## 4 Conclusions  
We compared LIT and PPT techniques, and how their results are affected by the wavelength range of the used camera, with respect to qualitative inspections of paintings. For this purpose, three paintings of different natures were measured with different combinations of measurement techniques and camera setups. The measurement results are presented as a side-by-side comparison. With two different thermal cameras, an MWIR and LWIR camera, three different camera setups were arranged, each with a different spectral range. It was shown that the camera setup in the 2.5-5 um range suffers from direct reflection artifacts, caused by reflections from the halogen lamps. As discussed, these artifacts were most severe for the continuous illuminated LIT measurements, but were also observed in PPT measurements, due to the residual heat of the lamps. Restricting the wavelength range to >4.1 um with a long-pass filter has effectively reduced the appearance of these artifacts. When combined with the long-pass filter, the images obtained with the MWIR camera looked very similar to those of the LWIR camera. An additional advantage of MWIR cameras is that they can be simultaneously used for IRR measurements. In this way, the same measurement setup can be used for both IRT and IRR measurements, while only having to swap out filters. Figure 7: LIT amplitude and phase images of each painting for the B-MWIR_F setup. Some of the surface defects are more pronounced in the amplitude images (**a,b**) when compared to the phase images (**d,e**). Several defect areas are circled in red. Phase images (**f**) are influenced less by the upper paint layer than amplitude images (**c**). The NETD, or thermal contrast, of the cameras also appeared to have a noticeable impact on the processed results, with the camera setups with a lower NETD producing higher contrast images. The maximum temperature fluctuation for the LIT measurements was 1.8 \({}^{\circ}\)C, compared to 2.7 \({}^{\circ}\)C for the PPT measurements. LIT and PPT measurements produce both phase and amplitude images. It has been shown that, in the phase images, the paint layers were nearly completely invisible, revealing the structure of the underlying wooden support. The upper paint layers were least visible in the PPT phase images. On the other hand, the pencil drawings were most clearly visible in the LIT phase images. Overall, LIT images appeared to have a higher contrast and appeared to be less noisy. Another attractive feature of LIT is its ability to tune the depth resolution based on the modulation frequency. This could potentially be useful when trying to determine the stratigraphy of multi-layered paintings, and is a possible avenue for future research. Conceptualization, M.H., S.V. and B.R. ; methodology, M.H., S.V. and B.R. ; software, M.H., B.R. and S.S.; validation, M.H. ; formal analysis, M.H. ; investigation, M.H. ; resources, G.S., G.V.d.S. and K.J. ; data curation, M.H. ; writing--original draft preparation, M.H. ; writing--review and editing, B.R., S.V., S.S., G.S., G.V.d.S. and K.J. ; visualization, M.H. ; supervision, G.S. and G.V.d.S. ; project administration, M.H. ; funding acquisition, G.S., G.V.d.S. and K.J. All authors have read and agreed to the published version of the manuscript. This research has been funded by the University of Antwerp (BOF/IOF). University of Antwerp BOF DOCPRO project: Depth-selective chemical imaging of Cultural Heritage Objects (DICHO) with project ID 39928. Industrial research fund of the university of Antwerp: 4D Thermal imaging of people using statistical shape models with ID 42602. Research Foundation-Flanders under Doctoral (PhD) grant strategic basic research (SB) with ID 1 SC0819N and through grants G054719N and G056619N. Data available on request. The data presented in this study are available on request from the corresponding author. The authors declare no conflict of interest. The following abbreviations are used in this manuscript: