The reflectance of dendritic-tungsten surfaces

The relatively low total optical reflectance measured from the surface of dendritic tungsten is quantitatively explained in terms of multiple reflections from the surface structure. A geometrical optical model is found to adequately describe the multiple reflections from dendrites which have an average spacing of about 5 μm. Coefficients representing the fraction of light reflected a given number of times are determined by analysis of the total reflectance of the dendritic surface on which an antireflective coating has been formed by anodization. At most wavelengths, second-order reflection (or light twice reflected) is found to dominate the reflectance of the dendritic surface for angles of incident light up to about 60° from normal, beyond which the first-order reflection becomes dominant. Based on the model, the reflectance of a geometrically characterized dendritic surface can be predicted for any type and thickness of antireflective coating.