Geometric Calibration for Thermography Cameras

The application of active thermography applied as Lock-in or pulse thermography has been widely used in aerospace industry. Over the last years a number of activities had been undertaken to achieve a higher level of automation. Industrial robots are very versatile and allow a comparably easy integration of a measurement system in robotic workcell further the control unit. It is therefore possible to cover a large range of geometrical complex parts even on larger dimensions. The 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. 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 spectrum the lenses of thermography cameras are made of Germanium. Although the high cost for these lenses, they are optimized for radiometric resolution, thus geometric precision or minimal distortion are of minor interest. The approach at DLR Augsburg of a robot based measuring system for industrial thermography applications and 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. Within this paper DLR-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 and results derived from experiments.