双向反射分布函数(BRDF)及其测量

在材料的光学性能研究领域。双向反射分布函数（BRDF）已经是一个被广泛认可的综合指标，在航天遥感、地理信息、海洋开发、自然灾害监测、气候研究、军工信息等领域得到广泛的应用。随着数字化和信息化的发展，BRDF的研究也开始了数学建模和数字仿真。不过．还有一个亟待发展的方向是BRDF的测量方法的统一和绝对量的标定。     

基于BRDF的在轨卫星反射特性

基于卫星的背景特性、材料特性、轨道特性及太阳帆板与卫星本体的相对运动特性,研究了在轨卫星的可见光反射特性.实验测量了三种卫星表面常用材料典型角度的双向反射分布函数,并构建了三种材料改进的SunBRDF模型.提出了一种单自由度运动的太阳帆板入射光线矢量计算方法.根据卫星的几何尺寸和表面材料特性,仿真获得了在轨卫星不同观测...     

复合材料压缩实验时应力分布的有限元计算

碳纤维增强树脂基复合材料（简称ＣＦＲＰ）薄板压缩实验时有三种不同的加载形式。本文对这三种加载形式进行了研究和分析，并有限元素法计算了这三种加载形式在１－３面内的压应力和层剪应力的分布? ?     

基于平板反射法的复合材料介电常数反演研究

本文采用平板反射方法,利用与无反射曲线具有同源性的条件选择曲线,探索出了一种反演等效复介电常数的新方法。研究表明,通过选取无反射曲线上的点进行的理论模拟,并对不同厚度样品吸收曲线进行模拟反演和实验值对比,结果相当吻合。本研究可为测试各种大尺寸、多孔材料、液体等样品等效介电常数提供了一种行之有效的方法,对内部多相大结构电磁功能复合材料的设计研究具有重要意义。     

双向梯度复合材料板瞬态热应力分析

该文通过编写一种8节点高阶双向梯度单元建立了结构的连续梯度有限元模型,采用细观力学方法结合混合律准则描述了双向梯度单元变化的热物理属性。分别采用有限单元-有限差分法和有限单元法分析了在循环热载荷作用下双向梯度板Al 1100/Ti-6Al-4V/ZrO2的瞬态热传导和热弹性问题,给出了温度场和瞬态热应力场的时间响应历程和空间分布形式。最后讨论了相关参数的影响规律,得到了一些重要的结论：冷却阶段更加容易萌生表面裂纹;沿x方向组分的分布形式影响了瞬态热应力的大小,但对其时间响应和分布形式影响很小;沿y方向组分的分布形式影响了瞬态热应力的时间响应和分布形式,但对其大小影响很小。     

多光谱BRDF测量系统转角装置研制

文中介绍了国内外双向反射分布函数的四种测量方案。采用固定光源和使样片转动以及探测器移动的方法实现了对双向反射分布函数的测量,进一步提出了利用可调机械臂结构以及三台步进电机的协调驱动实现对目标材料的高精度半球空间反射特性测量的改进方法。研制了多光谱双向反射分布函数测量转角装置,该装置的应用使双向反射分布函数测量系统的布局更合理,测量准确度更高。转角装置的探测臂旋转范围±180°,样片夹持台转动范围360°,角分辨率均为0.036°。经误差分析,所研制装置的转角精度约为0.75﹪。     

基于HASH函数的RFID安全双向认证协议研究

针对RFID技术在信息传递过程中容易遭受跟踪攻击、重放攻击等问题,为提高信息在传递过程中的安全性,在已有的RFID安全认证协议的基础上,提出一种基于Hash函数的RFID安全双向认证协议,并对其进行BAN逻辑分析和性能分析。该认证协议不仅运算量小、所需存储空间小、成本低,而且在一定程度上具有防窃听、前向安全性、防位置跟踪、不可分辨性以及防重放攻击等优点,较好地解决RFID技术在信息传输过程中可能遇上的安全隐患问题,适用于RFID环境。     

复合材料双向波纹夹层结构力学性能

为改进传统单向波纹夹层结构横向力学性能较差的缺点,设计了一种新型复合材料双向波纹夹层结构。考虑复合材料双向夹层结构制备困难,研究了整套真空辅助成型工艺(VARI)工艺制备方案,实现双向波纹夹层结构的高效制备,以满足工程应用的需要。对制备出的复合材料双向波纹夹层结构与单向波纹夹层结构分别进行面外压缩、弯曲和剪切实验,分析了双向波纹夹层结构在不同载荷下的破坏模式及其失效机制,计算了该结构在不同荷载条件下的强度和模量,并将其与单向波纹夹层结构进行对比分析。结果表明,在压缩荷载作用下,玻璃纤维/环氧树脂芯子为主要承载部分,结构的失效主要体现在芯子的屈曲、断裂和分层;在弯曲荷载的作用下,由于纤维的抗压强度远小于抗拉强度,所以压头下方的上面板最先达到破坏荷载,结构的弯曲失效形式主要为上面板的断裂和脱粘;结构的剪切失效主要以泡沫与面板的脱粘和压溃为主,芯子和面板未见明显的破坏现象;与单向波纹夹层结构相比,双向波纹夹层结构力学性能显著提升。      

复合材料激光辐照实验中的吸收特性研究

在复合材料激光烧蚀研究中,首要解决的问题是激光的吸收问题。利用积分球法测量了材料蒙皮层及材料整体结构对1.319μm红外激光的反射率和透过率,获得了蒙皮层材料激光吸收特性随厚度的变化规律,以及材料表面状态变化导致的反射与透射特性的变化。在此基础上研究了蜂窝夹层材料激光烧蚀前后的反射、透射性能变化,结果表明,材料表面碳化后对近红外激光的反射率由30%降为10%左右,透过率由40%降为0,平均吸收率由30%增加到90%,蜂窝夹层材料对激光的吸收大大增加。     

X射线小角散射法对纳米氧化锆粒度分布的研究

采用小角X射线散射法，在普通的X射线衍射仪上实现纳米级氧化锆粒度分布的表征；并按照国标（GB／T13221-2004）的标准，使用SAXS程序计算出纳米氧化锆的粒度分布范围在1-36nm之间。与纳米氧化锆的TEM图像相比较，对比结果表明该方法测得的纳米氧化锆粒度可靠。     

Instrument to measure the bidirectional reflectance distribution function of surfaces

A new instrument to measure the in situ bidirectional reflectance distribution function (BRDF) of surfaces is described. This instrument measures the BRDF for eight illumination angles from 0 to 65 deg, three colors (475, 570, and 658 nm), and at over 100 selected viewing angles. The viewing zenith angles range from 5 to 65 deg, and the azimuth angles, relative to the illumination direction, range from 0 to ?180 deg. Many tests of the system have been run and show that for flat surfaces the BRDF of a sample surface can be measured with a precision of 1-5% and an accuracy of 10% of the measured reflectance. The BRDF for a dry and wet sand sample is presented as a demonstration of the instrument.     

Image-based bidirectional reflectance distribution function measurement

We present a new image-based process for measuring a surface's bidirectional reflectance rapidly, completely, and accurately. Requiring only two cameras, a light source, and a test sample of known shape, our method generates densely spaced samples covering a large domain of illumination and reflection directions. We verified our measurements both by tests of internal consistency and by comparison against measurements made with a gonioreflectometer. The resulting data show accuracy rivaling that of custom-built dedicated instruments.     

Comparisons of bidirectional reflectance distribution function measurements on prepared particulate surfaces and radiative-transfer models

To understand the connection between single-particle optics and the optics of a closely packed surface, controlled laboratory measurements of bidirectional reflectance distribution functions on layers of polymer and glass spheres are carried out. The measurements are compared with predictions from five radiative-transfer models; the Hapke's models, the Lumme-Bowell model, the BRF algorithm of Mishchenko et al., and the discrete ordinate radiative transfer. It is found that models of strict numerical radiative-transfer equations (RTEs) predict measurements well in some regions but have errors in both forward- and backward-scattering directions. The improved Hapke's model, although it has an anisotropic multiple-scattering term, still produces considerable errors compared with the strict RTE. The difference can be attributed to the exclusion of a diffraction contribution in the Hapke model.     

Rayleigh scattering limits for low-level bidirectional reflectance distribution function measurements

The objective is to estimate the Rayleigh limit in bidirectional reflectance distribution function (BRDF) measurements caused by air in the laboratory, the wavelength, and the path length of light in the receiver field of view. Moreover, we intend to show the trend for the reduction of this limit by introducing a medium with small refractive index and by using a longer wavelength. Although the BRDF typically describes the angular distribution of scattered light from surfaces, the expression describing the equivalent BRDF caused by the optical scattering from gas molecules in the optical path is derived through the use of the Rayleigh scattering theory. The instrumentation is described, and the experimental results of the equivalent BRDF caused by gas scattering from molecules in clear air, nitrogen, and helium gases are reported. These results confirm the trends of the prediction.     

Optical properties (bidirectional reflectance distribution function) of shot fabric

To study the optical properties of materials, one needs a complete set of the angular distribution functions of surface scattering from the materials. Here we present a convenient method for collecting a large set of bidirectional reflectance distribution function (BRDF) samples in the hemispherical scattering space. Material samples are wrapped around a right-circular cylinder and irradiated by a parallel light source, and the scattered radiance is collected by a digital camera. We tilted the cylinder around its center to collect the BRDF samples outside the plane of incidence. This method can be used with materials that have isotropic and anisotropic scattering properties. We demonstrate this method in a detailed investigation of shot fabrics. The warps and the fillings of shot fabrics are dyed different colors so that the fabric appears to change color at different viewing angles. These color-changing characteristics are found to be related to the physical and geometrical structure of shot fabric. Our study reveals that the color-changing property of shot fabrics is due mainly to an occlusion effect.     

Structured surface bidirectional reflectance distribution function reciprocity: theory and counterexamples

I address the question of whether the bidirectional reflectance distribution function (BRDF) of a structured surface has the same value when the incident and reflected angles are reversed. In particular, I examine the validity of some recent counterexamples and contrary measurements. On the basis of a new definition for structured surfaces, I conclude that the BRDF should be reciprocal. I show that designed counterexamples are flawed and suggest that measurements that do not exhibit reciprocity can be attributed to uncertainty and uncontrolled factors.     

Bidirectional reflectance distribution function of specular surfaces with hemispherical pits

We derive the bidirectional reflectance distribution function for a class of opaque surfaces that are rough on a macroscale and smooth on a microscale. We model this type of surface as a distribution of spherical mirrors. Since our study concerns geometrical optics, it is only the aperture of the concavities that is relevant, not the dimension. The three-dimensional problem is effectively transformed into a much simpler two-dimensional one involving the possibly infinitely many reflections in a spherical mirror. We find that these types of surface show very strong backscattering when the pits are deep but forward scattering when the pits are shallow. Such surfaces also show spectral effects as a result of multiple reflections and polarization effects that are due to the orientation of the effective surface. Both this model and the locally diffuse thoroughly pitted surface model [Int. J. Comput. Vision 31, 129 (1999)] are superior to other models in that they allow for an exact treatment for physically realizable surface geometries.     

Reflectometer for measuring the bidirectional reflectance of rough surfaces

A fully automatic, four-axis gonioreflectometer is described. It has an angular accuracy of 0.3 degrees and a range of 90 degrees in both the theta(i) and the theta(r) zenith angles. The gonioreflectometer is simpler than previous designs because of its use of rotating arms rather than moving carriages to mount the optical components. Where possible, commercial components have been used to reduce the cost. A novel off-axis angular encoding scheme is also described.     

Directional statistics-based reflectance model for isotropic bidirectional reflectance distribution functions

We introduce a novel parametric bidirectional reflectance distribution function (BRDF) model that can accurately encode a wide variety of real-world isotropic BRDFs with a small number of parameters. The key observation we make is that a BRDF may be viewed as a statistical distribution on a unit hemisphere. We derive a novel directional statistics distribution, which we refer to as the hemispherical exponential power distribution, and model real-world isotropic BRDFs as mixtures of it. We derive a canonical probabilistic method for estimating the parameters, including the number of components, of this novel directional statistics BRDF model. We show that the model captures the full spectrum of real-world isotropic BRDFs with high accuracy, but a small footprint. We also demonstrate the advantages of the novel BRDF model by showing its use for reflection component separation and for exploring the space of isotropic BRDFs.     

Laboratory-based bidirectional reflectance distribution functions of radiometric tarps

Laboratory-based bidirectional reflectance distribution functions (BRDFs) of radiometric tarp samples used in the vicarious calibration of Earth remote sensing satellite instruments are presented in this paper. The results illustrate the BRDF dependence on the orientation of the tarps' weft and warp threads. The study was performed using the GSFC scatterometer at incident zenith angles of 0 degrees, 10 degrees, and 30 degrees; scatter zenith angles from 0 degrees to 60 degrees; and scatter azimuth angles of 0 degrees, 45 degrees, 90 degrees, 135 degrees, and 180 degrees. The wavelengths were 485 nm, 550 nm, 633 nm, and 800 nm. The tarp's weft and warp dependence on BRDF is well defined at all measurement geometries and wavelengths. The BRDF difference can be as high as 8% at 0 degrees incident angle and 12% at 30 degrees incident angle. The fitted BRDF data show a very small discrepancy from the measured ones. New data on the forward and backscatter properties of radiometric tarps are reported. The backward scatter is well pronounced for the white samples. The black sample has well-pronounced forward scatter. The provided BRDF characterization of radiometric tarps is an excellent reference for anyone interested in using tarps for radiometric calibrations. The results are NIST traceable.