Color variations of AR coatings caused by a leached layer on the substrate

Color differences of AR-coated prisms were found to be due to a surface layer with a refractive index of n = 1.46 and a geometrical thickness of ~41 nra on the glass substrate (BaK4, n = 1.5688). The existence of this layer is demonstrated by reflection spectroscopy measurements together with numerical calculations of the spectral reflectance of both the uncoated and coated substrates. Auger electron spectroscopy depth profiling analyses reveal that the Ba content of the glass had been leached out completely in the layer. The leaching process was caused by an improper cleaning agent used with the uncoated prisms in the optical shop. In addition to its inherent optical interference effect, the leached substrate surface seems to influence the condensation and growth of the first layer of AR coatings deposited onto it thus causing additional changes of the spectral characteristics of the coating.