辐射传递与边界面辐射换热的数理模型,并采用数值弯曲光线跟踪法求解介质内的热辐射传递。通过数值模拟,分析了正弦折射率下,边界面的反射特性、吸收率以及介质层光学厚度对介质内热辐射平衡温度场及热流分布的影响。结果表明,边界面的反射特性与吸收率对介质内辐射换热均有重要影响,吸收率的影响与边界面反射特性、介质层光学厚度及环境条件相关,呈现特征不同的作用。     

红外焦平面静态地平仪用地球模拟器光机系统设计

根据红外焦平面静态地平仪的特点,并考虑地球模拟器(地模器)光学系统视场大、成像要求高等因素,在合理耗资的前提下设计了地模器的光机系统.通过分析地平仪的光学系统及工作原理,合理地选择了地模器的光学系统参数及机械结构,对地平仪和模拟器光学系统的匹配进行像质评价,并对机械结构做误差分析.结果显示,匹配后光学系统的RMS直径<27μm,小于探测器像元尺寸.机械结构误差为0.03°,高于地平仪的探测精度.这些结果表明,该地模器基本满足检测红外焦平面静态地平仪的要求.     

热辐射输运问题的高效蒙特卡罗模拟方法

惯性约束聚变研究中,热辐射光子在介质中的输运以及热辐射光子与介质的相互作用是重要研究课题,蒙特卡罗方法是该类问题的重要研究手段之一.隐式蒙特卡罗方法虽然能正确地模拟热辐射在介质中的输运过程,但当模拟重介质(材料的吸收系数大)问题时,该方法花费的计算时间将变得很长,导致模拟效率很低.本文以离散扩散蒙特卡罗方法为基础,开发了"离散扩散蒙特卡罗方法辐射输运模拟程序",可以较好地解决重介质区的计算效率问题,但是离散扩散蒙卡罗方法在模拟轻介质区时精度不够高.辐射输运问题中通常既有轻介质也有重介质,为了能同时解决蒙特卡罗方法模拟的效率和精度问题,本文研究了离散扩散蒙特卡罗方法与隐式蒙特卡罗方法相结合的模拟方法,并提出了新的扩散区与输运区界面处理方法,研制了混合蒙特卡罗方法的辐射输运模拟程序.典型辐射输运问题模拟显示:在模拟重介质问题时,该程序能大幅缩短模拟时间,且能取得与隐式蒙特卡罗方法一致的结果;在模拟轻重介质均存在的问题时,与隐式蒙特卡罗方法相比,混合蒙特卡罗方法的模拟精度与其相当且计算效率同样能够得到显著提升.     

Simulation of the infrared radiation characteristics of high-temperature exhaust plume including particles using the backward Monte Carlo method

The analysis of the infrared radiation characteristics of high-temperature free-stream flow including particles is very significant for the field of target detection, combustion diagnosis and temperature measurement of flame. In this paper, the infrared radiation characteristics of high-temperature free-stream flow are calculated and analyzed using the backward Monte Carlo method, considering the effect of the directional radiation heat flux due to the particle scattering and the different boundary conditions. The calculation results of emitting, absorbing and anisotropically scattering media are compared with the forward Monte Carlo and finite-volume methods results, which shows the superiority on computational efficiency with the backward Monte Carlo method.     

Microwave radiative transfer intercomparison study for 3-D dichroic media

Three different numerical methods capable of solving the radiative transfer of microwave radiation within 3-D dichroic media are compared. A case study, represented by an intense rain shaft populated by perfectly oriented oblate raindrops, is analysed in detail, including a discussion of the behaviour of all four Stokes components.Results demonstrate an acceptable agreement between all Monte Carlo methods. The method based on a discrete ordinates scheme agrees only qualitatively with the Monte Carlo outputs. Because of its lower computational cost the backward Monte Carlo technique based on importance sampling represents the most efficient way to face passive microwave radiative transfer problems related to optically thick 3-D structured clouds including non-spherical preferentially oriented hydrometeors.     

Monte Carlo discrete curved ray-tracing method for radiative transfer in an absorbing-emitting semitransparent slab with variable spatial refractive index

A Monte Carlo discrete curved ray-tracing method is developed to analyze the radiative transfer in one-dimensional absorbing-emitting semitransparent slab with variable spatial refractive index, in which the Monte Carlo method is combined with the discrete curved ray-tracing method. A problem of radiative equilibrium with linear variable spatial refractive index is taken as an example to examine the accuracy of the proposed method. The temperature distributions and the dimensionless radiative heat flux are determined by the proposed method and compared with the data in references, which are obtained by other different methods. The results show that the Monte Carlo discrete curved ray-tracing method has a good accuracy in solving the radiative transfer in one-dimensional semitransparent slab with variable spatial refractive index.     

Markov chain formalism for vector radiative transfer in a plane-parallel atmosphere overlying a polarizing surface

We report on a way of building bidirectional surface reflectivity into the Markov chain formalism for polarized radiative transfer through a vertically inhomogeneous atmosphere. Numerical results are compared to those obtained by the Monte Carlo method, showing the accuracy of the Markov chain method when 90 streams are used to compute the radiation from a Rayleigh-plus-aerosol atmosphere that overlies a surface with a bidirectional reflection function consisting of both depolarizing and polarizing parts.     

Radiation symmetry test and uncertainty analysis of Monte Carlo method based on radiative exchange factor

Using Monte Carlo method, the paper investigates the radiative heat transfer in participating media. Based on the radiative exchange factor, an uncertainty analysis of Monte Carlo method is undertaken and the corresponding mathematical expressions are deduced to predict its accuracy. Furthermore, randomness properties of pseudorandom number generators are investigated, and a model to test radiation symmetry is adopted to validate the performance of some generators. The paper studies the effects of energy bundle numbers, discretization schemes, emission location, optical thicknesses, wall emissivity and CPU time on the numerical accuracy. In addition, the simulation results are proved to give a reference for using Monte Carlo method, which is applicable for calculation of the radiative exchange factor.     

Efficient inverse radiation analysis of temperature distribution in participating medium based on backward Monte Carlo method

An efficient numerical inverse radiation analysis based on the backward Monte Carlo (BMC) method is presented to determine the three-dimensional (3-D) temperature distribution in a large rectangular enclosure containing the participating medium, using radiative intensities in the visible range received by charge-coupled device (CCD) cameras. For large radiative sources and small detectors, when the radiation onto a small spot and onto a small direction cone is desired, the BMC method is more efficient than the forward Monte Carlo (FMC) method. Because the temperature reconstruction from the measured radiative intensities is an ill-posed inverse problem, least-square QR decomposition (LSQR) method is introduced to determine the meaningful temperature distribution. In order to gain insight into the effects on the accuracy of temperature distribution reconstruction, the detailed analyses are made using numerical simulations. The results show that the temperature distribution can be reconstructed accurately for the exact and noisy data.