Online fringe pitch selection for defocusing a binary square pattern projection phase-shifting method

A three-dimensional (3D) shape measurement system using defocusing binary fringe projection can perform high-speed and flexible measurements. In this technology, determining the fringe pitch that matches the current projection's defocus amount is of great significance for an accurate measurement. In this paper, we propose an online binary fringe pitch selection framework. First, by analyzing the fringe images captured by the camera, the defocus amount of projection can be obtained. Next, based on analysis of the harmonic error and camera noise, we establish a mathematical model of the normalized phase error. The fringe pitch that minimizes this normalized phase error is then selected as the optimal fringe pitch for subsequent measurements, which can also lead to more accuracy and robust measurement results. Compared with current methods, our method does not require offline defocus-distance calibration. However, it can achieve the same effect as the offline calibration method. It is also more flexible and efficient. Our experiments validate the effectiveness and practicability of the proposed method.