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11 November 1996 Contamination and related localized absorption in coated substrates
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Contamination of bare and coated substrates leads to strong degradation of their optical properties. We have developed an apparatus allowing precise positioning of a sample and simultaneous mapping of absorptance and scattering of a selected area on its surface. Thus we have characterized different cleaning procedures of bare substrates by mapping surface absorption. We have shown that cleaning processes displace polishing and cleaning residues without completely removing them. However, in all cases, multicomponent glasses (such as BK7) have a mean surface absorptance higher than that of fused-silica. Then we have studied coated substrates. For a coating on a multicomponent glass we have shown evidences of a photoinduced absorption in the substrate due to solarization during the evaporation process. Moreover metallic ions migrate from the glass substrate and gather at the film interfaces. We have developed a method of front and back illumination of samples in order to separate each part of the in-depth absorptance of single layer coated glasses that is to say bulk and interface absorptances of the film, and substrate absorption due to solarization. In all our measurements, absorptance of the substrate's solarized area prevails and, in the layer, absorptance at the interfaces is much higher than that of the bulk material. On contrary, for low absorbing films deposited on fused-silica substrates our results show that absorption of the film simply adds to that of the bare substrate. That is why we can observe precise correlation between absorption mappings of the same area on the bare and on the coated substrate.
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Pierre J. Roche "Contamination and related localized absorption in coated substrates", Proc. SPIE 2864, Optical System Contamination V, and Stray Light and System Optimization, (11 November 1996);

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