Inspection of specular and partially specular surfaces

The inspection of specular surfaces differs significantly from the case of non-specular surfaces. In contrast to the non-specular case, the appearance of a specular surface is dominated by the reflections of the environment that are visible in it. The transfer of this observation into automated visual inspection is called deflectometry. The main principle of deflectometric surface acquisition is to use a highly controllable environment, where a screen on which a well-defined pattern is presented is observed via the specular reflecting surface. Knowing that pattern, it is possible to inspect the surface qualitatively and - at least with certain additional knowledge - to reconstruct the surface under test. In this paper, we introduce the theoretical background of deflectometry. After presenting some properties of the deflectometric inspection itself, we describe the qualitative and quantitative evaluation of the deflectometric observation in detail. We will show that an inspection of specular and partially specular objects is feasible in an industrially applicable inspection system. For complexly formed and/or large objects, we propose a robot-based inspection setup.