Zonghua Zhang

Simple calibration of a phase-based 3D imaging system based on uneven fringe projection

Phase-based fringe projection metrology systems have been widely used to obtain the shape of 3D objects. One vital step is calibration, which defines the relationship between the phase and depth data. Existing calibration methods are complicated because of the dependence of the relationship on the pixel position. In this Letter, a simple calibration procedure is introduced based on an uneven fringe projection technique, in which the relationship between phase and depth becomes independent of the pixel position and can be represented by a single polynomial function for all pixels. Therefore, given a set of discrete points with a known phase and depth in the measuring volume, the coefficient set of the polynomial function can be determined. A white plate having discrete markers with known separation is used to calibrate the 3D imaging system. Experimental results demonstrate that the proposed calibration method is simple to apply and can build up an accurate relationship between phase and depth data.

Simple, flexible calibration of phase calculation-based three-dimensional imaging system

One important step of phase-based three-dimensional imaging system is calibration, which defines the relationship between phase and depth data. Existing calibration methods are complicated and hard to carry out because of using a translation stage or gauge block in a laboratory environment. This Letter introduces a new simple, flexible calibration method by using a checkerboard and a white plate having discrete markers with known separation. The checkerboard determines the internal parameters of a CCD camera. The plate gives phase and depth data of each pixel to establish their relationship. Experimental results and performance evaluation show that the proposed calibration method can reliably build up the accurate relationship between phase map and depth data in a simple, flexible way.

A simple, flexible and automatic 3D calibration method for a phase calculation-based fringe projection imaging system

An important step of phase calculation-based fringe projection systems is 3D calibration, which builds up the relationship between an absolute phase map and 3D shape data. The existing 3D calibration methods are complicated and hard to implement in practical environments due to the requirement of a precise translating stage or gauge block. This paper presents a 3D calibration method which uses a white plate with discrete markers on the surface. Placing the plate at several random positions can determine the relationship of absolute phase and depth, as well as pixel position and X, Y coordinates. Experimental results and performance evaluations show that the proposed calibration method can easily build up the relationship between absolute phase map and 3D shape data in a simple, flexible and automatic way.

Snapshot color fringe projection for absolute three-dimensional metrology of video sequences

We present a method to obtain simultaneous three-dimensional shape and color information from a single captured image using a composite red, green, and blue (RGB) projected fringe pattern. Previous attempts at single snapshot shape and color metrology have suffered from either poor dynamic range or have been highly dependent on the color of the artifact. An optimum multiwavelength process has been employed to maximize the dynamic range of the shape data and give absolute depth information independently at each pixel. Fringe processing is via a Fourier transform process with algorithms introduced to obtain RGB color texture. Simulated and experimental data demonstrate the algorithm’s robustness in the vicinity of surface discontinuities. Experimental results from a human hand show the applicability to dynamic scenes.

Accurate calibration method for camera and projector in fringe patterns measurement system.

The 3D measurement system based on fringe patterns is widely applied in diverse fields. The measurement accuracy is mainly determined by camera and projector calibration accuracy. In the existing methods, the system is calibrated by a dot calibration board with traditional image process algorithms. In this paper, an improved calibration method is proposed to increase camera and projector calibration accuracy simultaneously. To this end, first, a subpixel edge detection method is proposed to improve the detection accuracy of reference features for coarse calibration; second, an iterative compensation algorithm is developed to improve the detection accuracy of the reference feature centers for fine calibration. The experimental results demonstrate that the proposed calibration method can improve the calibration accuracy and measurement accuracy.

Flexible phase error compensation based on Hilbert transform in phase shifting profilometry

This paper makes use of Hilbert transform to analyze and compensate the phase error caused by the nonlinear effect in phase shifting profilometry (PSP). The characteristics of the phase error distribution in Hilbert transform domain was analyzed and compared with spatial domain. A simple and flexible phase error compensation method was proposed to directly process the phase-shifting fringe images without any auxiliary conditions or complicated computation. Experimental results demonstrated that the phase error can be reduced by about 80% in three-step PSP, and more than 95% in four or more step PSP, which verified the effectiveness, flexibility, robustness and automation of the proposed phase error compensation method.

Accurate projector calibration method by using an optical coaxial camera

Digital light processing (DLP) projectors have been widely utilized to project digital structured-light patterns in 3D imaging systems. In order to obtain accurate 3D shape data, it is important to calibrate DLP projectors to obtain the internal parameters. The existing projector calibration methods have complicated procedures or low accuracy of the obtained parameters. This paper presents a novel method to accurately calibrate a DLP projector by using an optical coaxial camera. The optical coaxial geometry is realized by a plate beam splitter, so the DLP projector can be treated as a true inverse camera. A plate having discrete markers on the surface is used to calibrate the projector. The corresponding projector pixel coordinate of each marker on the plate is determined by projecting vertical and horizontal sinusoidal fringe patterns on the plate surface and calculating the absolute phase. The internal parameters of the DLP projector are obtained by the corresponding point pair between the projector pixel coordinate and the world coordinate of discrete markers. Experimental results show that the proposed method can accurately calibrate the internal parameters of a DLP projector.

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

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 (PMD, 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 3D shape measurement of discontinuous specular objects by direct phase measuring deflectometry

AbstarctWith the advent of intelligent manufacturing, phase measuring deflectometry (PMD) has been widely studied for the measurement of the three-dimensional (3D) shape of specular objects. However, existing PMDs cannot measure objects having discontinuous specular surfaces. This paper presents a new direct PMD (DPMD) method that measures the full-field 3D shape of complicated specular objects. A mathematical model is derived to directly relate an absolute phase map to depth data, instead of the gradient. Two relevant parameters are calibrated using a machine vision-based method. On the basis of the derived model, a full-field 3D measuring system was developed. The accuracy of the system was evaluated using a mirror with known positions along an accurate translating stage. The 3D shape of a monolithic multi-mirror array having multiple specular surfaces was measured. Experimental results show that the proposed DPMD method can obtain the full-field 3D shape of specular objects having isolated and/or discontinuous surfaces accurately and effectively.

Phase error compensation methods for high-accuracy profile measurement

In a phase-shifting algorithm-based fringe projection profilometry, the nonlinear intensity response, called the gamma effect, of the projector-camera setup is a major source of error in phase retrieval. This paper proposes two novel, accurate approaches to realize both active and passive phase error compensation based on a universal phase error model which is suitable for a arbitrary phase-shifting step. The experimental results on phase error compensation and profile measurement of standard components verified the validity and accuracy of the two proposed approaches which are robust when faced with changeable measurement conditions.