Mishchenko M I, Mackowski D W, Travis L D
Appl Opt. 1995 Jul 20;34(21):4589-99. doi: 10.1364/AO.34.004589.
We use the T-matrix method as described by Mishchenko and Mackowski [Opt. Lett. 19, 1604 (1994)] to compute light scattering by bispheres in fixed and random orientations extensively. For all our computations the index of refraction is fixed at a value 1.5 + 0.005i, which is close to the refractive index of mineral tropospheric aerosols and was used in previous extensive studies of light scattering by spheroids and Chebyshev particles. For monodisperse bispheres with touching components in a fixed orientation, electromagnetic interactions between the constituent spheres result in a considerably more complicated interference structure in the scattering patterns than that for single monodisperse spheres. However, this increased structure is largely washed out by orientational averaging and results in scattering patterns for randomly oriented bispheres that are close to those for single spheres with size equal to the size of the bisphere components. Unlike other nonspherical particles such as cubes and spheroids, randomly oriented bispheres do not exhibit pronounced enhancement of side scattering and reduction of backscattering and positive polarization at side-scattering angles. Thus the dominant feature of light scattering by randomly oriented bispheres is the single scattering from the component spheres, whereas the effects of cooperative scattering and concavity of the bisphere shape play a minor role. The only distinct manifestations of nonsphericity and cooperative scattering effects for randomly oriented bispheres are the departure of the ratio F(22)/F(11) of the elements of the scattering matrix from unity, the inequality of the ratios F(33)/F(11) and F(44)/F(11), and nonzero linear and circular backscattering depolarization ratios. Our computations for randomly oriented bispheres with separated wavelengthsized components show that the component spheres become essentially independent scatterers at as small a distance between their centers as 4 times their radii.
我们采用米申科和马科夫斯基[《光学快报》19, 1604 (1994)]所描述的T矩阵方法，广泛计算固定和随机取向的双球体的光散射。在我们所有的计算中，折射率固定为1.5 + 0.005i，这接近于对流层矿物气溶胶的折射率，并且在先前对球体和切比雪夫粒子的光散射的广泛研究中被采用。对于固定取向且组分相接触的单分散双球体，组成球体之间的电磁相互作用导致散射图案中的干涉结构比单个单分散球体的干涉结构复杂得多。然而，这种增加的结构在很大程度上被取向平均所消除，并且导致随机取向双球体的散射图案接近于大小等于双球体组分大小的单个球体的散射图案。与其他非球形粒子（如立方体和球体）不同，随机取向的双球体在侧向散射角处不会表现出明显的侧向散射增强、后向散射减少以及正偏振。因此，随机取向双球体的光散射的主要特征是来自组分球体的单次散射，而双球体形状的协同散射和凹度的影响起次要作用。对于随机取向双球体，非球形和协同散射效应的唯一明显表现是散射矩阵元素的比值F(22)/F(11)偏离1，比值F(33)/F(11)和F(44)/F(11)不相等，以及非零的线性和圆后向散射退偏比。我们对具有分开的波长大小组分的随机取向双球体的计算表明，当它们中心之间的距离小至其半径的4倍时，组分球体基本上成为独立的散射体。
