In metallic fusion devices, parasitic light originating from multiple reflections on the wall is a major problem for the interpretation of optical diagnostics. Strong stray light affects several optical diagnostics in ITER. One possibility to cope with this reflected light is to use photonic simulation, which can accurately predict the behavior of light within complex 3D geometry. A prerequisite is to get a good description of the reflection model, represented by the Bidirectional Reflectance Distribution Function (BRDF), based on optical measurements of in-vessel materials. To avoid complicated measurements using goniophotometer to get the BRDF, one possibility is to link surface optical properties and topography characteristics, such as roughness measurements, for example, using the classical Bennett’s formula. Measurements were performed using two experimental goniophotometers to fully characterize the BRDF of tungsten samples with different roughness values. Surface topography was measured using a three-dimensional laser scanning confocal microscope. Several parameters were extracted from these measurements including the arithmetic average roughness (), the root mean square roughness (), the Surface Inclination Angle Distribution and furthermore its mean value and the power spectral density (PSD). The correlations of BRDF model parameters deduced from the measurements are compared with the previous topographic parameters. The initial results on several tungsten samples show that , which is the usual measure of surface roughness, is not the most suitable metric to link with the reflection behavior of the surface. In contrast, the PSD and the surface inclination angle are interesting metrics for describing the reflected light.

中文翻译：

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可见光谱中钨表面形貌参数与 BRDF 之间的关系

在金属融合装置中，源自壁上多次反射的寄生光是光学诊断解释的一个主要问题。强杂散光会影响 ITER 中的多种光学诊断。处理这种反射光的一种可能性是使用光子模拟，它可以准确预测复杂 3D 几何结构中的光行为。一个先决条件是基于容器内材料的光学测量，获得由双向反射分布函数 (BRDF) 表示的反射模型的良好描述。为了避免使用测角光度计进行复杂的测量来获得 BRDF，一种可能性是将表面光学特性和形貌特征（例如粗糙度测量）联系起来，例如使用经典的 Bennett 公式。使用两台实验测角光度计进行测量，以充分表征具有不同粗糙度值的钨样品的 BRDF。使用三维激光扫描共焦显微镜测量表面形貌。从这些测量中提取了几个参数，包括算术平均粗糙度 ()、均方根粗糙度 ()、表面倾角分布及其平均值和功率谱密度 (PSD)。将测量推导出的 BRDF 模型参数的相关性与之前的地形参数进行比较。几个钨样品的初步结果表明，作为表面粗糙度的常用度量，并不是与表面反射行为联系起来的最合适的度量。相比之下，PSD 和表面倾角是描述反射光的有趣指标。