Mapping non-destructive testing data on the 3D geometry of objects with complex shapes

Abstract

This paper presents an approach for measuring the geometry of objects with complex shapes with a 3D camera and, at the same time, collecting surface and subsurface information that is then mapped onto the geometry in good spatial alignment. Subsurface information can be captured with infrared thermography or ultrasonic probes. The approach can thus allow non-destructive testing procedures to be used for detecting, locating and measuring the size, shape and depth of undersurface defects of an object and to map these defects, and potentially other information such as reflectance or color, onto the geometric model of the object. 1. Motivation In a long-term monitoring context, the inspection of objects with complex geometries using non-destructive techniques such as thermal infrared imaging or phased array ultrasound sensing implies that the undersurface structure of the object to be observed is in good registration with the 3D object geometry. This paper presents an approach for measuring the geometry of objects with complex shapes using a handheld 3D scanner and for collecting subsurface information, in this case thermal images or ultrasonic recording, in registration with the captured object geometry. An important step leading to accurate registration of thermal and geometric data is the calibration of the intrinsic and extrinsic parameters of the pinhole camera model used to model the infrared camera. Intrinsic parameters describe “internal” properties of the camera such as focal length, principal point, radial/tangential distortion, etc. Extrinsic parameters describe the position and orientation (i.e. the pose) of the pinhole in a global reference frame (often called the “world” reference frame). Once calibration is complete, the “storage” of thermal data in the 3D map can be achieved with the help of a motion tracking system. The paper is organized as follows. The experimental setup and the various instruments used for the experiments are described in Section 2. Second, the model that was adopted for describing image formation by the thermal infrared camera is described briefly in Section 3. Then, the procedure for calibrating the pinhole modelling the infrared camera is described in detail and validation experiments are presented and discussed in Section 4. Section 5 explains the procedure that is adopted for acquiring and registering 3D and thermal data. Experimental results are presented. Section 6 presents an extension of the thermal mapping approach to the case of phased array ultrasound data. Finally, conclusion and perspectives of future work are given in Section 7.