## 7 Discussion and Conclusions
### Scanning in Continuous Motion  
The continuous scanning method is a fast way of inspecting long specimens. With the reorganization of the image sequence, the evaluation of a phase image is possible. The size of surface defects detectable with this method depends on various parameters and some of them are discussed in this section. 1. **Temperature increases and motion speed:** Figure 17 compares the temperature increase in the case of three different motion speeds, determined for the position close to the defect, after reordering the recorded IR sequence. The temperature increase due to the inductive heating is affected by the motion speed through the heating region of the inductor. Slower speed causes a longer excitation period and, therefore, higher temperature increase. In the transformed quasi stationary sequence, it can be observed that temperature curves look similar to that of stationary pulse measurements. The duration of these pseudo-pulses can be approximated by the time of how long the specimen needs to go through the effective heating range underneath the inductor. For motion speeds of 100, 150 and 200 mm/s and an effective heating range of inductor with a width of 20 mm, this results in pulse durations of approximately 0.1, 0.15 and 0.2 s, respectively (see Figure 17d). 2. **Camera frame rate and image resolution:** As described in [11], during the transformation to the quasi stationary sequence, the size of the images and the number of the frames are changed. The new image size in motion direction \(x\) is determined by the pixel columns running through the entire camera's FOV. This can be calculated by multiplying the number of frames in the original sequence and the shift \(s\), as in Figure 18. However, if the shift \(s\) significantly deviates from an ideal shift of 1 px, this calculation is inaccurate due to interpolation during the transformation. Therefore, through this transformation, the camera's frame rate impacts the spatial resolution of the transformed images. The number of frames in the transformed sequence is the quotient of \(n_{x}\), the number of pixels in motion direction in the original sequence and the shift \(s\) (see Figure 18). This means that the spatial resolution of the original image affects the temporal resolution of the created sequence. Figure 17: Evaluation of points in the transformed image for measurements with (**a**) 100 mm/s; (**b**) 150 mm/s; (**c**) 200 mm/s; (**d**) time vs. temperature graph of the three evaluated points. 3. **Evaluation with phase image:** For the calculation of the phase with Fourier transform, the images recorded during the heating pulse and afterwards during the cool-down are used. The following question arises: how many images during the cool- down should be considered? Earlier works show [7] that for phase images, the relationship between pulse length and cool-down-period influences the result. For stationary measurements, often a cool-down-period with the same length as the heating pulse itself is chosen. As described in point II for scanning, this is not so much a temporal length as it is rather a spatial length. This means that the recorded area underneath the inductor resembles the pulse length and the recorded area behind the inductor resembles the cool-down-period. In the case of the measurement in Figure 16, a distance of 45 mm in the moving direction was covert by the camera's FOV, from this 15 mm underneath the inductor and 30 mm behind the inductor. Therefore, the resulting phase image is an evaluation of a certain pulse length and additionally a cool-down-period, which is two times longer than the pulse length. Since the FOV needed for phase evaluation is rather small, it is possible to reduce the camera's image size in y-direction, which further allows for higher frame rates and, thus, increasing the resolution of the result. All the described parameters are interdependent. The frame rate of the used camera speed is an important factor to change and test different parameter settings.