It is well known that evaporated aluminum films develop a diffuse reflecting surface as their thickness increases, but it is not generally appreciated that the limiting thickness at which light scattering appears decreases as the incidence angle of the vapor atoms arriving at the condensing surface increases. Aluminum films formed at 80° incidence have a greater absorption than those condensed at normal incidence and develop a diffuse reflecting surface before becoming opaque. The film surface density at which diffuse reflection appeared at different vapor incidence angles was determined by measuring the specular reflectivity of the film for blue light at nearly normal incidence. It is shown that a diffuse reflecting surface is formed more easily as the vapor incidence angle approaches grazing incidence because the vapor atoms are mainly deposited on the upper surface of the film nuclei, the most elevated of which tend to grow in the direction of the vapor beam.
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