# An Overview of CNN-Based Image Analysis in Solar Cells, Photovoltaic Modules, and Power Plants
## 2 Imaging Methods and CNN Processing  
### Inspection Methods of Solar Cells and Modules  
Since CNNs are optimized for image analysis, the models need to be provided with some kind of record of the state of the solar panels. Furthermore, a large number of samples is needed for learning. There are basically three imaging modalities to choose from for the on-site and laboratory condition assessment of solar cells or panels:  
* Electroluminescence (EL) [42];
* Photoluminescence (PL) [43];
* Thermal imaging (IR) [44].  
Electroluminescence should not be confused with photoluminescence, although there are similarities between the two techniques. Both are based on the phenomenon of luminescence, but differ in the mode of excitation and the information content. In photoluminescence, the solar cell is illuminated by a laser or LED light, whose energy exciteselectrons, which are then excited to higher energy levels. When the electrons return to their ground state, they emit photons that can be observed with a camera or spectrometer suited to the purpose. The study provides information on the material quality and crystal structure defects of the PV cell. Electroluminescence, on the other hand, is a phenomenon where a material emits light in response to an electric current or electric field. When voltage is applied to the PV cell, photons are emitted. Figure 1 shows a schematic view of the EL test. [PERSON] [45] presented the difference between EL images of polycrystalline and monocrystalline solar cells in his publication. All cases are true where the emission is low or no photons can be detected at all, where a fault in the operation of the solar cell is suspected [46; 47; 48; 49; 50]. The wavelength of the emitted photons is in the near-infrared (NIR) range between 1000 and 1300 nm and its peak is around 1150 nm in the case of crystalline Si cells. Therefore, it cannot be detected by the human eye or by digital cameras made for everyday use [51].  
EL tests can provide a comprehensive and highly visual picture of the condition of an entire solar module. Nowadays, the technique can be used not only for silicon cells but also for perovskite and organic cells [52]. EL testing is beneficial when looking for structural or electrical problems, such as cracks, contact defects, or recombination anomalies. The end result of the test is a digital photograph. The EL test can be performed in both the opening and closing bias of the solar panel. Significant differences can be detected between the two methods. Open-end testing is more common and is therefore the standard practice for in-process quality assurance and condition assessment. Furthermore, efforts have been made to describe the relationship between EL images and the electrical parameters that can be measured [53; 54; 55; 56; 57].  
Figure 2 shows the typical defects that can be identified by EL testing [45; 46; 47; 48; 49]:  
(a) Crack;  
(b) Fracture;  
(c) Soldering failure;  
(d) Hotspot;  
(e) Finger interruption;  
(f) Tabbing disconnection;  
(g) Material defect;  
(h) Edge defect (contamination during the silicon ingot growth);  
(i) Corrosion;  
(j) PID phenomenon;  
(k) Black core;  
(l) Backsheet scratch.  
Figure 1: The schematic view of the EL test.