Heterodyne multi-frequency method for 3D profile measurement

A binocular structured light measurement system using heterodyne multi-frequency method is researched in this paper. Three steps must be done to recover 3D profile information: cameras calibration; points matching; 3D information reconstruction. The intrinsic and extrinsic parameters of cameras are calibrated by Zhengyou Zhan's method ^[1]. Stereo vision model is the basic rule of 3D recovering. Techniques of phase shift and heterodyne multi-frequency are used to aid points matching. Some coded digital fringes are projected to the free-form surfaces and corresponding images are captured by two cameras simultaneously. The wrapped phase map is obtained through four-step phase shift. In a single cycle, phase value is calculated using arctangent function and it is between -π and π. The heterodyne multi-frequency method is applied to get the absolute phase map. When the number of frequencies is changed from 2 to 3, or 4, the unwrapping accuracy and matching results are improved. The processing theory and experimental results are illustrated and analyzed. The experimental results show that accurate and reliable phase result can be obtained on phase map boundaries and break points, which proves its feasibility in industrial situations.