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In the production of optical elements, surface layers can be created on optical glass in various ways, particularly if we are dealing with sensitive glasses of low acid or alcali resistivity. Basically, any exposure of a glass surface to a wet or a humid environment can cause a surface layer, although different mechanisms may be determining its thickness and composition. In the production of optical elements, their surface can be endangered before coating by polishing, cleaning, general handling and even by storage. Some of these cases will be discussed in general and illustrated by examples. It is shown that all of these surface layers - independent of how they originate - alter the optical properties of subse-quently deposited thin film interference coatings, particularly of antireflection coatings, in a much disturbing manner. Besides more or less homogeneously extended surface layers, localized topographic defects on optical surfaces such as polishing or cleaning residues even much below any scratch and dig specification can degrade the performance of optical coatings deposited thereon. Such minute defects are not visible to the naked eye and perhaps also not detectable with the standard 4 - 8 x magnifiers which are still in use in many optical shops. However, these submicron defects can initiate structural defects in optical coatings which are preferably disturbing in multilayers as the size of the defects in the coating increases with its over-all thickness.
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