An appearance-based measure of surface defects

Surface defects on sheet metal panels such as local out-of-plane geometric deviations, cause unwanted deviations from the intended visual appearance. In the automobile industry, these defects are commonly graded for visual acceptability through reflection analysis on the hardware. This widely used method is unfortunately subjective and thus has low repeatability. The same method can be applied on simulated sheet metal panels by the use of ray-tracing. In this paper we present an objective, appearance-based, measure by formalising the approach of subjective reflection analysis. Hence, a natural connection between an objective measure and the appearance is obtained making it easy to establish acceptable levels. The performance of this measure and three other objective measures is discussed and demonstrated for a real panel and a panel geometry obtained through sheet forming simulations. These examples illustrate that different measures yield different information about the geometry, and the proposed measure gives a useful quantification of the visual appearance deviation.