## 3 Aerial Infrared Thermography for Photovoltaic Applications  
IRT can be used for the detection of a great number of defects in PV cells, modules, and strings, since the majority of the defects have an impact on their thermal behavior. Such thermal patterns have been identified and classified in previous studies (e.g., [21; 22]) and they are now standardized in the international norm IEC TS 62446-3 Edition 1.0 2017-06. In this sense, the most common IR image patterns can be seen in Figure 4. The IR patterns can be classified into two groups. In the first group (represented by the upper part of the image), the defects correspond to both a module or one row (sub-string) warmer than the others. On the other hand, the second group (represented by the lower part of the image) is in connection with single cells that are warmer than the others. Figure 2: Image undistortion for visible images. (**a**) Natural landmarks on orientation 1, (**b**) Natural landmarks on orientation 2. (**c**) Image undistorted after calibration for orientation 1. (**d**) Image undistorted after calibration for orientation 2. Figure 4: Summary of PV module IR image patterns observed in outdoor measurements (adapted from [22]). Figure 3: Image undistortion for infrared images. (**a**) Natural landmarks on orientation 1, (**b**) Natural landmarks on orientation 2. (**c**) Image undistorted after calibration for orientation 1. (**d**) Image undistorted after calibration for orientation 2. Therefore, the objective of a IRT inspection is to detect and classify these defects in the thermal images. Traditionally, IRT inspections for PV applications are performed with handheld IR cameras on the ground or on lifting platforms to increase the coverage. This procedure is dependent on human labor and competence and very time-consuming and labor-intensive. As a result, the monitoring accuracy is prone to human error and the uncertainty of the method increased. IRT can be combined with aerial technologies, like UAV, in order to increase the cost-effectiveness and employ IRT for large-scale PV plants or roof-mounted PV systems with limited access. Furthermore, and in connection again with the international norm IEC TS 62446-3, some considerations, which are briefly commented, as follows, should be taken into account when acquiring the thermal images in photovoltaic IR inspections. First of all, The IR-camera image shall be taken as perpendicular to the PV module surface as possible avoiding reflections from the drone. In cases where the image cannot be taken perpendicular to the PV module surface the angle between the camera and the PV module plane should still be greater than 30\({}^{\circ}\) to minimize the effects of reflected background. Moreover, fast camera carriers (e.g., aerial drones) can cause a negative effect on the image quality, as the typically used bolometer detectors have a long time constant and high camera movement velocities above 3 m/s [23] may cause smearing. In connection with the environmental conditions, the recommended values are: (a) a minimum irradiance value of 600 W/m\({}^{2}\) in the plane of the PV module is necessary, (b) a maximum wind speed of 28 km/h, (c) clear sky, (d) and no soiling (cleaning is recommended, e.g., if bird droppings exist). Finally, the GSD should have a maximum value of 3 cm/pixel. Standard c-Si PV panels have a cell of 15 \(\times\) 15 cm (see Figure 5). Therefore, norm IEC TS 62446-3 recommends that all PV modules should be recorded with a minimum resolution of 5 \(\times\) 5 pixels per cell. However, this resolution seems to be not enough to properly register all of the thermal defects when performing aerial inspections of photovoltaic plants. More specifically, in [24], the authors proved that the GSD strongly influenced the ability to detect the thermal defects. They considered that GSD of 1.8 should be necessary to properly detect all thermal defects.