## 3 Results  
The measurements were performed on a PVC flat bottom hole plate with a thickness of 9.8 mm and a hole pattern consisting of 12 holes with various depth and diameter. The diameters increase step by step between 6 and 25 mm: 6 mm, 12 mm, 20 mm and 25 mm. The hole depths vary between 2.5 and 8.5 mm: 2.5 mm, 5.5 mm and 8.5 mm. A visual representation can be found in Figure 8. As the used measurement technique is dynamic line scan thermography, the pulse length is solely dependent on the movement velocity. The heating is initiated at the beginning of the measurement and remains until the end of the movement. While the sample moves underneath the heater, the pulse length for a specified region can be calculated using the movement speed of the sample. The parameter set is optimised for hole 2 and resulted in following combination: \(d_{heat-cam}\) = 50 mm, \(P_{heat}\) = 16.7 W/cm\({}^{2}\), \(v_{source}\) = 5 mm/s, \(d_{height}\) = 300 mm, \(T_{ambient}\) = 22 \({}^{\circ}\)C, \(d_{start}\) = 2.5 mm, \(D_{hole}\) = 12 mm and Maximum use temperature of 80 \({}^{\circ}\)C. Optimising the parameter set for hole 2 will result in the upper horizontal row to be detectable in the inspections. The diameter difference has minimal influence on the optimised parameter set. The diameter however will determine the maximal height placement of the camera. One could see the influence of varying the measurement set-up parameters in comparison to the optimal parameters as predicted by the response surface in Figure 9. Performing measurements at higher speeds without varying the rest of the parameters will result in a loss of contrasts. Placing the camera at a bigger distance from the heat source than  
Figure 8: Visual representation of the PVC flat bottom hole plate with various depths and diameters. Holes 1–4 have a depth of 2.5 mm, 5–8 have a depth of 5.5 mm and 9–12 a depth of 8.5 mm. Every vertical set of holes have a different diameter starting with 6 mm for holes 1, 5 and 9. The distance between the centre of the holes is 45 mm in order to minimise the influence of one defect on another. prescribed by the optimised parameter set will result in a minor contrast loss but will majorly result in a loss of detail about the size and shape of the detected defect. The optimal parameter set for each hole can be found in Figure 10.  
In order to compare the contrast objectively the contrast is calculated between a pixel in the centre of defect on eight different pixels around the defect. The location of the pixels is shown in Figure 11. The contrast is calculated as the absolute value of the sum of the temperature difference between a pixel and the centre pixel divided by 8. Figure 9: Comparison between measurements performed with alternating parameter set. The values on the \(x\)- and \(y\)-axis are pixel values and the colours represent temperatures in \({}^{\circ}\)C. The range is fixed between 29 and 70 \({}^{\circ}\)C for measurements (**a-c**) in order to facilitate the comparison between different measurements. Image a shows the detection of hole 2 using the optimal parameter set (\(d_{heat-cam}\) = 50 mm, \(P_{heat}\) = 41.7 W/cm\({}^{2}\), \(v_{source}\) = 5 mm/s, \(d_{height}\) = 300 mm, \(T_{ambient}\) = 22 \({}^{\circ}\)C, image b shows the detection of hole 2 with the only difference that the inspection was performed with higher source velocity (15 mm/s) and image c shows the detection of hole 2 using a bigger distance between the camera and heat source (170 mm). As this is a snapshot taken from the DLST measurement other defects are not clearly visible in these images. The defects 1–4 were all clearly visible in the movie what can be explained by the fact that they have the same depth. Other holes such as 7 and 8 could be detected in the movie as well, but were not as clear as hole 2.