Interference and diffraction in globular metal films

The optical properties of globular Sn films on Al2O3 substrates are analyzed. These films are composed of Sn particles with sizes of the order of the wavelength of visible light, and are of interest because of their solar-selective properties. The experimental data show interference-film properties at long wavelengths, and a large diffracted component at short wavelengths superimposed on a Lambertian angular distribution. The calculated properties of finite-conductivity wire gratings in general agree well with the observed film properties; however, at short wavelengths a scalar diffraction model describes the angular distribution of the transmitted diffracted light. In this model the globular film appears to have film-thickness variations, and a coherence length of approximately two globule diameters. With an empirical “shadowing” term removed from the experimental data, this scalar model works well even at relatively long wavelengths (λ/4 ≤ globule radius).