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Theses

Vision conoscopique 3D : Calibration et reconstruction

Abstract : Industrial metrology is a key challenge in all fields of activity when an accurate dimensional control is required to guarantee norm adequacy of product specifications and to aim at ``zero default'' quality. Current systems are based on such technologies as stereoscopy, photogrammetry, structured light projection, active triangulation or time-of-fly methods. In general, these systems are task-specific, bulky, fragile and cannot operate in hostile environments. Moreover, they suffer from a geometrical limitation because of a death angle in which no measurement can be performed.

In this thesis, a new probe, called conoscopic scanner and based on the use of spatially incoherent light, is presented. It offers such unique features as (i) ability to provide extremely accurate estimates of shape profiles at a variety of scales ranging from the microscopic to the macroscopic domain, (ii) efficiency in any -even hostile- environment, (iii) compacity and reduced dimensions.

Based on conoscopy principles, and therefore relying on crystal birefringence properties, the conoscopic scanner comprises a Coupled Charge Device (CCD) camera, a conoscopic kernel and an information processing unit. In order to achieve demanding performance specifications that require a millionth of millimeter accuracy in 3D measurements, every component in the measurement chain has to be precisely analyzed and characterized. Within this framework, calibration issues prove to be crucial.

A calibration method for the CCD camera has been first developed. Based on landmark registration using a pinhole viewing model that combines intrinsic and extrinsic camera parameters, it results in a 1.5% of field of view accuracy. In a second step, two calibration methods for the conoscopic device, adapted to specific configurations of the conoscopic scanner, have been proposed. In the first one, depth estimates are obtained through a direct analytic phase computation using standard optics and crystal biregringence equations. In the second one, phase is related to fringe patterns. Depth measurements are then obtained by applying a fringe counting method to the digitized hologram and using experimental material-specific calibration curves. These curves take into account temperature and lighting variations and integrate an acquisition noise global modelling. Extensive tests, performed both on simple geometric profiles (crenel, staircase...) and on real 3D objects, have demonstrated similar performances for the two methods, namely a 1.5% of field of view accuracy.

Having established the technological feasibility of system specifications by means of adequate calibration procedures, we have developed two different configurations of the conoscopic scanner, called fix-probe and scan-probe and adapted to small and large size objects, respectively. Their performance on different diffusive and awkward materials have been analyzed, demonstrating a one thousandth of the analyzing field measurement accuracy.

The various contributions of this thesis have been integrated to the currently developed conoscopic system, which is today available and operating in industrial and medical applications.
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  • HAL Id : tel-00273319, version 1

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Didier Gava. Vision conoscopique 3D : Calibration et reconstruction. Informatique [cs]. Université René Descartes - Paris V, 1998. Français. ⟨tel-00273319⟩

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