When you look at a solar panel, it looks like a uniform grid of blue or black cells. But inside, individual cells can be cracked, disconnected, or degraded — and you would never know by looking. Electroluminescence (EL) testing is the only reliable way to see inside a solar panel without dismounting it — revealing defects that are completely invisible to thermal cameras, visual inspection, and even performance monitoring.
How Does EL Testing Work?
Solar cells are semiconductors — they convert light into electricity, but they can also do the reverse: when an electrical current is passed through them, they emit light. EL testing exploits this property. An external power source passes a small current (typically at the panel's short circuit current) through the panel in a darkened environment. Healthy cells emit a uniform near-infrared (NIR) glow. Damaged, cracked, or inactive cells emit less or no light — appearing as dark areas in the EL image captured by a specialised near-infrared camera.
What Defects Does EL Testing Reveal?
- Micro-cracks (hairline fractures in silicon cells from transport, installation, or hail)
- Finger interruptions (broken current-collecting fingers on cell surface)
- Inactive cell areas (dark patches where cells are not generating)
- Snail trails (oxidation of silver paste causing dark S-shaped patterns)
- Delamination (separation of cell layers visible as dark regions)
- Shunting (localised current leakage appearing as bright spots)
- Potential Induced Degradation (PID) patterns across strings
EL Testing vs Thermal Imaging — Key Difference
The most important thing to understand is: EL testing and thermal imaging detect different faults. A cracked cell may not generate enough heat to appear in a thermal image — especially if the crack is fresh or only partially interrupting the circuit. EL testing will clearly show the crack regardless. Conversely, a bypass diode failure creates a very clear thermal hotspot but may show minimal EL anomaly. The two methods complement each other — not replace each other.
When is EL Testing Performed?
- Pre-commissioning acceptance — establish a baseline before COD
- Post-delivery inspection — verify modules arrived undamaged from factory
- Post-hail / storm damage assessment — detect invisible micro-cracks
- Warranty claims — manufacturers require EL evidence for replacement
- Annual O&M (with thermal) — comprehensive plant health audit
- Pre-refinancing or M&A due diligence — full technical assessment
Can EL Testing Be Done by Drone?
Traditional EL testing required panels to be removed and taken to a lab — or tested at night with heavy ground equipment. Varuna Aerotech uses drone-mounted near-infrared EL cameras for nighttime aerial EL testing, which combines the systematic coverage of drone inspection with the diagnostic power of EL imaging. This is significantly faster than ground-based EL testing for large plants and provides GPS-tagged, module-level results across the entire array.
Frequently Asked Questions
Can EL testing be done during the day?
EL requires low ambient light — ideally darkness or very low light conditions — because the EL emission is very faint compared to sunlight. We perform EL inspections at dawn, dusk, or nighttime.
Does EL testing damage solar panels?
No. EL testing is completely non-destructive. The current levels used are within the panel's normal operating parameters and cause zero damage.
Do all module manufacturers accept EL test results for warranty claims?
Most major manufacturers (Jinko Solar, Longi, Trina, Waaree, Adani Solar) accept EL testing reports following IEC 60904 methodology. Always confirm with your specific manufacturer before testing.
