3D-design-model-assisted shiny surface shape measurement in fringe projection profilometry

In fringe projection three-dimensional (3D) shape measurement, image saturation is an important influencing factor for shiny surfaces, for example, an aero-engine blade. This paper proposes a novel method assisted by the 3D design model to resolve image saturation. The proposed algorithm has the following characteristics: (a) Based on the high matching between the measured finished object and its 3D design model, the design model is transformed into the discrete point cloud with sufficient accuracy. (b) Based on the spatial coordinate transformation principle, a multi-coordinate system transformation method is proposed to convert discrete point cloud data from the design coordinate system to the world coordinate system. (c) Considering the influence of the actual measurement conditions on the measured object, the existing illumination model is improved by integrating the system's greyscale nonlinear response, camera exposure time and sensitivity. (d) Combined with the captured grating images and the 3D design model, the parameters are accurately calibrated by the least-squares algorithm. (e) Using the calibrated reflection model, the optimal rotation angle is determined to compensate for the spatial points corresponding to the image saturation area, and finally, the entire measured surface is registered with point clouds in the calculated optimal rotation locations. Our method is used in the actual pressure surface measurement experiment. The experimental results show that our method can effectively measure the space points corresponding to the image saturation area in the optimal rotation angle and improve the measurement accuracy of shiny surfaces.