This article provides an in-depth explanation of the different visual causes of the thermal anomalies in your Solar Thermography and Solar Thermography Pro data product.

If you are looking for more information on the Solar Thermography Pro or the Solar Thermography data product, please check the support article here.

This cause is used when a clear hotspot or heated area is visible in the thermal data, but no corresponding issue can be identified in the RGB imagery. In such cases, the anomaly is attributed to a physical internal issue.

This cause is commonly associated with anomalies such as hotspots or bypass diode-related heating, where the most likely explanation is an internal defect within the PV module (e.g. cell, interconnection, or internal component failure). These issues cannot be visually confirmed from external imagery and typically require further investigation or corrective action.
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Soiling occurs when dirt or debris accumulates on the surface of a PV module and is often visible in the RGB imagery. This condition commonly corresponds with a hotspot or multiple hotspots in the thermal data.
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In some cases, soiling may not be clearly visible in the RGB imagery. However, it can still be identified based on the characteristic hotspot patterns observed in the thermal data, typically concentrated along the lower edge of the module. Due to the inclination of PV modules, dirt tends to settle and accumulate in the bottom sections, leading to localized heating.
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Soiling is generally a non-permanent issue and can be resolved through module cleaning.
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The following examples show soiling build-up concentrated in the lower corners of PV modules.
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This cause is used when vegetation is visible in the RGB data at the same location as a hotspot identified in the thermal data. In such cases, the anomaly is attributed to vegetation obstructing part of the PV module and causing localized heating.
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This classification applies specifically to small or localized obstructions (e.g. grass, weeds, or low plants) in direct contact with or very close to the module surface. Larger obstructions that cast broader shadows, such as trees, are not classified as vegetation and should instead be categorized as shadowing.
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Shadowing occurs when an object casts a shadow onto a PV module, partially obstructing incoming irradiance. In thermal data, this condition may appear as a hotspot, multiple hotspots, heated substrings, entire modules, or even affected strings.
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It is important to note that shadowing can be temporary and dependent on the time of day, sun position, and surrounding objects. As a result, the analysis reflects the conditions present at the moment the images were captured.
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In the following real examples, both a hotspot and a bypassed substring can be identified. This behavior occurs because bypass diodes within the PV module are designed to protect the module from damage caused by partial shading. When shading affects a substring, the bypass diode activates, causing the affected substring to be bypassed and resulting in a characteristic thermal signature.
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Self-shading occurs when one row of PV modules casts a shadow onto an adjacent row. Unlike general shadowing caused by external objects, this type of shading is created by the PV array layout itself.
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Self-shading is most commonly observed when thermography is performed at low sun angles, such as early morning, late afternoon or winter time. In RGB imagery, it typically appears as a narrow shadow along the edge of the affected module. In thermal data, the shaded area is visible as a cooler, linear feature corresponding to the shaded section of the module.
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The following example illustrates a thin edge shadow in the RGB image and its corresponding thermal signature.
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This cause is used when bird droppings are clearly visible on the module surface in the RGB imagery. In some cases, droppings are visible in both the RGB and thermal data, where they may correspond to localized heating.
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However, there are situations where droppings are visible in the RGB imagery but do not produce a distinct thermal signature. In such cases, the anomaly type classification is based solely on the thermal observation.
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Bird droppings are generally more persistent and more difficult to remove than typical soiling.
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The following examples show bird droppings on PV modules:
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The front glass of a PV module serves as the primary protective barrier against environmental elements such as rain, dust, sand, and hail. This glass can be damaged by external impacts, for example from maintenance or cleaning equipment operating between module rows.
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Broken or cracked glass increases the module’s susceptibility to environmental exposure, particularly water ingress. When the glass integrity is compromised, light transmission to the solar cells is reduced, and foreign elements such as water or dust may enter beneath the glass surface. This can lead to partial shading, reduced energy output, and, in some cases, internal corrosion of module components.
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An industry trend toward thinner protective glass has contributed to an increase in cracked glass incidents, ranging from severe fractures to fine hairline cracks. Smaller cracks often do not produce a detectable thermal signature and may therefore not be visible in thermography. For this reason, very small or fine cracks are identified using high-resolution RGB inspections, such as our Solar UHD Inspection Product.
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PV modules are designed to be air- and water-tight. To achieve this, the module components—including the front glass, solar cells, and backsheet—are laminated together under vacuum. If this lamination process is insufficient or defective, delamination may occur during operation. Delamination refers to the separation or detachment of these laminated layers.
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Delamination can allow moisture to penetrate the module or cause air bubbles to form within the laminated structure. Moisture ingress may lead to internal corrosion, which often appears as darker or discolored areas on the module surface. This degradation typically begins at the edges of the PV module and can progressively spread across the module.
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On crystalline silicon modules, delamination is often difficult to identify and may require high-resolution RGB imagery, as it does not always produce a detectable thermal signature in standard thermography products. On thin-film modules, delamination is generally easier to detect due to more visible surface and thermal characteristics.
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In some cases, birds may be present on PV modules during the flight. This is a temporary condition, as the birds will leave the modules after a short time. Such cases are classified as a multi-hotspot anomaly with the cause Birds (alive).
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We report this anomaly to document the presence of birds on site, as they can temporarily create hotspots and lead to short-term performance losses. Additionally, highlighting bird activity helps raise awareness of potential risks, such as soiling or physical damage caused by repeated bird presence. Since this is a temporary condition, these anomalies can easily be filtered out in reports if required.
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Very often, we spot a UFO on a PV module. No, not a flying one 😀, but an Unidentified Field Object. Examples include items such as notepads, gloves, or tools left behind after maintenance activities on site.
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These objects obstruct irradiance and can cause localized heating, which appears as a hotspot in the thermal data. Such cases are classified as a hotspot anomaly with a removable object cause, as the issue can be resolved by simply removing the object from the module surface.
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This cause is used when an anomaly is detected in the thermal data and a visible feature is present on the PV module in the RGB imagery, but the exact cause cannot be confidently identified.
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In such cases, there is a visible element on the module surface that correlates with the thermal anomaly, yet it does not clearly match any defined anomaly cause category. To avoid incorrect classification, the cause is labeled as Visible Unknown.
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