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.
Deflectometry has been widely used in topography measurement of specular surface due to the merits of full-field data acquisition, automatic data processing, high accuracy, and large dynamic range. Existing deflectometry-based methods, such as model phase measuring deflectometry, direct phase measuring deflectometry, stereo deflectometry, employ a plane screen. Due to the size and shape of the used screen, reconstruction of specular surface with large curvature can’t be carried out in single measurement. The characteristics of curved screens effectively expand the ranges of the gradient and height field of mirror. Ideally, curved screens are a part of normal cylinders. However, initial experiment shows that the commercialized curved screen changes in radius, instead of a constant value. In order to obtain the model of the curved screen, two CCD cameras capture fringes displayed on the curved screen. Stereo technique is applied to compute screen’s point cloud containing curved screen’s location and phase. The stereo technique is rectifying pairs of fringe map based on epipolar constraint to the same rotating matrix. The epipolar line of fringe map pair lies in the same row. Then disparity is computed through block match in column coordinate and refined through curve fitting. Some experiments have been carried out and results show that rectifying fringe map helps get more accurate and stable point cloud of curved screen to be suitably employed in deflectometry by a curved screen.
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.
Aiming at the problem that the center extraction accuracy of the circular coding target is easily affected by camera angle, a method to extract the center based on the radial line fitting of the coding ring is proposed in this paper. Firstly, the radial straight line edge of the circular coding target ring is obtained by edge extraction after image preprocessing, The extracted sub-pixel edge points set is mapped to the parameter space into curves, and the intersection points between all curves are found. Finally, the RANSAC (Random Sample Consensus) algorithm is used for fitting the line to the above points, and the actual center is obtained by mapping the fitted line to the original coordinate space. Compared with the ellipse fitting method, the reprojection error of the camera calibration by the extracted targets centers is reduced by 20%, and the calibration result is more accurate and reliable.
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.
KEYWORDS: 3D metrology, Imaging systems, 3D acquisition, Infrared imaging, Light, Complex systems, Reflectivity, Deflectometry, Data processing, Data integration
Phase sensitivity plays a vital role in accurately retrieving three-dimensional (3D) shape of diffused objects in fringe projection profilometry (FPP). With regard to traditional means that simply project horizontal or vertical fringe patterns, the fringe frequency and direction are not the best choice to acquire optimal sensitivity to depth variations. This talk will present a novel method to obtain 3D shape with high accuracy by combining the optimal fringe frequency and the optimal fringe direction (OFF + OFD). The simulated and actual experiments have been carried out to demonstrate that the proposed optimization method is feasible and effective in increasing phase sensitivity and measurement accuracy.
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.
With the fast development of integrated circuits, photovoltaics, automobile industry, advanced manufacturing, and astronomy, it is particularly important to obtain the three-dimensional (3D) shape data of specular objects quickly and accurately. Owing to the advantages of large dynamic range, non-contact operation, full-field and fast acquisition, high accuracy, and automatic data processing, phase-measuring deflectometry (PMD, also called fringe reflection profilometry) has been widely studied and applied in many fields. Direct PMD (DPMD) can directly establish the relationship between phase and depth data without gradient integration process, so it can be used for 3D shape measurement of specular objects having discontinuous surfaces. However, compared with gradient data, depth data is more sensitive to measurement noise, so the measurement accuracy obtained by gradient field integration is higher. In order to make use of the anti-noise property of gradient measurement structure, some PMD techniques usually abandon the height data obtained from the intermediate process and use the gradient data to complete the surface reconstruction. Stereo deflectometry is a typical representative to retrieve 3D shape of the measured object according to the gradient integration. It calculates the gradient field based on the uniqueness of the normal vector of the object points by using two cameras. Although obtaining high-precision measurement results, this method cannot be used for the measurement of discontinuous objects. In this paper, a new stereo DPMD method is proposed to obtain 3D shape of specular objects having discontinuous surfaces. Some experimental results on measuring discontinuous specular objects verify the high precision of the proposed stereo DPMD method.
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