Robot-assisted BRDF measurement and surface characterization of inhomogeneous freeform shapes

The quality of optical measurements is significantly affected by the reflection properties of the measured component. Therefore, it is important to consider the properties of the reflective surface to obtain accurate measurement results. A common method for the mathematical representation of reflections is the bidirectional reflection distribution function (BRDF). Typically BRDFs are measured via a gonioreflectometer. However, these are often only applicable on flat specimens or objects with previously known geometric properties. This paper presents an approach for the measurement of the BRDF on inhomogeneous freeform surfaces. For this purpose, a robot-assisted multisensor system is used consisting of a fringe projection sensor and an industrial camera, which is modified with six light sources that are evenly distributed around the optical axis and point at the measuring object. The reflection measurement consists of the sequential image acquisition of individual lighting configurations by successively switched on light sources. With the assumption of isotropic surface properties and known position of each individual light source, the relative BRDF can be determined pixel by pixel. This enables the BRDF measurement of freeform surfaces with varying reflection properties. Knowing the transformation between both sensor coordinate systems, the resulting BRDF data can be projected onto the points of the fringe projection measurement for geometrical representation. As an application example, a damage characterization of surfaces, based on the measured BRDF data is presented. For this purpose, a worn turbine blade of an aircraft engine is characterized so that burnt regions on the components' surface can be detected.