## 2 Materials and Methods
### Imaging Acquisition Set-Up  
The conventional setup for acquiring EL images under daylight conditions consists of a programmable power supply (EA ELEKTRO-AUTOMATIK EA-PSB 10200-25 2U 1500W) directly connected to the PV module under analysis. In this study, a 235 W\({}_{\rm P}\) polycrystalline PV module (Trina Solar TSM-235-PC05A) was examined. This conventional approach requires disconnecting the PV module from the string and using either a generator or a grid connection to power the supply. The self-powered method relies on an electronic board that can be integrated into each PV module within a string. The board topology, shown in Figure 1, is based on a half-controlled H-bridge and an LC (L-inductor C-capacitor) low-pass filter. The half-controlled H-bridge enables bidirectional current flow through the tested PV module. When MOSFETs M\({}_{1}\) and M\({}_{2}\) are turned on, current flows through the module in the normal direction, allowing it to operate as a generator. Conversely, when M\({}_{1}\) and M\({}_{2}\) are off, current flows in the opposite direction, causing the module to behave as a load, thereby inducing EL effect. Finally, if only one of the MOSFETs (M\({}_{1}\) or M\({}_{2}\)) is turned on, the module is bypassed, and no current flows through it. By controlling the MOSFETs (Metal-Oxide-Semiconductor Field-Effect Transistor) with pulse-width modulation (PWM) signals and incorporating an LC low-pass filter with a cutoff frequency lower than the PWM frequency, an arbitrary current signal can be modulated within the tested PV module. The present implementation involves a 5 kHz PWM signal and a LC low-pass filter with 0.8 kHz cutoff frequency. Since EL signal emission is directly proportional to the current injection, signal modulation is inherently achieved. Figure 1: Topology of the electronic board for self-powered signal modulation. In the self-powered approach, the tested PV module (Trina Solar TSM-235-PC05A) is integrated into a string of 10 modules optimized by a PV string inverter. Modulation is performed while the inverter remains in operation, enabling real-time measurements without requiring module disconnection. The acquisition system for the self-powered approach is shown in Figure 2, where a laptop is used as the control system to automate the acquisition of the image sequence and to control the electronic board for proper signal modulation. Additionally, the board must be connected to both the tested PV module and the remaining modules in the string. The acquisition system consists of a tripod and an InGaAs camera (Hamamatsu C12741-03) with a resolution of 640 \(\times\) 512 pixels, a maximum frame rate of 60 frames per second (fps) and a 14-bit depth. The camera lens is equipped with an optical bandpass filter that blocks wavelengths outside the 1100 nm-1250 nm range. The camera exhibits high sensitivity at the EL emission wavelength (1150 nm for silicon solar cells), which, combined with the optical filter, enhances the signal-to-background ratio. This is particularly critical when measurements are performed under high irradiance conditions. While the current and signal are modulated either by the power supply or the electronic board, the camera captures a sequence of consecutive images at its maximum frame rate, with a fixed exposure time of 2 ms for all measurements. The camera's shutter has been adjusted to ensure an appropriate histogram, with the average pixel intensity centered around the mid-range. All images were acquired under high irradiance conditions (1030-1150 W/m\({}^{2}\)) around midday, with stable irradiance and clear-sky throughout the acquisition period. It has been done to achieve similar irradiance conditions during all the measurements. In both approaches, the modulated signal has a frequency of 4 Hz. This frequency was selected to acquire enough images per cycle to observe signal variations. At 4 Hz and a maximum frame rate of 60 fps, 15 images are captured per cycle, which is adequate to represent the periodic variation of the signal.