## 4 Discussion
The lower boundary of the rectangle corresponds to the 25 th percentile, the upper to the 75 th percentile, and the line between them to the 50 th percentile (median). The percentile indicates the number of defects in an area smaller than the specified one. For instance, the 25 th percentile with an area of 5000 pixels means that 25% of all fragments have an area smaller than 5000 pixels. Outlier lines on this diagram show the values, which fall within \(\pm\)1.5 of the interquartile range \(IRQQ=a_{75}-a_{25}\) (where \(a_{75},a_{25}\) are the 75 th and 25 th percentiles, respectively). For the incandescent lamp (Figure 14c,d), the scatter of the recognised defect areas is approximately in the same range (average light above 108 lx). Thus, after reaching a certain \"saturation\" level, the scatter of the \"damage area\" values is the same for both light sources. Therefore, in addition to having only slight changes in the areas of fragments recognised,  
Figure 14: Scatter plot (**a,c**) and box plot (**b,d**) of defect areas for LED (**a,b**) and incandescent lamp (**c,d**) sources. only a few new fragments are recognised. This is especially noticeable in the case of the incandescent lamp. Bar plots drawn up for the LED source allow us to conclude that regardless of the light level, half of all damage fragments recognised have an area of up to 7000 pixels (the median). The number of very big fragments increases with the light level. Thus, with 220 lx on average, fragments of damage with an area of 40,000 pixels and more appear in the image. This is because many more small defects can be recognised in higher light. Moreover, when located close, they merge into one large fragment. The lowest scatter of fragment areas is observed at the light level up to 100 lx (for both light sources). Speaking about the object of interest, only the most pronounced horizontal damages, which have approximately the same shape, direction, and area, are recognised in low light. As the light level increases, the scatter of areas also increases, showing approximately the same results for all light levels. Figure 15 contains scatter plots showing the orientation (tilt angle) of the damage fragments found. In the diagram, each marker represents a piece of damage recognised. Three groups of damage can be distinguished, the slope of which relative to the \(x\) axis is close to 0 rad (zone 1) and \(\pm\frac{\pi}{2}\) rad (zones 2 and 3). This corresponds to the visually recognisable directions of scratches on the test specimen (Figure 13), which contain mainly horizontal and vertical scratches. Angles \(\pm\frac{\pi}{2}\) and \(\pm\frac{\pi}{2}\) have the same orientation, given that damage is not represented by a direction vector. Note that the \(x\) axis is vertical and the \(y\) axis is horizontal in Figures 11 and 12. The incandescent lamp helped recognise defects that have the same orientation. Given this, the scatter plot became even more compact (Figure 15b). Defects of different orientations could be found only at the highest light (\(\sim\)270 lx). To assess the general influence of the illuminance level on defect recognition, the calculated results were compared with the data marked by the expert. The Dice similarity coefficient (13) was used as a metric. Figure 16 shows the dependence of the DSC coefficient on the illuminance level applied to the test surface under the LED source. The best result (\(DSC=0.93\)) was obtained at 196 lx. A lower DSC at lower light levels means that some defects are still not recognised. DSC decreases at a high light because the surface formations become more visible, and the recognised fragments become wider. In addition, small fragments appear in high light, erroneously identified as damage. Figure 15: Scatter plot showing the orientation of damage fragments recognised for LED (**a**) and incandescent lamp (**b**) sources.