This paper proposes an approach for the full-field surface reconstruction of three-dimensional (3-D) objects. The method uses a fringe projection profilometry system to obtain a 3-D point cloud with a multiview and proposes an objective function that involves all of the parameters of rotation and translation. The feature points of the surface of the object are extracted to determine the initial value of the objective function, which is then optimized to achieve accurate registration of the point clouds. The experimental results demonstrate that the proposed method is accurate and effective.
We propose a calibration method of a fringe projection system for surface profile measurement. The calibration method is divided into two parts: (1) phase to z calibration based on measuring the absolute phase distributions of a flat calibration board placed at several different known z positions. (2) Camera calibration based on Zhang’s method of imaging a checkerboard with different poses. Experiments of a flat plane, a sphere, and a Gorky sculpture demonstrate the validity and accuracy of the proposed technique.
This paper develops a set of projection Moiré system. It projects the grating onto the surface of specimen and forms grid lines of which the pitch is adjustable on large area through light source and projection grating. At the same time, the imaging system images the grid lines precisely onto the reference grating and then the Moiré fringes are formed. The modulation of out-of-plane displacement to grid lines transform into the distortion of Moiré fringes. The greatest advantage of this projection Moiré system is that what the CCD camera captures is the Moiré fringes, not the gird lines. The Moiré fringes can magnify the variation of out-of-plane displacement of the specimen with no distortion and the measuring resolution is improved a lot. Besides, this system is appropriate for large area measurement by using a wide-angle lens in imaging system. The full-field out-of-plane displacement can be acquired through image processing. The measuring resolution of this system can reach 10μm on a large area of 3m*3m.
Fringe projection profilometry (FPP) using a digital video projector is widely used for three-dimensional shape measurement. However, the gamma nonlinearity, system vibration, and noise cause the captured fringe patterns to be nonsinusoidal waveforms and have a grayscale deflection from their true value. This leads to an additional phase measurement error for a general phase-shifting algorithm. Based on the theoretical analysis, we propose a method to eliminate the phase error considering two factors. In this method, four-step phase-shifting is done four times with an initial phase offset of 22.5 deg and the average of these four phase maps precisely results in the real phase. As a result, phase error caused by gamma nonlinearity can be effectively suppressed. In addition, every image in phase shifting is replaced by the average of 20 fringe images continuously captured at the same state to avoid the phase error caused by system vibration and noise. Experimental results show that this method is effective in eliminating the phase error in practical phase-shifting FPP. In general, more than 90% of the phase error can be reduced.
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