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In recent years we investigated optical inspection methods for quality control of printed matter. Special imaging processes like reflected shadow imaging in the specular reflex, scanning through focal planes using switched delay elements and narrowing the depth of field by using central stops, and also cross polarization photography were used to separate printed layers from semi-macroscopical structures or textures of glossy surfaces and vice versa. These methods are interesting not only for printed matter, but for several kinds of more or less 2-1/2-D surfaces, like textured metal sheets, lacquered surfaces or laminated material. A similar problem arises in the automated inspection of natural roof tiles, especially from clay slate with fine mica like lamella structure, which is usually the preferred material due to cleavage and easy machining. The typical inspection process involves visual and acoustic checks for possible disintegration of the lamella structure throughout the tile, but it does not necessarily quantify the integrity of the lamellae near surface. Further, the orientation and quality of the breaking edges of the lamellae determines the water flow conditions on the roof and the resistance against frost damages, which is to be considered in the finishing process when the tile is cut from the blank and again when installing the tile. While traditionally the slate production involves manual and visual inspection in every step, an automated inspection method for pre-selection and orientation of blanks as well as documented and certified onsite inspection are desirable. The present paper is dedicated to the development of a robust and rugged imaging system, which emphasizes the slate structure in orthogonal view an allows the discrimination of elevated edges from color variations and the extraction of the edge orientation in both longitudinal and transversal directions. In the paper we discuss our current investigations, the system concept and application examples and present some illustrative measurement results.
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