Phase measuring deflectometry (PMD) has been widely used to measure three-dimensional (3D) shape of specular objects because of its advantages of non-contact, large dynamic range, high-precision and fast-acquisition. However, due to the limited depth of field (DOF) of a camera lens, a liquid crystal display (LCD) screen and a tested specular object cannot be clearly imaged together, which affects the measurement accuracy. To tackle this issue, this paper proposes a novel PMD method of auxiliary imaging LCD screen by using a concave mirror. The LCD screen and a reference are in the same plane via the center of the concave mirror, and are perpendicular to the optical axis of the mirror. Both the LCD screen and the reference are symmetric each other through the optical axis, so that the imaging of one is the location of the other. Based on the imaging principle of concave mirror, the LCD screen is reflected and an inverted real image of the equal size is formed at the reference, and then reflected into the camera by the reference mirror during calibration. During the measuring procedure, a tested specular object is placed at the position of the reference. The displayed fringe patterns on the LCD screen are modulated by the tested specular object and the deformed fringes are captured by the camera. After phase calculation from the captured fringe patterns, 3D shape of the tested specular object can be obtained. The proposed PMD method can avoid the limited DOF of the camera lens, because fringe patterns displayed on the LCD screen and the tested specular surface can be clearly imaged together at the same position. Some simulated experiments on measuring specular objects have been carried out and the results demonstrate that the proposed PMD method can effectively and accurately measure 3D shape of specular surfaces.
Phase measuring deflectometry (PMD) is a superior technique to obtain three-dimensional (3D) shape information of specular surfaces because of its advantages of large dynamic range, noncontact operation, full-field measurement, fast acquisition, high precision, and automatic data processing. We review the recent advances on PMD. The basic principle of PMD is introduced following several PMD methods based on fringe reflection. First, a direct PMD (DPMD) method is reviewed for measuring 3D shape of specular objects having discontinuous surfaces. The DPMD method builds the direct relationship between phase and depth data, without gradient integration procedure. Second, an infrared PMD (IR-PMD) method is reviewed to measure specular objects. Because IR light is used as a light source, the IR-PMD method is insensitive to the effect of ambient light on the measured results and has high measurement accuracy. Third, a proposed method is reviewed to measure the 3D shape of partial reflective objects having discontinuous surfaces by combining fringe projection profilometry and DPMD. Then, the effects of error sources that mainly include phase error and geometric calibration error on the measurement results are analyzed, and the performance of the 3D shape measurement system is also evaluated. Finally, the future research directions of PMD are discussed.
Phase measuring deflectometry (PMD) is a superior technique to obtain 3D shape information of specular surfaces because of its advantages of large dynamic range, non-contact operation, full-field measurement, fast acquisition, high precision and automatic data processing. This paper reviews the recent advance on PMD. First, the basic principle of PMD is introduced following several fringe reflection methods. Then, a direct PMD (DPMD) method is presented for measuring 3D shape of specular objects having discontinuous surfaces. The DPMD method builds the relationship between phase and depth data, without gradient integration procedure. Next, an infrared PMD (IR-PMD) method is reviewed to measure specular objects. Because IR light is used as a light source, the IR-PMD method is insensitive to the effect of ambient light and has high measurement accuracy. The following will analyze the effects of error sources, including nonlinear influence, lens distortion of imaging and projecting system, geometric calibration error, on the measurement results and evaluate the performance of the 3D shape measurement system. Finally, the future research directions of PMD will be discussed.
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.
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