Superlarge depth range 3D measurement based on dual-focal optimization strategy.

Fringe projection profilometry (FPP) is a widely applied high accuracy three-dimensional measurement technique, and the binary defocusing technique is commonly used to improve measurement speed. However, the measurement accuracy is affected by the defocusing region. In the traditional binary defocusing system, only the front section or the back section of proper defocusing region can be used as the measurement depth range. Moreover, existing optimization methods mainly focus on extending the front section of proper defocusing regions. To achieve a larger depth range, this paper proposes a dual-focal optimization strategy, in which the proper defocusing region can be expanded by introducing a cylindrical lens group to adjust the projector focal lengths simultaneously in the meridian and sagittal planes. This approach creates a larger proper defocusing region by optimizing the focal lengths based on the light field distribution properties. Compared to the traditional binary defocusing method and existing optimization methods, the proposed method could overcome the limitation of the measurement depth ranges of only using the isolated front section or back section. The experimental results show that the proposed method increases the measurement depth range from 120 mm and 650 mm (two isolated depth ranges in the traditional binary defocusing method) to 950 mm, effectively creating a superlarge measurement depth range and acquiring better measurement results.