Infrared reflectance spectra of semi-transparent SiO₂ rich films on silicate glasses: Influence of the substrate and film thickness

The influence of the substrate and film thickness on the IR reflectance spectra of silicate glasses with silica rich surface films is demonstrated experimentally and with mathematical simulations. SiO₂ films, both 0.3 and 0.6 μm thick, are sol-gel coated onto one metal and three glasses. The measured IRRS curves are definitely different, in agreement with predictions based on a Fresnel-type equation considering the optical constants of both the substrate and the surface layer, the film thickness and the wavelength. Mathematical simulations show that the IRRS curves are insensitive to the substrate only for SiO₂ coatings with a thickness of several micrometers. These results show that the traditional interpretation of the IRRS spectra of leached and surface coated glasses, assuming that the incident beam is fully absorbed before reaching the film/substrate interface and therefore attributing all the observed bands to some species in the surface film, can be grossly approximated.