Three-Dimensional Shape Measurements of Specular Objects Using Phase-Measuring Deflectometry

Self Cite

Phase-measuring deflectometry (PMD)-based methods have been widely used in the measurement of the three-dimensional (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 is affected by the external environment in actual measurements. To overcome this problem, an infrared PMD (IR-PMD) method is proposed to measure specular objects by directly establishing the relationship between absolute phase and depth data for the first time. Moreover, the proposed method can measure discontinuous surfaces. In addition, a new geometric calibration method is proposed by combining fringe projection and fringe reflection. The proposed IR-PMD method uses a projector to project IR 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 are captured by an IR camera. A multiple-step phase-shifting algorithm and the optimum three-fringe number selection method are applied to the deformed fringe patterns to obtain wrapped and unwrapped phase data, respectively. Then, 3D shape data can be directly calculated by the unwrapped phase data on the screen located in two positions. The results here presented validate 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.

Section: Introductionmentioning

confidence: 99%

Measurement of the Three-Dimensional Shape of Discontinuous Specular Objects Using Infrared Phase-Measuring Deflectometry

Chang

Zhang

Gao

et al. 2019

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“…However, these 3D measurement techniques usually fail to deal with mirror-like specular surfaces subject to the strong reflectance. Instead of avoiding the specular reflection by coating the surface with powders, the phase measuring deflectometry (PMD) technique [10][11][12] takes full advantage of the surface reflective property, which captures the virtual image reflected by the target surface. By analyzing the deformed strip patterns captured by the camera, precise discrete slope variations of the test surface can be calculated.…”

Section: Introductionmentioning

confidence: 99%

An Accurate Calibration Means for the Phase Measuring Deflectometry System

Han

Song

2019

Calibration is a critical step for the phase measuring deflectometry system. Existing calibration methods are mainly optimizing the calibration parameters with respect to the 2D re-projection error criterion. However, such a procedure cannot reduce metric errors in the practical application. Therefore, an accurate and practical calibration method is proposed. In which, conventional calibration means is first applied for the primary calibration. Then, a precise square planar mirror is used for the optimization of system calibration parameters. All the intrinsic and extrinsic parameters are considered as a global multi-objective optimization problem. Three metric error criteria are introduced to evaluate the 3D reconstruction accuracy of the reference mirror. Compared with classical calibration means, which apply the parameter optimization in 2D image space to minimize the re-projection errors, the proposed optimization approach is executed in 3D space directly. An experiment and comparison are conducted to verify that the proposed optimal calibration approach can effectively reduce the system deviation and to improve the system measurement accuracy.

“…PMD has been applied to measure aspheric mirrors and dynamic specular surfaces, detect subsurface cracks, and has been used on specular surfaces from micro-scale to large scales [ 8 , 9 , 10 , 11 , 12 , 13 ]. For example, aspherical and spherical mirrors, silicon wafers and ball grid array, automobile industry (automotive glass, car body) [ 14 ]. This technique must display straight sinusoidal fringe patterns on a liquid crystal display (LCD) screen or project a structured pattern onto ground glass.…”

Section: Introductionmentioning

confidence: 99%

Distance Calibration between Reference Plane and Screen in Direct Phase Measuring Deflectometry

Huang

Liu

Gao

et al. 2018

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The recently developed direct phase measuring deflectometry (DPMD) method can directly measure the three-dimensional (3D) shape of specular objects with discontinuous surfaces, but requires a calibrated distance between a reference plane and liquid crystal display screen. Because the plane and screen are different distances from the imaging device, they cannot be clearly captured given the limited depth of field (DOF) of the lens. Therefore, existing machine vision-based methods cannot be used to effectively calibrate a DPMD system. In this paper, a new distance calibration method that uses a mirror with a hollow ring matrix pattern and a mobile stage is presented. The direction of the mobile stage in the camera coordinate system is determined by the mirror’s pattern at several positions in the camera’s DOF so that the reference position outside of the DOF can be calculated. The screen’s position can also be calibrated by displaying patterns at a known scale. Therefore, the required distance is accurately obtained in the camera coordinate system. Evaluation results show that the maximum value of the absolute error is less than 0.031 mm. The experimental results on an artificial stepped mirror and a reflected diamond distribution surface demonstrate the accuracy and practicality of the proposed method.