## 4 Database and Acquisition Procedure  
All if the images were automatically collected by a drone over a photovoltaic plant. In connection with the drone, the DJI Matrice 210 RTK V2 was used, which can be seen in Figure 7a. As can be seen, there are two cameras attached to the aircraft. The camera on the left is the DJI Zenmuse Z30 (not used in the acquisition process). The camera on the right is the DJI Zenmuse XT2. This camera, which can be seen in Figure 7b enlarged, integrates a high-resolution FLIR thermal sensor and a 4K visual camera with DJI's stabilization. Therefore, both a thermal and a visual (RGB-Red-Green-Blue) image are recorded by the XT2 camera. Under the scope of this publication, only the thermal image is used and, hence, the RGB image is not used for the detection of defects in the photovoltaic panels. The RGB image is only used for visualization purposes. The XT2 can be mounted with different lens models: 9 mm, 13 mm, 19 mm, and 25 mm. In this case, the selected lens model is the one offering 25 mm. These models provide a 25\({}^{\circ}\)\(\times\) 20\({}^{\circ}\) Field Of View (FOV) angle with 0.680 mrad. In connection with the thermal camera, an uncooled VOx Microbolometer sensor is used, providing a resolution of 640 \(\times\) 512 pixels. The sensor also provides a 17 \(\upmu\)m pixel pitch size and the spectral band is in the range of 7.5-13.5 \(\upmu\)m. The thermal camera can be configured in the range of \(-\)25\({}^{\circ}\) to 135 \({}^{\circ}\)C (High Gain) or in the range of \(-\)40\({}^{\circ}\) to 550 \({}^{\circ}\)C (Low Gain). In this case, the camera was configured in the first range with a High Gain. In Figure 8, some key points in connection with the acquisition process are shown. The photovoltaic plant is located in the province of Leon, Spain. This plant has a size of 1.56 hectares. In Figure 8 left, it can be seen that the DJI mission planner estimated a total of 298 photos, but a total of 321 images was finally acquired. Figure 8 right shows the GPS position associated with each image. All of the images were acquired with a relative altitude of (approximately) 25 m. The exact relative altitude associated with each image is stored using the Exchangeable Image File Format (EXIF) [25]. In this sense, all of the necessary information that will be used in the processing procedure is stored using this image file format. As an example, in Table 1 the main parameters associated with  
Figure 6: Considering only severe failures, two hotspot defects are detected. Figure 7: DJI Matrice 210 RTK V2 was used to collect the images from the photovoltaic plant. (**a**) Drone. each thermal image can be seen. Additionally, images taken by the XT2 camera are recorded pointing to the nadir direction (the camera axis--in the direction of the lens--is perpendicular to the ground). Based on the height and width of the camera thermal sensor, focal length and aircraft relative altitude, the Ground Sample Distance (GSD) can be calculated (see Equation (5)). \[ \begin{split} GSD\_w=(sensor\_width\times relative\_altitude\times 100)/( focal\_length\times image\_width)\\ GSD\_h=(sensor\_height\times relative\_altitude\times 100)/( focal\_length\times image\_height)\end{split}  \tag{5}\]  
In the case of the XT2 thermal camera, \(sensor\_width=10.880\) mm, \(sensor\_height=8.704\) mm, \(focal\_length=25\) mm, and \(relative\_altitude\) value depends on the specific value stored for each image, but it can be considered 25 m for clarification purposes. Using these values, a \(GSD\_w\) of 1.7 cm/pixel and a value of \(GSD\_h\) of 1.7 cm/pixel is also obtained. This way, it can be considered that a \(GSD=1.7\) cm/pixel is obtained. It should be noted that the smallest distinguishable feature will be larger than the GSD. In this sense, the smallest distinguishable element to consider is the photovoltaic cell. As previously commented, the photovoltaic cell has a size of 15 \(\times\) 15 cms.