Three-dimensional shape measurement of specular objects based on infrared phase-measuring deflectometry

Phase-measuring deflectometry (PMD)-based methods have been widely used in the measurement of the threedimensional (3D) shape of specular objects, and the existing PMD methods utilize visible light. However, specular surfaces are sensitive to ambient light. As a result, the reconstructed 3D shape results are mostly affected by the external environment in actual measurements. To overcome this problem, a novel infrared-PMD (IR-PMD) method is proposed to measure specular objects by directly establishing the relationship between the absolute phase and depth data. In addition, a new calibration method for the measurement system has been proposed by combining fringe projection and fringe reflection. The proposed IR-PMD method uses an IR projector to project sinusoidal fringe patterns onto a ground glass, which can be regarded as an IR digital screen. The IR fringe patterns are reflected by the measured specular surfaces and the deformed fringe patterns captured by an IR camera. The multiple-step phase-shifting algorithm and the optimum three-fringe number selection method are applied to the deformed fringe patterns to obtain the wrapped and unwrapped phase data, respectively. Then 3D shape data can be directly calculated by the unwrapped phase data on the screen located at two positions. The results have validated the effectiveness and accuracy of the proposed method. It can be used to measure specular components in the application fields of advanced manufacturing, automobile industry, and aerospace industry.