Defocus-Driven Variable-Shape Fringe for Accuracy Improvement in Fringe Projection Profilometry

Sinusoidal fringe projection profilometry (FPP) is a promising 3-D measurement technique. However, the accuracy of sinusoidal FPP will be seriously reduced due to the impact of the defocusing effect. Different from existing methods with constant fringe shapes, this article proposes a defocus-driven variable-shape method that adapts the fringe shape to the defocusing degree to improve accuracy. The defocusing effect reduces the modulation of each harmonic in the fringe, especially the high-order harmonics. Taking advantage of this property, high-order harmonics can be introduced to the fringe to improve the fundamental harmonic modulation, thereby achieving a higher signal-to-noise ratio (SNR) without causing significant harmonic errors. With the increase of defocusing degree, more high-order harmonics can be introduced to the fringe, leading to a varying optimal fringe shape. This article employs a parameterized fringe shape and an error optimization method to determine the optimal fringe shape for each defocusing degree. Then, the variable-shape fringe (VSF) is generated according to the defocusing distribution of the measurement target. The experimental results demonstrate that the VSF can achieve higher accuracy than the traditional methods with constant fringe shapes.