## 3 Results
### Passive and Active IRT Results  
PIRT is useful to identify potential detachments, part of the wall structure, and a presumable humidity area. Figure 4 shows a part of the apse between two panels of the frescoes and precisely between the central one and that on the left (between panels A and B), which is the one with the worst degree of conservation and that was probably retouched in the past several times. A quite large and abrupt average measured temperature discontinuity between the central-right side of the apse and the left side can be seen, which was confirmed by the other images collected. As mentioned above, this can be due to the position of the left side relative to the cardinal points, even if at the time of the measurement, the left side was exposed to the sun, but probably also to differences in the structure that affect the emissivity (e.g., materials, construction techniques, plaster characteristics, and later works such as the construction of the sacristy). This difference between the left and right sides is confirmed by the PIRT images shown in Figure 5 and collected over the same areas imaged with SPV and HT, reported in Figure 3. The subplots are named according to Figure 3.  
In general, large temperature drops that could indicate large voids, anomalous moisture levels, or mass transfer (e.g., water) are not visible, even considering that the measurements were collected in the winter just after a huge snowfall (the church and all the area was covered by a thick layer of snow). There was also no evidence of severe inhomogeneities,  
Figure 4: Passive IRT image of the attachment point of panels A and B. An abrupt discontinuity in the average emitted radiation can be seen. Figure 5: Passive IRT images of some frescoes’ parts. The subplots (**a,c**,**e**,**g**) are named according to Figure 3.  
moisture, or detachments, as the wall texture was visible in all the images, while large and long cracks were not. However, the wall texture was more visible on the left side of the apse, which is also less homogeneous with respect to the measured temperature. The active IRT focused on two figures of the frescoes cycle. One is the figure that is going to be slaughtered in the second panel of the frescoes, starting from the left of the apse (panel A), and the other is the soldier lying at the foot of the tomb in the scene of the resurrection, which is in the first panel from the right (panel E). These two figures were chosen since passive IRT images evidenced some inhomogeneous spots on them, and further, for the soldier, the pictorial layer is significantly damaged, mainly scratched. In particular, the regions of interest (ROIs) were suggested by an art historian, Dr. [PERSON], a renowned expert of the art master [PERSON]. The results of the PuCT experiments are reported in Figures 6 and 7. In particular, after the PuCT, virtual wave processing was applied [24]. These technique maps the time thermal signals, i.e., the intensity time trends of each pixel of IRT images, which are due to a diffusion process, into virtual signals associated to a propagation phenomenon, in which velocity is related to the thermal diffusivity. In this way, it is possible to estimate the depth corresponding to a specific image in the sequence after virtual wave application. This was made by assuming a thermal diffusivity of 0.3 mm\({}^{2}\)/s. To the authors' knowledge, this is the first application of the virtual wave approach to the thermography inspection of CH. Figure 6: Results of PuCT and PuCT + virtual wave processing for the first area investigated. The red boxes in the bottom-right images indicate the shape of the bricks, which also barely visible in the passive IRT image (see Figure 5a). In the case of PuCT + virtual wave the bricks’ shape appear from a certain value of depth and the maximum contrast is reached for a depth d = 10 mm. Figure 6 reports the results for the first ROI.