Incidence Angle Influence Between a UAV-Based Thermal-Infrared Camera and Solar Panel on Hotspot Detection

Hotspots on PV (PhotoVoltaic) modules reduce energy yield and risk physical damage, but reliable detection in UAV (Unmanned Aerial Vehicle)–based TIR (Thermal Infrared) imagery is hindered by viewing-angle effects. This study investigates how changes in viewing direction (−90° to +90° in 10° steps) alter radiometric responses even after geometric rectification. Using CLAHE (contrast-limited adaptive histogram equalization)–assisted SuperPoint–SuperGlue matching and RANSAC (Random sample consensus) homography, 19 images were aligned to a 0° reference coordinate system with a mean RMSE (Root-Mean-Square Error) of approximately 0.74 pixels. Hotspots within a fixed ROI (Region of Interest) were segmented and assessed via IoU (Intersection over Union), soft-IoU, DIoU (Distance-IoU), centroid displacement, and area ratio. IoU peaked at the frontal view and declined with increasing |angle|, while Soft-IoU reduced boundary errors but not internal intensity shifts. DIoU often overestimated compact detections, and centroid displacement grew with obliquity, indicating radiometric drift. The area ratio fluctuated unpredictably. These outcomes confirm that geometric correction alone cannot counteract incidence-angle–driven radiometric distortions that obscure true hotspot morphology and intensity. The study recommends near-frontal image acquisition and incidence-aware temperature normalization to enhance reproducibility and operational reliability in UAV–TIR hotspot inspection.