## 2 Applied Measurement System and Description of Samples  
Thermographic measurements, despite their simplicity, require good preparation of both: the sample and the appropriate test stand. The test stand should provide adequate stability and protection against environmental impact that may significantly influence the results of observations. VIGOcam v.50 (VIGO System S.A, Ozarow Mazowiecki, Poland) thermal imaging camera was used for the measurements. Its basic technical parameters are presented in Table 1.  
In order to isolate test objects from external impacts, while taking tests, they were placed in a casing that was protecting them against direct effects of other external temperature fields. The inner surfaces of the protective casing were covered with a graphite layer in order to minimize reflections and to stabilize the emissivity of the background at a constant level. It was done because one of the basic problems is the variability of the emission factor (\(\epsilon\)) of the tested objects, which is due to the secondary physical properties of the tested samples (such as the structure of the emitting surface, radiation angle, wavelength or temperature of the object). The above mentioned properties can significantly change the emissivity in the analyzed area. At the same time, it is known that the emissivity of metals increases along with the temperature rise, and in case of nonmetals, it decreases [11]. Therefore, the value of emissivity stability of the tested object is a prerequisite for proper temperature measurement when using thermal imaging cameras [12]. The emissivity coefficient is taking values between 0 and 1. For tests were chosen Samxon's DRL106 SOTI25R supercapacitors (Man Yue Electronics Company Limited, Hong Kong, China) that are characterized by a variable structure (texture) of the observed surface (Figure 1). Their main technical data can be found in Table 2.  
With the purpose minimizing the impact of surface structure changes, it was decided to cover the supercapacitor with a graphite layer (Graphite 33 from Contact Cheme). Then, the emissivity (\(\varepsilon\)) was measured in relation to the available reference field, which allowed for the temperature measurement and had the known emissivity \(\varepsilon\approx 0.96\). Technical measurements of the emissivity coefficient variation, in the assumed range of temperature increments (\(\Delta T\) = 25 \({}^{\circ}\)C), were also performed. Samples, in the tested scope, were characterized by emissivity coefficient of emissivity \(\varepsilon\approx 0.80\). Furthermore, it was decided to place the model reference field in the observation field of thermographic camera and to perform the procedure of nonuniformity correction (NUC) before taking each thermographic image. Based on parameters of the camera that is included in [10] and publications [12], the uncertainty of determining temperature increments (\(\Delta T\)) was estimated at \(\sim\)210 m \({}^{\circ}\)C. Atlas-Solitech (Multichannel Potentiostat Galvanostat) module was used to force charging/discharging processes in the tested capacitor samples. This module is able to perform both: power and measurement functions to record the present conditions of the test [14]. Figure 2 shows a schematic diagram of the applied test stand. In order to minimize external influences, it was decided to determine the temperature increments in relation to the available reference field. Such an approach enabled the most effective use of the thermal resolution of the available measurement equipment. It needs to be bear in mind that thermographic methods are characterized by a considerable error when it comes to determining the temperature. For the used equipment it was estimated at \(\pm\)2.5 \({}^{\circ}\)C (without reference field in the observed area).