## 3 Results and Discussion
### Icon: Pigments  
The Russian icon will be used to first compare the ability to see pencil drawings within the IR-A and IR-B+C' spectra and second to identify the different blue coloured pigments as indicated in Figure 9. Therefore, a comparison between the IR-B+C' and MA-rFTIR results will be made to evaluate the received accuracy of the IR-B+C'device. Reflectography:  
An IR-A result of the Russian icon is shown in Figure 10a, where the halogen illumination source is used. It is shown that the pencil drawings occur prominently, including some details that were not visible in the regular photograph. For example, the beard of the icon contains a significant amount of pencil details and a cross becomes visible. The pencil marks absorb the illuminated radiation in contrast to the paint, which becomes semi-transparent. The paint is still semi-opaque, which makes it possible to compare the pencil drawing with the painted icon on top. The IR-B+C' reflectography alternative is shown in Figure 10b. A similar amount of detail in the pencil drawings can be observed although some horizontal marks (head, eyes and chin) become transparent, which were visible in Figure 10a. It should be noted that the pencil colouring is more prominent due to the higher transparency of the pigments in general. It should be noted that these results are an average of the full sensitive spectrum. By subdividing the full spectrum in  
Figure 8: Active thermography results on the Mondrian painting. (**a**) IR-C’ results of Mondrian compositions with halogen sine wave of 0.1 Hz and glass filter after Fourier transform and phase analysis. Composition 1 lines are barely visible; (**b**) IR-C’ results of Mondrian compositions with flash excitation after principal component thermography (PCT) analysis (bin 3). Composition 1 is clearly visible. smaller sub-spectra, multi-spectral analysis might deliver more in-depth information and increase the accuracy. A multi-spectral analysis of the IR-B+C' spectrum is performed using the four filters. The results are shown in Figure 11, which could be compared with similar measurements performed by the MA-rFTIR spectroscopy measurements of Figure 12. Both results are consistent with respect to the pigment appearance. The _cobalt green_ pigment is only fully transparent around approximately 2.1 \(\upmu\)m. For lower and higher wavelengths, the pigment absorbs the incoming radiation and differentiates from the background. The _utramarine_ and _prussian blue_ in contrast behave differently. The ultramarine on the book is fully transparent in the IR-C' spectrum and the prussian blue, which looks visually very similar, absorbs more radiation in the IR-C' spectrum. In this way, there can be made a clear difference between the three blue pigments. Figure 10: IR-A (**a**) and IR-B+C’ (**b**) reflectography results without the use of filters. Figure 9: Visual photograph of [PERSON] icon with known pigment characteristics indicated. Active thermography:  
The active thermography measurements are performed on the sample using a low power flash excitation of 400 J and principal component thermography (PCT). Using PCT, the measured data is divided into principal components that are used to disseminate certain artefacts. The wood  
Figure 11: IR-B+C’ reflectography results by using the four different filters. Differences in the blue pigments are present on the stole and book. Figure 12: Validation results performed making macroscopic reflection mode Fourier transform infrared (MA-rFTIR) to compare the IR-B+C’ filtered results. grain structure can be disseminated from the picture using the same MWIR camera within the IR-C' spectral range only and not looking to the illumination, but to the thermal diffusion. This is shown in Figure 13, where in (b) only the picture is visible and in (c) only the grain structure. This is an added value that is not possible with MA-rFTIR or IR-A measurement systems.