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.

EL and other field tests are part of IEC 62446 acceptance testing.