Two different signals
Both methods use infrared cameras, which is why they are sometimes confused. They answer different questions because they observe different physical effects.
Infrared thermography records the temperature of the module surface. A module that is generating power and has a fault often shows a warmer area where energy is lost as heat. The array is working normally while it is scanned.
Electroluminescence, or EL, works the other way round. Current is passed through the module so that the solar cells emit weak infrared light, much like an LED. A camera records the light. Healthy cell areas glow evenly, while cracked, disconnected or damaged regions appear dark or patterned. The image is a picture of the cell structure, which is why it can show damage that heat does not.
What thermal imaging is good at
- Hot cells. A cell that is dissipating power shows as a warm spot.
- Failed bypass diodes. A substring that is bypassed or conducting incorrectly appears as a distinct thermal pattern.
- Connection problems. Poor terminations and connectors that run hot.
- Mismatch and string issues. Modules or strings behaving differently from their neighbors.
- Speed across a large area. A drone can cover a big array quickly, which is why thermal is the natural broad screen.
The conditions matter. The array has to be producing under steady sun, as described in when to fly a drone thermal scan.
What thermal imaging can miss
Many microcracks do not create a hot spot at the moment of the scan. Cells that are partly disconnected may not change the surface temperature enough to stand out. Defects can exist for a long time without a thermal signature, and an all-clear thermal image does not rule them out.
What EL is good at
- Microcracks. Fine cracks in cells appear as dark lines.
- Inactive cell regions. Areas that no longer contribute current appear dark.
- Broken interconnects and finger interruptions. Disconnected parts of a cell.
- Handling and manufacturing damage. Problems that began before or during installation.
The EL test sees cell structure directly, so it can show a defect before it grows into a heat problem.
What EL can miss or cost you
EL requires driving current through the modules, which usually means taking them offline or testing sections, and it is often done in low light or at night so the weak emission can be seen. That makes it slower and more involved than a thermal flyover. It also shows the cell-level condition but not the behavior of the whole array under load, so it does not replace thermal for connection and string issues.
Using both
A common approach for a commercial array is to use thermal as the broad screen and EL as the closer look.
Thermal first
Scan the whole array and identify the locations with heat-related findings. Pair the thermal images with visible images so each finding is located.
EL where the question is cell damage
Use EL on modules of interest: those flagged by thermal, those from a questionable lot, modules after a hail or handling event, or a sample for a warranty discussion. The result tells you whether the cells are cracked and how extensive the damage is.
Visual for the surface
Add visual inspection for soiling, physical damage and mounting problems. The visual inspection checklist shows what to record.
How to decide
- Need a fast screen of a whole array: thermal.
- Suspect cracks after an event or during handover: EL.
- Production dropped and the cause is unknown: thermal first, then EL on suspect modules.
- Planning a warranty claim: ask the manufacturer which evidence they accept, and use that method.
For the scheduling side of the thermal program, see how often a commercial array should get an infrared inspection.
What to bring to the conversation
Whichever method you choose, the provider will do better work with a few documents: the as-built layout with module positions, the module make and model, monitoring data showing where output dropped, and a description of any events such as storms or work on the array. With those, the choice between thermal, EL or both becomes a short conversation instead of a guess, and the findings can be tied back to the exact modules involved.
Where Solar Survey AI fits
Our electroluminescence (EL) inspection service drives current through the array so the cells emit infrared, making microcracks and inactive cell regions visible that visual and thermal inspection can miss. Combined with thermal and visual work, it gives a complete picture of the array. Tell us what you are trying to settle and we will recommend the combination.