Preliminary study on the 3D digitization of millimeter scale products by means of photogrammetry

Photogrammetry is a passive 3D digitization technique, mainly oriented to large sized objects, since its origins are in architectural and civil engineering. With the continuos development of digital imaging hardware and software, photogrammetric applications are involving smaller and smaller fields of view, with some critical aspects such as the depth of field getting narrower. In this conditions the lack of focus becomes important and affects heavily the possibility of accurately calibrate cameras. Bi-dimensional calibration patterns are affected by this problem when the camera principal axis has an angle with the pattern plane higher than a critical value. Moreover, the accuracy of the pattern, in terms of both shape and 3D positions of the targets, becomes critical decreasing the size of the pattern. In this paper the authors address these problems through a comparison of several calibration patterns included into the open source computer vision software library called OpenCV. 3D digitization of a small object is presented to test the best resulting calibration, using a consumer reflex camera equipped with macro lens and extension tube