## 6 Results for Scanning in Continuous Motion
### Evaluation of Scanning Measurements in Continuous Motion  
The evaluation process for measurements in continuous scanning is described in Figure 10. Two transformations of the entire image sequence are necessary to compute a resulting phase image of the whole work-piece. In the first step, each IR image of the recorded sequence is rectified via transformation, calculated from the calibration image. Figure 11 shows the original recorded IR image in the position as the squat defect is moving through below the inductor. In the camera's FOV, the inductor is on the left side and the specimen is moved from left to right in x-direction. Due to the first performed transformation, the sides of the images cannot be used and the images are cropped to a size of 623 \(\times\) 424 pixels. This results in a new image sequence shown in Figure 11b. This sequence looks in a way as if it was recorded by a camera with  
Figure 8: Phase image with fading in (0–100%) of pixel values on the left and fading out (100–0%) of pixel values on the right side. Figure 10: Postprocessing of measurements in continuous motion. Figure 9: Panoramic phase image computed from five measurements for a whole length of 250 mm. a perpendicular view of the specimen's surface, but with the difference that the position below the inductor is visible. In the next step, this transformed IR sequence is reorganized with the same method as described in an earlier publication [11]. The pixel-columns are shifted and interpolated according to the scanning speed and the camera's FOV. In this way, a new image sequence is calculated where each pixel corresponds to a real position on the specimen's surface. \(s\), the shift of the specimen between two consecutive images of the rectified image sequence, is calculated as follows:  
\[s=\frac{v\cdot\overline{x}}{f}, \tag{1}\]  
where \(v\) is the motion speed in mm/s, \(f\) is the camera's frame rate in Hz and \(\overline{x}\) is the spatial resolution of the transformed image in px/mm. In the case of measurements with the cooled infrared camera with a 300 Hz frame rate, the resolution \(\overline{x}\) is 5 px/mm. For the scan with a speed of 200 mm/s, this results in a shift s of 3.3 pixels between the consecutive images. Figure 12 shows one temperature image of the sequence after re-ordering the film sequence, i.e., after the scanning motion is eliminated. This quasi-static sequence is now evaluated to a phase image using Fourier transformation (see Figure 13). In this phase image, both large squat defects and also small shallow cracks in a line with the squats can be recognized, proving the advantages of using a phase image instead of one single temperature image for defect detection. The phase image results from the scanning measurement with conversion to the quasi-static sequence are also comparable with results from the stationary measurements. Figure 11: (**a**) IR image of the squat defect during scanning, inductor is visible on the left side; (**b**) image after applied rectification transformation. Figure 12: Temperature image of the specimen after re-organizing the pixel-columns. Figure 13: Resulting phase image of a measurement with a scanning speed of 200 mm/s. For stationary measurements, the heating and cooling of a specimen are recorded and evaluated, which is now also possible for scanning measurements, due to the tilted camera position. Figure 14 shows a comparison of two measurements performed on RP02. On the left side, a region of the evaluated scan shown in Figure 13 is shown with adapted scaling. The right side depicts the same ROI of a stationary measurement, which was previously published in ref. [9]. It can be observed that in both cases, the detected smaller cracks are shown as bright lines with brighter spots at each end of the crack. The significant advantage of the scanning technique is that it allows for continuous movement, which is more realizable in practical applications aiming at crack detection with a movable setup and it is also quicker.