Large depth-of-field 3D measurement with a microscopic structured-light system

Abstract Fringe projection techniques have been widely used in three-dimensional (3D) microscopic measurement; however, the working volume is severely restricted due to the shallow depth-of-field (DOF) of the telecentric lenses used. In this paper, a large-DOF 3D measurement method for use with a microscopic structured-light system was proposed. By employing a precisely controlled moving platform, the height of the camera can be adjusted to capture several sets of fringe patterns with different amounts of defocusing. Then the proposed focusing map generation algorithm was employed to calculate the focusing degree of each pixel and generate a focusing map for each set of fringe patterns. Subsequently, local 3D shape data corresponding to in-focus pixels were merged with the guidance of the focusing maps, and global fine 3D measurement data without the influence of lens blur can be obtained. Specifically, to guide the compositing of the 3D shape data, a novel, simple focusing map generation algorithm was proposed to calculate the focusing degree of each set of captured fringe patterns. In contrast to existing DOF-extending 3D measurement techniques using multiple focal distances, the proposed algorithm does not need the object’s texture to calculate the gray-level gradient; hence, complex convolution or iteration is not required. The experimental results demonstrate that the proposed technique can increase the measurement DOF to approximately three times that of a conventional system.

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