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

Zonghua Zhang, Yuemin Wang, Shujun Huang, Yue Liu, Caixia Chang, Feng Gao, Xiangqian Jiang
2017 Sensors  
The fast development in the fields of integrated circuits, photovoltaics, the automobile industry, advanced manufacturing, and astronomy have led to the importance and necessity of quickly and accurately obtaining three-dimensional (3D) shape data of specular surfaces for quality control and function evaluation. Owing to the advantages of a large dynamic range, non-contact operation, full-field and fast acquisition, high accuracy, and automatic data processing, phase-measuring deflectometry
more » ... , also called fringe reflection profilometry) has been widely studied and applied in many fields. Phase information coded in the reflected fringe patterns relates to the local slope and height of the measured specular objects. The 3D shape is obtained by integrating the local gradient data or directly calculating the depth data from the phase information. We present a review of the relevant techniques regarding classical PMD. The improved PMD technique is then used to measure specular objects having discontinuous and/or isolated surfaces. Some influential factors on the measured results are presented. The challenges and future research directions are discussed to further advance PMD techniques. Finally, the application fields of PMD are briefly introduced. full-field fringe pattern projection profilometry [13] . However, the coating will slightly change the geometry of the measured specular objects. Moreover, some objects, such as antiques and highly accurate optical components in aircrafts and automobiles, should not have powder sprayed onto their surfaces [14, 15] . Therefore, interferometry and deflectometry have recently been used to study specular objects because these two methods do not require contact or changing the surface. Interferometry uses the phenomenon of interference to obtain distance information. It is a superior technique for measuring simple surfaces such as spheres, planar surfaces, and weakly aspheric surfaces. A reference is normally required for interferometry; therefore, it is hard to measure complicated aspheric mirrors or free-form specular objects. White light interferometry [16] , wavelength scanning interferometer [17] and multiple wavelength interferometer [18] can be used to measure specular objects having discontinuous surfaces. However, the field of view of the objective lens, the range of the scanner and the limited synthetic wavelength are restricted to the measurement range. These modes of interferometry are normally used for surface finish measurements instead of surface form measurements. Therefore, interferometry is not suitable for measuring a greatly curved and/or large scale surface, although it can measure specular surfaces with a very high accuracy and resolution. To measure free-form specular objects with a steep slope and/or large size, in recent years, deflectometry has been investigated by many researchers, which has led to the development of different deflectometric techniques, such as Moire deflectometry [19] [20] [21] [22] , the Ronchi method [23,24], phase-measuring deflectometry (PMD, also called fringe reflection profilometry) using structured illumination of the surface [25] [26] [27] [28] [29] [30] , and laser scanning deflectometric techniques [30] [31] [32] [33] [34] [35] . Compared with interferometry, deflectometric techniques do not need to locate the specular objects under investigation at a precise position. Although these techniques overcome the shortcomings of interferometry to measure specular objects with a high dynamic range, it remains a challenge to reconstruct a specular surface shape by using deflectometric methods. Deflectometry calculates the slope (surface gradient) distribution of the surface, and finally the 3D shape is usually obtained by numerically integrating the slope distribution. Because the phase can yield continuous and highly accurate data, PMD-based methods have been widely studied for measuring specular objects. Other benefits of PMD are a large dynamic range, non-contact operation, full-field measurement, and automatic data processing. Therefore, this paper focuses on reviewing the recent developments and the existing problems of PMD. Most of the existing PMD-based methods have problems when measuring the specular objects having multiple discontinuous and/or isolated surfaces. Many improved PMD methods have been studied to overcome the challenges in classical PMD. This paper will also briefly introduce these improved PMD methods and their recent development, especially two new PMD methods. One is called the DPMD (direct PMD) method which uses two liquid crystal display (LCD) screens plus a beam splitter to realize the parallel design of the two screens. The other is MPMD (model PMD) which involves a mathematical model to simultaneously reconstruct both the height and the slopes from the measured phases, instead of integrating just the slopes. The following section will review the relevant techniques in classical PMD, mainly including the generation of fringe patterns, the geometric calibration, and the slope data integration. Section 3 introduces some improved PMD methods to measure the 3D shape of specular objects having discontinuous and/or isolated surfaces. The influential factors on the measurement results are given in Section 4. Section 5 discusses the challenges and the future research directions for the advancement of the PMD techniques. Some application fields of PMD are briefly reviewed in Section 6. The conclusive remarks are provided in Section 7. Principle The classical PMD technique uses the full-field fringe patterns to measure the slope information and then slope integration to obtain the 3D shape data of the specular objects. Digital sinusoidal Sensors 2017, 17, 2835 3 of 26 fringe patterns are commonly generated using computer software. The generated fringe patterns are displayed on a digital screen, such as an LCD, or projected onto a ground glass plate. From a different viewpoint, the reflected fringe patterns from the specular surfaces appear deformed with regard to the slope variation of the measured surfaces and the modulated fringe patterns are captured by an imaging device, for example, a charged couple device (CCD) camera. In PMD systems, one, two and multi-cameras can be used. However, few researchers studied PMD by using multi-cameras because the reflected ray at one point by the specular surface is along a certain direction according to the law of reflection. The reflected ray can be only captured theoretically from one certain direction. Therefore, most of the existing PMD systems contain one or two cameras. A stereo-camera PMD system can simplify the geometric calibration comparing to using mono camera. Wrapped phase information in the captured fringe patterns is demodulated by using multiple-step phase-shifting algorithms or transform-based algorithms. Spatial phase unwrapping and temporal phase unwrapping methods can be used to obtain the absolute phase data. Phase calculation and unwrapping methods are out of the scope of this paper and the readers should refer to the published literature for more details on these procedures [36] [37] [38] [39] [40] [41] [42] [43] [44] . The absolute phase data are applied to calculate the slope of the measured specular surfaces using the system parameters. Finally, the 3D shape of the specular surfaces under investigation is reconstructed by integrating the slope data. The procedure of the classical PMD is illustrated in Figure 1 . The following subsections will elaborate on each step in the classical PMD.
doi:10.3390/s17122835 pmid:29215600 pmcid:PMC5750795 fatcat:o7xrdw43kzetrgeihapxn2xtim