Dark line in UVF image

In a UV fluorescence image, a cracked cell shows a dark bar or line across it. The crack is often easier to spot here than in an EL image. The line forms because oxygen diffuses in through the backsheet and photobleaches the fluorescent encapsulant products along cell edges and cracks, turning them non-fluorescent. Images are taken in the dark, usually at night, with UV light below 400 nm, and the modules do not need to be disconnected or powered. The module needs enough prior sun exposure to give a usable signal, roughly a UV dose of 80 kWh/m², or about 1.5 years outdoors in Germany. A new crack shows only in EL because the encapsulant has not yet had time to bleach, so comparing UVF and EL images helps date the damage. Cracks at the cell edge cannot be seen because the cell border is already bleached, and grey zones along the interconnectors make cracks near them hard to identify. The main failure mode behind this signature is cell cracks, for example after a severe hailstorm. The fluorescence signal also depends on the defect type, such as micro cracks, hotspots or glass breakage.

<!-- review notes

• sentence 6: support check: partial - Edge cracks undetectable due to bleaching at cell frame and interconnector grey zones make nearby cracks hard to identify are stated, but 'already bleached' is roughly implied; fine detail minor.
• sentence 7: support check: partial - Cell cracks are linked to the FL signature and the hailstorm figure and defect types (micro cracks, hotspots, glass breakage) are stated, but 'hail causes cracks' is only implied by a figure caption and 'main failure mode' is an inference. -->