## 3 Results and Discussion  
For the A-MWIR_O camera setup there was a substantial overlap between the emission spectrum of the halogen lamps and the spectral range of the sensor. As a result, reflections of the primary radiation of the halogen lamps by the object caused direct reflection artifacts during the measurement. LIT measurements were affected the most, since the lamps were turned on for almost the entire duration of the measurement. Nevertheless, the PPT measurements were significantly hampered by these artifacts as well, which were caused by infrared radiation being emitted by the lamps even after they were turned off. The effect of these artifacts was most noticeable for the measurements of the St George painting,with the surface composition being clearly distinguishable, even in the phase images (Figure 4). This type of artifact is a known issue that is covered in the literature [33; 34]. The measurements of the other artworks were also affected by these artifacts, but to a lesser degree (Figures 5 and 6). Adding a long-pass filter with a cut-on wavelength of 4.1 um to the MWIR camera (setup B-MWIR_F) eliminated most, if not all, direct-reflection artifacts. After these artifacts had been eliminated, the spectral range of the sensor did not seem to have much of an impact. As shown by Figures 4-6, the phase images for the measurements taken with B-MWIR_F and C-LWIR all look very similar to each other. However, B-MWIR_F phase images had a slightly better contrast compared to C-LWIR ones, which resulted in a more pronounced visualization of the wooden support structure (Figure 5). This was likely caused by the lower NETD of B-MWIR_F compared to C-LWIR. In the phase images of the [PERSON] copy, the upper paint layer became almost entirely invisible and the structure of the wooden board below was revealed (Figure 5). For the purpose of visualizing the wooden structure of the board, PPT seemed to be preferable to LIT, since less of the upper paint layer remained visible in the PPT phase images. On the other hand, the lines of the pencil drawings of the painted icon and other small surface defects appeared to have a higher contrast in the phase image of the LIT measurement (Figure 6). Overall, the LIT images appeared to have a higher contrast, and appeared to be less noisy, when compared to the PPT images. Figure 4: LIT and PPT phase images of the St George painting for all camera setups. Both LIT and PPT measurements were heavily influenced by reflections from the halogen lamps (**a**,**d**). The influence of these reflections was eliminated when using a long-pass filter (**b**,**e**) and the results look very similar to those of the LWIR camera (**c**,**f**). Figure 5: LIT and PPT phase images of the [PERSON] copy for all camera setups. The upper paint layers are almost entirely invisible and the structure of the wooden board is revealed. PPT phase images (**d–f**) show less of the paint layer when compared to LIT phase images (**a–c**). The wood grain contrast seems higher for the images obtained with the B-MVIR setup _F setup (**b,e**) when compared to the C-LVIR setup (**c,f**). Figure 6: LIT and PPT phase images of the St. Nicholas icon for all camera setups. By eliminating the influence of the reflections, we can clearly visualize the pencil drawing (**b,c,e,f**). In the LIT phase images (**a–c**) the pencil drawing and smaller surface defects have a higher contrast, and the images appear less noisy overall, when compared to the PPT phase images (**d–f**). As previously discussed, phase images are affected less by thermal and optical non-uniformities. This includes non-uniformities caused by the uneven heating of the upper paint layer, because the different paints absorb different amounts of illumination during thermal excitation [11]. As a result, the upper paint layer is also less visible, which is of particular interest when inspecting the wood grain of the support (Figure 7f).