The limits of traditional lightweight engineering are being extended by the development of hybrid components. Lightweight potential is especially high when using dissimilar materials, e.g., a friction-welded steel-aluminum combination. An important factor for the mechanical properties of such a combination is the geometry and location of the joining zone between the materials. The geometry of these objects can be reconstructed by optical triangulation techniques such as fringe projection profilometry. In this paper, we present a method to robustly detect the different material zones on the surface of a hybrid steel-aluminum component. We use reflection models and data from a fringe projection profilometry system. The intensity values and 3D geometry data from the fringe projection system are used to estimate material-specific reflection parameters for each 3D point and detect different material areas based on a global threshold.