Geometric Calibration in Active Thermography Applications

Abstract

The application of active thermography such as Lock-in or pulse thermography has been widely used in aerospace and other industries. Over the last decade, the level of automation has increased by using various manipulators like classical NC machines or even industrial robots. Meanwhile integration of measurement systems and automated manipulators achieved industrial standard. It is therefore possible to cover a large range of geometrical complex parts even on larger dimensions. Manipulators like a robot can be used for the positioning of the thermography camera and delivers the location of the robot flange coordinate system at the time of measurement automatically. Or one can think of having a stationary camera and the part is moved around. In principle thermography cameras used in NDT applications as well as other fields can be handled as standard photogrammetric cameras. According to the longer wavelengths of the measured infrared spectrum the lenses of thermography cameras are mainly made of Germanium. Although the high cost for these lenses, they are optimized for radiometric resolution, thus geometric precision or minimal distortion had not been targeted especially in the field of NDT. The approach at DLR Augsburg of a robot based measuring system for industrial thermography applications in the context of CFRP production technology including a hand-eye-calibration procedure for a thermography end-effector has been presented in previous papers. In order to achieve higher accuracy in terms of geometric precision a number of distortion parameters have been evaluated. An automated procedure has been developed to compare different calibration strategies as well as different camera characteristics e.g. focal length. Within this paper DLR’s Center for Lightweight-Production-Technology in Augsburg will give insight into current work in the area of geometric calibration for thermography cameras for a high resolution camera, results derived from experiments and applications for such calibration procedure.