Introduction
Photovoltaic arrays can look static from a distance. At module scale they are live electrical systems operating under irradiance, voltage, and weather. Infrared thermography is one way to document apparent temperature differences that are not visible in ordinary photographs.
This article is about documenting those patterns with RGB and location context so a qualified reviewer can decide what deserves further inspection. It is not a diagnosis of failed panels, defective modules, electrical faults, or root cause. Reporting language for PV thermography is addressed in thermal reporting standards in solar PV inspections.
The purpose of this article is to describe apparent temperature patterns that commonly appear in drone-based photovoltaic records and to state what those records are for. They locate a difference and keep the visible context around it. Apparent temperature is the value the camera reports for a surface under the settings and conditions of that capture. Where the file is a radiometric thermal image, each pixel stores that value for later measurement. SterFlies records the pattern for qualified review.
Apparent temperature difference, not a color palette
Under load, current that cannot move evenly through a module can appear as heat. Infrared cameras record emitted radiation and display temperature variation across modules, strings, and nearby components. The useful quantity is usually the apparent temperature difference between an area and comparable nearby modules captured under the same conditions.
A warm pixel means little in isolation. Environmental variables matter. Low irradiance, wind, intermittent cloud cover, viewing angle, reflections, and camera settings all affect contrast. That is why IEC 62446-3 treats operating conditions as part of a valid PV thermography inspection, and why a report should state those conditions.
With those conditions written down, the record can be read for the patterns reviewers commonly ask to have located.
Patterns that commonly appear in PV thermal records
Localized warmer areas, repeating bands that follow substring layout, and broader warming across a group of modules are patterns that later reviewers often ask about. Those patterns can have several explanations, including soiling, shading, mismatch, connection resistance, or internal module stress. Documentation can locate the pattern. It cannot assign the cause.
Connection points such as connectors, junction boxes, and combiner locations can also show localized apparent temperature differences. Those observations are areas for further inspection. They are not, by themselves, proof of overheating, fire risk, or impending failure.
An orthomosaic and a site-wide thermal map show whether a pattern is isolated or repeats across a block. Repetition can focus where a reviewer looks next. Surface conditions, connections, and the module itself remain possible explanations. The map documents that distribution for qualified review.
Soiling and shading can look like electrical issues
Uneven soiling, debris, vegetation, and shading can create current mismatch that appears as elevated temperature. The corrective action, if any, is a qualified operational decision. The documentation task is to show where the pattern occurred and what the visible context looked like at the time.
Soiling and shade can reduce the current part of a module is able to pass, so cells that share the circuit operate under mismatch. The additional heat can appear as an apparent temperature difference. Vegetation, debris, or a shadow line from a nearby structure may be plain in the RGB photograph and absent from the thermal frame alone. The observation is the pair: the difference, and the visible condition at the same place and time.
Cleaning, vegetation work, or an electrical test can each be a reasonable next question for the same kind of contrast. Choosing among them is an operational decision for the people responsible for the plant. The thermal record shows where the pattern was recorded and what the module looked like then, and it holds that record for qualified review.
Why location context matters more than a single frame
Individual thermal frames are hard to use in the field. A geo-referenced thermal map, paired with RGB imagery, lets later reviewers see whether a pattern sits on one module, repeats across a string, or aligns with a known shading source. That is mapping and labeling, not diagnosis.
An absolute temperature reading can also mislead because ambient conditions shift the baseline. Comparing a module to its neighbors in the same operating block is usually more informative than quoting one number. Example values from a specific capture should stay attached to that capture. They are not universal thresholds.
For a technician walking the array back, the useful result is a module that can be found again. For an engineer or an attorney reading the file later, the useful result is the same comparison: a pattern attached to a place, a time, and the neighboring modules captured with it. A single frame, separated from that map, drops the pattern back into an unlabeled temperature.
Field examples are shown in the PV farm thermal documentation project and thermal case studies. Service scope is under thermal documentation. A shorter primer is Thermal Imaging 101.
Conclusion
Drone-based thermal documentation of a photovoltaic array records apparent temperature differences that ordinary photographs leave out, together with the RGB and location context a later reviewer needs. Localized warmer areas, bands that follow substring layout, broader warming across a group of modules, and localized differences at connectors and combiner locations are patterns to place on the plant and describe.
Soiling, shading, mismatch, and connection conditions can produce similar contrast under load. Neighbor comparison inside one operating period, with the conditions stated, is the comparison the record can support. A single apparent temperature, detached from neighboring modules and from the weather at the time, is a weak basis for that comparison.
IEC TS 62446-3 remains the photovoltaic-specific reference for treating operating conditions as part of the inspection. The deliverable described here is a located pattern held for qualified review. Module failure, an electrical fault, and root cause are determinations that review may reach. The capture records the pattern those determinations can be checked against.