A scanning deflectometry scheme for online defect detection and 3-D reconstruction of specular reflective materials

In this paper, a new deflectometry approach well suited for online inspection of specular reflective materials is proposed. Based on a simple hardware setup combining two linear light sources, a camera and a conveyor, our approach allows to detect, to localize, and to reconstruct in 3-D, surface aspect defects. It is easy to implement and particularly well suited for large objects in an industrial context (production line, for example). When the camera and light sources are fixed, the first step consists in calibrating the camera and estimating the light sources positions in the camera frame. Then, a full scanning of the object can be done, and the defects are detected in all images. In the last step, the defect is reconstructed in 3-D by slope integration. Thanks to a second light source added to the setup, the initial depth required by this kind of method is automatically estimated. Our approach has been tested in real experimental conditions with different plastic reflective parts and compared to the results provided by a metrology machine to validate the calibration and the reconstruction accuracy. The reconstructions of few millimeters defects on 4 different plastic parts show errors below 50 \(\upmu \) $\mathrm{\mu }$ m. This accuracy meets the usual requirement in the industrial context as the 3D industrial metrology machine that we use to validate our method is more complex to handle with specular objects and has an accuracy of around 20 \(\upmu \) $\mathrm{\mu }$ m.