BRDF characterization of Al-coated thermoplastic polymer surfaces

In this paper, we present a combined morphological and optical characterization of aluminum-coated thermoplastic polymer surfaces. Flat plastic substrates, obtained by means of an injection molding process starting from plastic granules, were coated with ultra-thin aluminum films evaporated in vacuo, on top of which a silicon-based protective layer was plasma deposited in order to prevent oxidation of the metal reflective surface. Different sample treatments were studied to unravel the influence of substrate chemistry, substrate thickness, aluminum and protective layer thickness, and surface roughness on the actual optical reflectance properties. Bidirectional reflectance distribution function measurements, corroborated by surface morphological information obtained by means of atomic force microscopy, correlate reflectance characteristics with the root-mean-square surface roughness, providing evidence for the role of the substrate and the thin films' morphology. The results unravel information of interest within many applicative fields involving metal coating processes of plastic substrates as an example in the case of automotive lighting.