In one line
A diagnostic that pushes current backwards through a module in darkness and photographs the resulting infrared emission, revealing cracks, broken fingers and dead cells invisible to the eye.
What it means
Driven in reverse, a solar cell behaves like a very inefficient LED, emitting near-infrared light in proportion to local material quality. Photographing that emission with a sensitive camera produces a greyscale map in which healthy silicon glows evenly and defects appear dark: micro-cracks show as fine dark lines, broken metallisation fingers as dark stripes, and fully disconnected regions as black patches. EL imaging is the standard acceptance test for incoming module batches, the definitive evidence in transport-damage claims, and the confirmation step after an I-V trace suggests a problem. It requires darkness — meaning night work or a shrouded enclosure — and a DC source, which is why field EL campaigns are typically scheduled overnight.
Worth knowing
- Reveals micro-cracks long before they cost measurable energy.
- The standard evidence in transport and handling damage disputes.
- Requires darkness and a current source — usually a night-time activity.
- Pairs naturally with I-V tracing: I-V finds the string, EL finds the cell.
Field notes
- 1EL a statistically meaningful sample of every incoming container — transport micro-cracking is common and only visible this way.
- 2Photograph before and after installation on projects where handling disputes are likely; the before-image is your negotiating position.
- 3Drone-mounted EL is now practical at utility scale and much faster than module-by-module inspection.
Common mistakes
- Accepting a delivery on visual inspection alone, then discovering pervasive micro-cracks after the claim window closes.
- Reading every dark line as a warranty defect — some faint cracks never propagate or affect output.
Frequently asked
What does electroluminescence testing show in solar panels?
It reveals defects invisible to the eye — micro-cracks, broken metallisation fingers, disconnected cell regions, and cell damage from PID or handling — by imaging the infrared light a cell emits when current is driven through it in darkness.
When should EL testing be done?
At factory acceptance, on incoming batches after transport, after any suspected handling or hail event, and as a confirmation step when I-V tracing or thermal imaging suggests a module-level problem.
Sources
Related terms
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