## 2 Materials and Methods  
A broad range of measurements is performed within this contribution, mainly focused on the IR-B spectral range (between 1.5-3 \(\upmu\)m) as it is less frequently investigated for artwork inspection but promising results are expected. A first subdivision of the measurements can be made within a measurement technique:  
**Infrared reflectography** is well-established for art work inspection, in order to reveal _pentiment_, touch-ups and other artefacts [16]. It is a photographic technique at a longer wavelength, which makes it feasible to use the transparency of most of the pigments in this spectral range to evaluate the layers underneath optical opaque layers. The technique is mostly performed in the IR-A spectrum, but it is possible in the IR-B spectrum as well. The degree of paint opacity is mostly determined by two factors: the absorption of light by the pigment and the scattering of light by pigment particles [23]. A full description can be found in the work of [PERSON] et al. [23]. Within the measurement set-up, a halogen float lamp of 1000 W or IR LED 940 nm with a power of 800 W is made use of as an illumination source. The acquired measurement data is not further post-processed before interpretation. **Active thermography** uses the transient behaviour of the response on a thermal excitation (heating or cooling) of the object to identify anomalies within the structure as shown in Figure 1. The study of active thermography has become an important Non-Destructive Evaluation technique (NDE) for damage detection and material updating in metallic structural components [24; 25; 26], as well as CFRP (Carbon Fiber Reinforced Polymer) and GFRP (Glass Fiber Reinforced Polymer) composites [27]. In state-of-the-art research, the technique is upcoming for the inspection of art works in the IR-C spectrum [16]. Figure 1: Schematic view of active thermography on a painting, as used within this work. IR: Infrared Radiation. Thermography has many advantages including operating in a non-contact mode, it can be performed in situ, it can cover large areas and it is a quantitative method [28; 29]. The disadvantages are that anomalies deeper in the structure are more difficult to inspect [30] and that the results are highly dependent on the used experimental set-up conditions, for example emissivity variations and non-homogeneous surface heating. Efficient signal processing and filtering of the anomaly information in the field of active thermography methods are very important. With the help of adapted image- and signal processing algorithms, it is possible to detect small discontinuities inside structures or extract material characteristic information. By reducing the amount of acquired data, the process is accelerated, but a certain decrease in accuracy has to be kept in mind. Within this work, two main post-processing techniques are used based on image correlation and principal component analysis [31] using the IR-View software (V2015.5.29, VisioolImage, Quebec-City, QC., Canada, 2015), described in [32]. * SWIR: Short Wave Infrared radiation, only for reflectography measurements. spectral range: 0.9-1.7 \(\upmu\)m (IR-A), measurements performed with a Goodrich SU640 SDWH-1.7 RT camera, CMOS InGaAs uncooled sensor and a spatial resolution of \(640\times 512\). * MWIR: Mid-wave Infrared radiation. Global spectral range of 1.5-5 \(\upmu\)m (IR-B+C'), measurements performed with FLIR X6540 sc camera with a cooled InSb sensor, Noise Equivalent Temperature Difference (NETD) of 20 mK and a spatial resolution of \(640\times 512\). The IR-C' measurements are compared with a FLIR Phoenix camera, which is only sensitive in the IR-C' spectrum:
* reflectography measurements within the spectral range of 1.5-5 \(\upmu\)m (IR-B+C') with separate filter options:
* 1.615-2.280 \(\upmu\)m,
* 2.055-2.145 \(\upmu\)m,
* 2.5-2.7 \(\upmu\)m,
* 1.5-5 \(\upmu\)m,
* active thermography measurements within the spectral range of 3-5 \(\upmu\)m (IR-C').