## 1 Introduction  
In the case of inductive thermography, a short inductive heating pulse in the range of 0.05-1 s is applied to the work-piece and the surface temperature is recorded during and after the heating pulse by an infrared (IR) camera. Surface defects deflect the induced eddy currents and disturb the heat diffusion; therefore, the defects in metallic materials can be excellently detected by analyzing the infrared images [1; 2; 3; 4; 5; 6]. In addition, this method has the advantage compared to other methods such as magnetic particle testing or penetration testing that it also provides information about the defect depth and about the inclination angle of a surface crack [1]. Metallic surfaces often have a low emissivity value and inhomogeneous surface properties. Evaluating only one infrared image, e.g., at the end of the heating pulse, is strongly affected by this inhomogeneity and also by the inhomogeneity of the heating. This could cause false detections or on the other hand, missing detection of defects. However, recording a whole infrared sequence and evaluating the temporal change in the temperature provides the advantage of suppressing the inhomogeneity effects and it reduces the background noise in the images, resulting in a higher detection reliability. For the evaluation of the infrared sequence, often Fourier transformation is used to calculate a phase image, which is then used to localize and to characterize the defects [7]. For the inspection of long work-pieces, scanning in a stop-and-go way, which is also called step scanning [8], can be used. During one measurement, both the inductor and the recording infrared camera are stationary and after one measurement, either the work-piece or the heat source together with the IR camera are moved further to the next position. Afterwards, the separate measurement results are stitched together to one panorama image [8; 9]. In this paper, a method is presented to optimize this technique by applying a calibration object for rectifying the images. As we have applied this method to a rail piece where the defects occur at the curved surface of the gauge line, the calibration object was also used to reduce errors in the stitching caused by the rail curvature. Another possibility for the inspection of long pieces is a scan with continuous motion [9; 10; 11; 12], where either the work-piece is moved with a constant speed and the heat source together with the infrared camera are stationary, or the reverse. During this relative motion, the infrared camera always records the temperature of different parts of the inspected work-piece; therefore, an additional step is necessary to generate a phase image. The pixels in the recorded infrared images are reorganized in a way, as they belong to the same position of the work-piece, as it would be the case for a stationary measurement. This allows for the application of the same evaluation method of Fourier transformation, as it has been developed for stationary measurements [11]. The scanning pulse phase thermography, SPPT [11], works very well for the detection of subsurface defects. In this case, only the temperature changes after the heating pulses are evaluated. The heat is generated due to the induced eddy currents in a thin depth at and below the surface and the heat travels into the body. If there is a defect, then the heat diffusion is disturbed and the perturbation in the surface temperature can be observed with a time delay after the heating pulse. Therefore, the most relevant information regarding the defect detection is contained in the temperature change recorded after the heating pulse [13]. For surface cracks, the situation is different and the most important part of the information is included in the temperature change during the heating, as the perturbation due the surface crack occurs instantly at the surface. If the induction coil during the scanning covers the surface, then due to this occlusion, the infrared camera cannot record the temperature during the heating.