The effects of grain size on bidirectional polarized reflectance factor measurements of snow

The linear polarization of light reflected from snow surfaces was measured by an instrument composed of a semi-automatic goniometer and an ASD spectroradiometer under a direct lamp to determine its potential to detect differences in grain size. The Stokes parameters were computed for all snow samples. The results confirmed that the degree of polarization increased in bright bands and decreased in dark bands with an increase in grain size. The trend was more obvious in the near-infrared bands and in the forward direction. As the snow grain size varied the contribution of Q parameter to the degree of polarization was more than the U parameter, which was slight. Our results confirmed that the degree of polarization may be useful to distinguish the snow grain size. All above measurements were carried out in the laboratory where the atmospheric contribution was ignored.     

Polarised bidirectional reflectance factor measurements from soil, stones, and snow

The bidirectional reflectance factor and degree of linear polarisation of selected snow, soil, and gravel types have been measured using Finnish geodetic institute field goniospectropolariphotometer (FIGIFIGO). It was observed that with all measured samples the degree of linear polarisation is weakest 0–30 backwards from nadir, turning 1–5% negative in the backward direction, and growing larger (5–50%) in the forward and Brewster directions. Polarisation was found to be inversely but non-linearly proportional to reflectance. In addition, a wavelength-dependent trend was found to exist in some data, but in general, the wavelength dependence was smooth. Dry old snow polarises clearly more than melting snow or new snow. White gravel polarises somewhat similarly to snow in visible region, and black gravel resembles snow in infrared.The authors conclude that polarisation observations may be useful in land surface remote sensing, but that only far-forward angles (60–70) contain a strong enough signal for easy interpretation. The polarisation spectrum brings little additional information to reflection spectrum, but in conditions where the reflectance spectrum cannot be accurately normalised or incident irradiation is not known, polarisation spectrum could still be usable. The directional signal is strong and may yield refractive index or microstructure, and must be understood in any application. Polarisation near nadir is low for all known samples, and can thus be safely ignored with most non-polarised imaging applications. However, for accurate atmospheric polarimetry, e.g. by polarisation and directionality of the earth's reflectances (POLDER) or aerosol polarimetry sensor (APS) sensors, it is recommended to take into account land surface polarisation.     

植被双向反射分布函数的模型及其发展

介绍了双向反射分布函数(BRDF)的概念和反映植被特性的归一化植被指数(NDVI)、增强植被指数(EVI)、差值植被指数(DVI)等一些重要指数参数。评述了近年发展起来的植被BRDF模型的构建方法,例如统计反演方法,MODIS植被指数合成法和集合卡曼滤波法(EnKF)。分析了目标表面BRDF数据的获取方法和改进的驱动核算法。对BRDF模型的发展趋势进行了展望。     

Simulation of the polarimetric bidirectional reflectance distribution function

The dependences of the polarization parameters of probe radiation on the observation geometry and relations between the Stokes vector parameters have been simulated based on the bidirectional reflectance distribution function as applied to active polarimetric systems.     

An NMR protocol for determining ice crystal size distributions during freezing and pore size distributions during freeze-drying

Nuclear magnetic resonance water proton relaxometry is widely used to investigate pore size distributions and pore connectivity in brine-saturated porous rocks and construction materials. In this paper we show that, by replacing water with acetone, a similar method can be used to probe the porous structure of freeze-dried starch gels and therefore the ice crystal size distribution in frozen starch gels. The method relies on the observation that the starch surface acts as a powerful relaxation sink for acetone proton transverse magnetization so that Brownstein-Tarr theory can be used to extract the pore size distribution from the relaxation data. In addition the relaxation time distribution is found to depend on the spectrometer frequency and the Carr-Purcell-Meiboom-Gill pulse spacing, consistent with the existence of large susceptibility-induced field gradients within the pores. The potential of this approach for noninvasively measuring ice crystal size distributions during freezing and pore size distributions during freeze-drying in other food systems is discussed.     

Simulation of radiative heating of snow and ice coating

Numerical simulation of thermal state was performed for semitransparent medium; the thermal state is formed under impact of incident radiation and convective heat transfer to the ambient medium. The model takes into account the heat transfer to the semi-infinite opaque substrate below the layer of semitransparent material. Computation was performed for governing parameters typical of snow and ice coating during winter period. Solving the radiative part of problem uses the modified average flux method. This method takes into account the dependence of optical properties on the wavelength of incident radiation, scattering, and reflective properties of the layer boundaries.     

Analysis of one-dimensional rough surface scattering based on bidirectional reflectance distribution function model

Through in-depth analysis of the scattering properties of a one-dimensional random rough surface, a method to calculate the spatial bidirectional reflectance distribution function (BRDF) characteristics of a randomly rough surface is proposed. The scattering characteristics of an electrically large rough surface can be substituted by combination of subsection electromagnetic fields. The radar cross-section (RCS), which is transformed from the BRDF, is compared with numerical results from the Method of Moment (MoM). The agreement of the results verifies that this method could calculate the electromagnetic scattering field efficiently and straightforwardly. Moreover, the resulting combined fields from the BRDF of subsections in the rough surface show a good match with the entire scattering field of a whole surface, which indicates that a large electrically rough surface could be feasibly and effectively substituted with the combination of scattering elements using the BRDF method and the statistical method.     

基于双向反射分布函数的目标激光测距方程

导航系统中广泛采用激光测距确定星地间距离,双向反射分布函数可精确描述实际物体反射光的情况,是研究激光测距方程的有效手段。结合目标反射几何模型,计算得到透镜各点接收目标反射光强的表达式。对目标光斑和接收透镜进行网格划分,仿真得到透镜面元接收目标反射的光强分布和透镜接收的瞬时光强,并讨论了平面目标的网格步长选取和计算次数。当光束从不同空间位置入射至平面目标表面时,根据探测器-反射棱镜之间距离与接收光强的对应关系得到激光测距方程的待定参数。实测数据验证了理论计算的正确性。     

Measurement of bidirectional reflectance distribution function with a linear light source

This paper proposes a practical measurement system for bidirectional reflectance distribution functions (BRDFs) of a three-dimensional (3D) object with a linear light source. Using the linear light source, the proposed system can reduce the number of image acquisitions which are necessary for an estimation of the spatially-varying BRDFs of the object. Furthermore, the size of the proposed system is much smaller than a conventional system which uses a parallel light. In this proposed system, the light field of the linear light source is previously measured to determine direction and radiance of incident rays to each point of the object, because the direction and radiance are not constant at each point. Using the proposed system, the BRDF of a point of flat objects was experimentally measured, and results showed validation of the estimation accuracy of the proposed system. Measurement efficiency of the proposed system was also evaluated by comparing reflectance model parameters estimated by the conventional and proposed systems. For the estimation, the reflectance function of a 3D object was measured by both systems. The estimation accuracy of the proposed method was also evaluated by comparing among a real image and rendered 3D objects of the conventional and proposed methods.     

Estimation of surface snow properties using combined millimeter-wave backscatter and near-infrared reflectance measurements

Knowledge of surficial snow properties such as grain size, surface roughness, and free-water content provides clues to the metamorphic state of snow on the ground, which in turn yields information on weathering processes and climatic activity. Remote sensing techniques using combined concurrent measurements of near-infrared passive reflectance and millimeter-wave radar backscatter show promise in estimating the above snow parameters. Near-infrared reflectance is strongly dependent on snow grain size and free-water content, while millimeter-wave backscatter is primarily dependent on free-water content and, to some extent, on the surface roughness. A neural-network based inversion algorithm has been developed that optimally combines near-infrared and millimeter-wave measurements for accurate estimation of the relevant snow properties. The algorithm uses reflectances at wavelengths of 1160 nm, 1260 nm and 1360 nm, as well as co-polarized and cross-polarized backscatter at a frequency of 95 GHz. The inversion algorithm has been tested using simulated data, and is seen to perform well under noise-free conditions. Under noise-added conditions, a signal-to-noise ratio of 32 dB or greater ensures acceptable errors in snow parameter estimation.     

Equilibrium shape of nickel crystal

The equilibrium shape of pure nickel and the effect of carbon on changes in the equilibrium shape at 1200°C were investigated. A statistical observation on the size-dependent, time-dependent and carbon-induced morphological evolution of crystallites suggested that the equilibrium crystal shape (ECS) of pure nickel is a polyhedron consisting of {111}, {100}, {110} and {210} surfaces. However, crystals with an extensive proportion of {320} surfaces were also frequently observed. The appearance of {320} surfaces was interpreted as kinetically stabilized metastable surfaces, which survived during the thermal equilibrating process, possibly due to a high nucleation energy barrier for their removal. On the other hand, the ECS of pure nickel was observed to change dramatically into a spherical shape with facets of {111}, {100}, {110} and {210} without exception under a carburized atmosphere, which indicates that carbon not only facilitates surface diffusion by which energetically more stable surfaces can be easily developed but also decreases the surface energy anisotropy. Together with X-ray photoelectron spectroscopy studies, it was proposed that the carbon-induced changes in the ECS are possibly due to a solid solution effect, which could lead to a reduction in the binding energy among atoms in the bulk as well as on the surfaces.     

Omni directional reflectance properties of superconductor-dielectric photonic crystal

The omnidirectional reflection properties in one dimensional superconductor-dielectric photonic crystal have been studied theoretically. In this present communication, the superconductor-dielectric photonic crystal in one dimension having alternate regions of superconductor-dielectric. The reflectance behaviors from these periodic multilayered structures are calculated for different angles of incidence. The reflectance and band structure is obtained by solving a Maxwell's equation using a translational matrix method. The study of reflectance bands of such superconductor-dielectric photonic crystal show that it can be used as broad band reflector.     

Dependence of ice crystal optical properties on particle aspect ratio

The single-scattering properties of randomly oriented hexagonal ice crystals have been extensively used to study the bulk radiative properties of ice clouds. It has been reported in the literature that the asymmetry factors of these particles vary with their aspect ratios in a “V-pattern” with the minimum at a unit aspect ratio. However, this phenomenon was not explained in the previous studies. The present paper reports on an in-depth analysis of the optical properties of hexagonal ice crystals. It is shown that the delta-transmission is primarily responsible for the aforementioned “V-pattern” variation of the asymmetry factor as a function of the aspect ratio. Additionally, the term (1?f_δ)g is also partially responsible for the large values of the total asymmetry factor in the case of small aspect ratios, where f_δ indicates the ratio of the amount of the energy associated with the delta-transmission rays to the total amount of the scattered energy and g is the asymmetry factor associated with the geometric optics rays.     

Spectral reflectance characteristics of different snow and snow-covered land surface objects and mixed spectrum fitting

The field spectroradiometer was used to measure spectra of different snow and snow-covered land surface objects in Beijing area.The result showed that for a pure snow spectrum,the snow reflectance peaks appeared from visible to 800 nm band locations;there was an obvious absorption valley of snow spectrum near 1 030 nm wavelength.Compared with fresh snow,the reflection peaks of the old snow and melting snow showed different degrees of decline in the ranges of 300～1 300,1 700～1 800 and 2 200～2 300 nm,the lowe...     

Effect of particle size and shape on nonresonant random laser action of dye-doped polymer random media

We present an experimental study of the light emission from dye-doped polymer random media dispersed with TiO₂ particles of various sizes, shapes, and structures. Random lasing with nonresonant feedback, similar to that for spherically shaped particles that are used for conventional random lasers, is observed for almost all types of particles and aggregates. The efficiency of random lasing for each medium is analyzed using the relationship between the emission spectrum and the transport mean free path (TMFP), which is measured by enhanced backscattering experiments. Results show that the peak emission intensity depends strongly on the particle shape and structure, whereas the spectral linewidth is governed by the TMFP.     

不同积雪深度与面积对积雪覆盖遥感反演的影响

针对利用多源遥感数据进行积雪覆盖监测的需要,在分析不同深度与面积的积雪实测光谱基础上,探讨了在利用NDSI方法进行积雪覆盖遥感监测时,不同积雪深度对其的影响,并以HJ-1B卫星以及MODIS卫星为例,研究了不同积雪面积对遥感积雪反演的影响。结果表明,积雪深度几乎对遥感积雪覆盖监测的结果没有影响,而在积雪面积方面,受所使用的遥感影像空间分辨率的制约,对遥感积雪监测的结果有一定影响。     

Band structure and reflectance for a nonlinear one-dimensional photonic crystal

We consider a model for a one-dimensional photonic crystal formed by a succession of nonlinear Kerr-type equidistant spaceless interfaces immersed in a linear medium. We calculate analytically the band structure of this system as a function of the incident wave intensity, and find two main tendencies: the appearance of prohibited bands, and the separation and narrowing of these bands. We consider as well a finite version of this photonic crystal for a limited number of alternating linear and non linear set of stacks for which we calculate reflectance as a function of the electromagnetic wave intensity, band index and number of periods. A system with these features can be constructed by alternating very thin slabs of a nonlinear soft matter material with thicker solid films, which can be used to design a device to control light propagation for specific wavelength intervals and light intensities of the same propagating signal.