Electroluminescence vs. Thermography: Which Method for Which Defect?
Two inspection methods, two fundamentally different measurement principles – yet they are often confused or equated. Thermography and electroluminescence (EL) are not competitors but tools for different questions. Anyone who understands what each method can and cannot do makes better decisions: about the right moment, the right method and the question of when a combination of both makes sense.
Comparing the two measurement principles
Both methods inspect solar modules non-destructively – but they measure physically completely different quantities and require fundamentally different conditions:
Infrared measurement during live system operation under solar irradiance
Camera measurement with injected current – at night or in darkness
The decisive difference lies in the measurement principle: thermography measures heat that is generated by an electrical defect. This presupposes that the defect is actively effective – i.e. current loss is already occurring and converting into heat. EL measurement, by contrast, makes Zellstruktur selbst sichtbar, by injecting current into the modules and capturing the resulting light emission. Intact cells glow brightly, damaged cells or cracks appear as dark areas.
Welche Defekte erkennt welches Verfahren?
The following table shows the most important PV defects and their detectability with both methods. ✓ stands for reliable detection, ~ for limited detectability and ✗ for not detectable:
| Defekt | Thermografie | Elektrolumineszenz | Empfehlung |
|---|---|---|---|
| Hotspot (aktiv) | ✓ | ~ | Thermografie ausreichend |
| Bypass-Diode defekt | ✓ | ✓ | Thermografie ausreichend |
| PID-Degradation | ✓ | ✓ | Beide ergänzend sinnvoll |
| Substring-Ausfall | ✓ | ✓ | Thermografie ausreichend |
| Mikrorisse (aktiv, Zellausfall) | ~ | ✓ | EL bevorzugt, Thermo ergänzend |
| Mikrorisse (inaktiv, kein Verlust) | ✗ | ✓ | Nur EL erkennbar |
| Fingerunterbrechungen | ✗ | ✓ | Nur EL erkennbar |
| Inaktive Zellen | ~ | ✓ | EL bevorzugt |
| Delamination (fortgeschritten) | ✓ | ~ | Thermografie ausreichend |
| Anschlusskasten-Defekte | ✓ | ✗ | Nur Thermografie (IR-Handkamera) |
| Überhitzung Wechselrichter | ✓ | ✗ | Nur Thermografie |
| Verschmutzung/Beschattung | ✓ | ~ | Thermografie ausreichend |
Kernaussage: Thermography detects all defects that currently cause a measurable yield loss. EL measurement additionally detects structural damage that is not yet thermally visible – but will be in the future. Anyone wanting to assess the current condition needs thermography. Anyone additionally wanting to capture latent risks supplements with EL.
Wie die EL-Messung technisch funktioniert
The measurement principle of electroluminescence is based on the reciprocal property of solar cells: while a solar cell converts light into current in normal operation, it can conversely also convert current into light – when current is injected into it. This emitted light is in the near-infrared range (approx. 900–1,200 nm) and is not visible to the human eye.
For the measurement the PV system is shut down and a direct current is injected into the modules – typically the rated short-circuit current or a multiple thereof. A highly sensitive camera with a corresponding NIR sensor captures the emitted light. Intakte Zellen leuchten gleichmäßig hell, damaged areas appear dark or as characteristic patterns – cracks as dark lines, inactive areas as black surfaces, PID as a gradual darkening from the outside inward.
The main disadvantage: the measurement requires absolute darkness or at least very weak ambient light. Outdoor EL therefore depends on night measurements or requires special shielding. This makes its use more laborious and more expensive than thermography.
Entscheidungshilfe: Wann welche Methode?
Annual or biennial condition inspection of the system. Thermography captures all actively effective defects quickly and across the entire site – without operational interruption.
When the monitoring shows a yield drop thermography identifies the causes directly and localises them to the exact module.
For insurance settlements thermography is the recognised standard per VdS 2858 and IEC TS 62446-3.
After hailstorms or when transport damage is suspected EL also shows fresh cracks that are not yet thermally measurable.
At commissioning EL makes manufacturing defects and transport damage visible at cell level before they reduce performance.
For a system acquisition or for systems from 10 years of age: thermography for active losses, EL additionally for latent structural risks.
Kostenvergleich
EL measurement is generally more laborious than thermography since it must take place at night or in a dark chamber. Exact costs depend on system size, location and scope – contact us and we will advise you which method makes sense for your case and prepare an individual quote.
EL measurement is generally more expensive and logistically more demanding than thermography. It is not an economical replacement for the regular maintenance inspection but a valuable supplement for specific questions. The decision for a combination of both methods should always follow the specific occasion – and not the principle "more is more".
Wann eine Kombination sinnvoll ist
There are three situations in which we recommend a combination of thermography and EL measurement:
Nach Hagelschlag: Hailstones can cause microcracks that develop mechanically – often without immediate yield impact. Thermography shows which modules are currently producing losses; EL shows which modules have structural damage that will follow in the near future. For a complete damage assessment against the insurer the combination of both reports is the strongest basis.
Technical Due Diligence: When acquiring an existing system one wants to know not only what is currently costing yield but also what latent risks the modules contain. Thermography delivers the current losses; EL provides an outlook on future development – particularly relevant for systems from 10 years of operation.
Qualitätssicherung Neuanlage: After installation the combination gives the most complete guarantee of the delivered condition: thermography checks electrical integrity under operating load; EL documents the structural condition of the modules at cell level. Both together form the most solid baseline for subsequent comparative measurements.
Request thermography
For most systems thermography is the first and most important step. We prepare an individual quote for every system size on request.
Request now →Frequently asked questions
Thermography measures heat during operation and detects active defects (hotspots, PID, bypass diodes). EL measures light emission in the dark and shows structural cell damage such as microcracks. Thermography needs sun; EL needs darkness and current injection.
Advanced microcracks with cell failure are thermographically detectable. Early, inactive cracks without current loss are not thermally measurable – for these EL is the appropriate method.
A combination is recommended after hailstorms, for technical due diligence before a system acquisition and for quality assurance after new installation. For regular maintenance inspections thermography alone is sufficient in most cases.
Thermography is clearly standardised through IEC TS 62446-3 and VdS 2858 and recognised by insurers. EL measurement is not an independent outdoor standard but can be usable as a supplementary expert assessment.
Conclusion
Thermography and electroluminescence are not competitors – they answer different questions. Thermography is the standard method for ongoing maintenance, yield-loss diagnosis and the insurance case. It is standardised, drone-suitable and delivers a complete picture of all actively effective defects during normal operation.
EL measurement supplements where thermography reaches its physical limits: with inactive microcracks and early structural damage. It is more laborious but indispensable when latent risks need to be captured – particularly for system acquisitions or after mechanical impacts.
For most operators Thermografie is the right first step. In specific situations the combination is worthwhile. Contact us – we will advise you which method makes sense for your system and your occasion.