EL Imaging (Electroluminescence)

What EL Imaging Reveals

Because a defect-free silicon cell emits infrared light almost uniformly across its surface when forward current is applied, anything that disrupts current flow within a cell shows up as a dark region against that even glow. In practice this makes EL imaging one of the few methods that can catch a cell microcrack while it is still sub-visible to the naked eye and before repeated thermal cycling turns it into a measurable power loss. The same test also flags cold solder joints at the ribbon-to-cell interconnect, dead or disconnected cells, and cells affected by potential-induced degradation, which tend to dim in a pattern tied to their position in the string rather than showing the sharp line of a crack.

How the Test Works

The module is taken off the array (or isolated on the production line) and a DC current is applied in the forward direction — roughly the module’s rated short-circuit current. Working cells respond by emitting a faint near-infrared glow that is invisible to the eye but readable by a cooled CCD or InGaAs camera. Because the emission is faint, the test has to be run in full darkness: an indoor booth on the manufacturing line, or outdoors at night for field and commissioning work. The resulting image is essentially a current-flow map of every cell in the module — even, bright cells are healthy, and dark lines, patches, or fully black cells mark exactly where current isn’t flowing as it should.

When EL Imaging Is Used

• Manufacturing QC — sampled or 100% inline testing to catch cell and interconnect defects before modules leave the factory.
• Pre-shipment inspection — a common requirement on utility-scale procurement, to document module condition before container loading.
• Field commissioning — re-testing after transport and installation to confirm no new cracking occurred en route or during racking, before the plant is handed over.
• Warranty and underperformance investigations — when a string or block is underperforming and the cause isn’t obvious from string-level monitoring alone.

EL Imaging vs. IR Thermography

The two tests are often confused because both produce a false-color image of a module and both catch cell-level problems, but they test opposite conditions. EL imaging is a powered-off, dark-room test that applies current artificially to look for defects before they cause heat. IR thermography instead images a live array under sunlight to find hot spots that are already generating excess heat during normal operation. A well-run QC program typically uses both: EL at manufacturing and commissioning to catch latent defects, and periodic IR sweeps during operation to catch problems — including cracks that were missed or that develop later — before they become a fire or yield risk.

Key Takeaways

• EL imaging reveals PV cell defects via IR emission under applied current.
• Detects microcracks, soldering defects, PID, dead cells.
• Performed at manufacturing and field commissioning.
• Best done at night or in dark conditions.
• IR thermography is complementary; EL is more sensitive to specific defects.

Related Reading

EL imaging findings are only useful once they’re documented in a way a lender, EPC, or module manufacturer can act on — see how Heaven Designs structures that documentation on the Reports services page. The defects EL imaging is built to catch don’t stay static: a microcrack found at commissioning can progress into a full hot spot after enough thermal cycling if it isn’t tracked and re-inspected. For a closer look at how the individual defect types this test catches actually develop and affect long-term output, the cell microcrack and PID glossary entries above walk through the mechanisms in more depth.