## 3 Results and Discussion
### Mondrian: Underdrawing  
The IR-A results, presented in Figure 7a, are performed with halogen illumination and give a clear view of the _prussian blue_ lines of the composition below the lead white cover layer. The _massicot_ and _ultramarine_ pigments appear only vaguely.  
Figure 5: Canvas B with the replica of [PERSON]. On the left is a photograph of the figure (recto) and on the right is a picture of the canvas structure (verso).  
Figure 6: Visual photograph of both marquetry test samples. The right sample contains some surface piercing defects, and the left has only corrected surface piercing anomalies present.  
The IR-B+C' results, measured with the FLIR X6540 sc camera, are subdivided in reflectivity (IR-B) and active thermography (IR-C') measurements:  
IR-B reflectivity:  
The measurements deliver the best result when the filter around 2.2 um is used in combination with halogen illumination. The results are presented in Figure 7b, with prior knowledge, a few of the _prussian blue_ lines can be distinguished, but the results remain vague. There could be concluded that it is not possible to distinguish a subsurface composition, and the results are worse with respect to the IR-A measurements.  
IR-C' active thermography:  
For the thermal inspection, the camera is used without camera filters within the IR-C' spectrum. First, the halogen excitation source is used as active heat source directly aimed at the painting, exposing the painting to a sine wave excitation of 0.1 Hz, blocking the thermal power using a double glass filter. The result is shown in Figure 8a after Fourier transformation and phase analysis. The _prussian blue_ lines are visible, but excessive post-processing is needed.  
A second similar inspection is performed using pulsed thermography with a high power Xenon flash excitation of 5 ms. The results, post-processed with Principal Component Analysis (PCA) (bin 3),are presented in Figure 8b. It can be seen that not only the _prussian blue_ lines but the _massicot_ and _ultramarine_ pigments could be clearly seen as well and the _minium_ pigment could be vaguely distinguished. This can conclude that it is possible to evaluate the full sub-layered composition and that the results are similar or even better than the commonly used IR-A reflectography measurement.  
Figure 7: Reflectographic results on the [PERSON] painting. (**a**) IR-A results of [PERSON] compositions with halogen illumination. Composition 1 appears mixed with composition 2; (**b**) IR-B reflectographic results using the filter around 2.2 μm and halogen illumination. Composition 1 remains hidden.  
With those results, we can conclude that the inspection of composition no. 1 is possible with a similar or even better accuracy using active thermography and an IR-B+C' camera with respect to the well-known IR-A reflectography technique. It should be marked that the selected test sample contains a very thick intermediate layer of lead white. Unfortunately, with state-of-the-art techniques, those paintings below a painting with a thick lead white layer are very difficult to inspect. With these results, we can conclude that the use of active thermography can provide a relatively fast way to evaluate the presence of sub-layer compositions.