Abstract
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.
Highlights
Double-sided telecentric lenses have been widely used in many machine-vision applications such as noncontact measurement and inspection systems [1,2,3]
A non-linear optimization is performed to refine the all camera parameters considering lens distortion, which is a common step in camera calibration
Camera parameters with a distortion-free telecentric camera model are achieved by a closed-form solution, in which an additive stage is employed in the calibration pattern to avoid the ambiguity of orthographic projection
Summary
Double-sided telecentric lenses have been widely used in many machine-vision applications such as noncontact measurement and inspection systems [1,2,3]. Unlike the perspective projection of the pinhole camera model, double-sided telecentric lenses provide purely orthographic projections of scene points by locating an aperture stop at the focal point of a lens [4]. These lenses provide constant perspective and a large magnifica‐ tion depth of field. Chen proposed an effective calibration method for a telecentric camera in an object space, considering lens distortion [20]. In this solution, the problem of sign ambiguity from the planar object-based calibration technique is successfully solved.
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