By combining a fringe projection setup with a telecentric lens, a fringe pattern could be projected and imaged within a small area, making it possible to measure the three-dimensional (3D) surfaces of micro-components. This paper focuses on the flexible calibration of the fringe projection profilometry (FPP) system using a telecentric lens. An analytical telecentric projector-camera calibration model is introduced, in which the rig structure parameters remain invariant for all views, and the 3D calibration target can be located on the projector image plane with sub-pixel precision. Based on the presented calibration model, a two-step calibration procedure is proposed. First, the initial parameters, e.g., the projector-camera rig, projector intrinsic matrix, and coordinates of the control points of a 3D calibration target, are estimated using the affine camera factorization calibration method. Second, a bundle adjustment algorithm with various simultaneous views is applied to refine the calibrated parameters, especially the rig structure parameters and coordinates of the control points forth 3D target. Because the control points are determined during the calibration, there is no need for an accurate 3D reference target, whose is costly and extremely difficult to fabricate, particularly for tiny objects used to calibrate the telecentric FPP system. Real experiments were performed to validate the performance of the proposed calibration method. The test results showed that the proposed approach is very accurate and reliable.

With their constant perspective and large magnification in the working distance, double-sided telecentric lenses have been widely used in machine-vision applications. This paper puts forward a flexible calibration approach for the double-sided telecentric camera. Based on an orthographic projection model considering the major sources of lens distortions, a two-step calibration procedure is proposed. In this approach, the camera parameters apart from the lens distortions are achieved by a closed-form solution. Then a double non-linear optimization is performed to refine all the parameters, including the distortion coefficients and distortion centre. In addition, to achieve a flexible calibration procedure, the calibration pattern used is a cheap print product rather than a professional customized calibration pattern. Simulation and real-world experiments are performed to validate the performance of the proposed calibration approach. In addition, the comparison experiments between the print calibration pattern and professional calibration pattern are carried out, and the accuracy of calibration results are at the same level.

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Non-contact measurement technology based on triangulation with cameras is extensively applied to the development of computer vision. However, the accuracy of the technology is generally not satisfactory enough. The application of telecentric lenses can significantly improve the accuracy, but the view of telecentric lenses is limited due to their structure. To address these challenges, a telecentric surface reconstruction system is designed for surface detection, which consists of a single camera with a telecentric lens, line laser generator and one-dimensional displacement platform. The designed system can reconstruct the surface with high accuracy. The measured region is expanded with the used of the displacement platform. To achieve high-accuracy surface reconstruction, we propose a method based on a checkerboard to calibrate the designed system, including line laser plane and motor direction of the displacement platform. Based on the calibrated system, the object under the line laser is measured, and the results of lines are assembled to make the final surface reconstruction. The results show that the designed system can reconstruct a region of 20×40 mm2, up to the accuracy of micron order.

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