Few-pattern defocusing fringe projection profilometry for high-speed 3-D imaging

Fringe projection profilometry (FPP) based on the binary defocusing technique (BD) shows great potential in high-speed 3-D imaging. Due to the constant defocusing degree, existing binary defocusing operations require adopting similar-wavelength fringe patterns, thereby forming a long imaging sequence in multi-frequency temporal phase unwrapping (TPU). In this paper, we propose a few-pattern defocusing FPP for efficient and accurate 3-D imaging. The imaging sequence consists of only 6 hybrid images, namely 2 unit-frequency ramp images, 2 low-frequency, and 2 high-frequency sinusoidal fringe images. Combining unit-frequency ramp and low-frequency fringe images, the unknown average intensity and fringe orders of fringe images can be determined. Consequently, the final absolute phase map can be extracted from the high-frequency fringe images. Moreover, a kernel-optimized dithering technique is presented to generate the projected patterns of hybrid images. In this dithering technique, a dynamic kernel and a dual-objective function ensure the optimal binarization of defocused images with different grayscale variations. Experiment results verify the proposed few-pattern defocusing FPP achieves efficient 3-D imaging with a measurement accuracy of 0.02 mm.