A thermal imaging survey uses a long-wave infrared (LWIR) sensor to map the apparent temperature of every surface in its field of view. The output — a thermogram — shows hot and cold features in a false-colour image, revealing what the eye can’t: a slipped roof tile letting wind into a loft, a bypass-diode failure in a solar PV array, a glowing busbar joint in an 11 kV switchgear panel.
The technique only works under the right conditions, and the data is only as good as the person interpreting it. This guide explains the physics, the standards (Cat 1 vs Cat 2 thermographers, IEC 62446-3, BS EN 13187), and the applications that make commercial sense. If you need a survey delivered, see our thermal imaging survey service.
What a thermal sensor actually measures
All objects above absolute zero (-273 °C) emit infrared radiation. The amount and wavelength of that radiation depend on the object’s temperature and its emissivity — a material property that describes how efficiently it radiates compared to a perfect blackbody. Brick has an emissivity around 0.93; polished aluminium is around 0.05.
An LWIR sensor (operating in the 7.5–14 µm wavelength band) measures the radiation arriving at the detector and back-calculates a temperature. The calculation needs three inputs:
- Emissivity of the target surface — set per material on the camera or in post-processing
- Reflected temperature — what the target is reflecting from its surroundings (the sky, a nearby wall)
- Atmospheric transmittance — affected by humidity and distance
Without correcting for these, the absolute temperature reading can be wrong by several kelvin. A thermal survey is rarely about absolute temperature — it’s about the temperature difference (Δt) between adjacent points. A 14 K difference between two PV modules in identical irradiance is a clear anomaly, even if the absolute readings are slightly off.
The conditions a survey needs
Thermal surveys fail when conditions don’t allow the temperature difference to develop. Three rules:
- Stable, dry weather — rain, dew or moisture on surfaces equalises temperature and washes out the anomaly. Surfaces must be dry for at least 1–2 hours before survey.
- A driving thermal gradient — for buildings, an internal-to-external Δt of at least 10 K (typical heating-season morning). For PV arrays, an irradiance of at least 600 W/m² with the array under load. For substation work, the apparatus must be carrying load (a switched-off transformer shows nothing useful).
- Low wind — wind chills surfaces and disrupts the thermal signature. Standard limits are <5 m/s for building work, <8 m/s for PV array work.
A surveyor who turns up on a wet, windy August afternoon and submits a thermal report has not done a valid survey. Conditions are recorded in the report alongside every thermogram so the client can audit validity.
Cat 1 vs Cat 2 thermographers
Thermography qualifications follow BS EN ISO 18436-7 (the European condition-monitoring standard) and are issued by independent training bodies (ITC, the Infrared Training Center, is the dominant UK route).
- Category 1 — a 32-hour course covering camera operation, the basic physics, and how to capture a useful thermogram. Sufficient for routine inspection of straightforward targets where pass/fail criteria are well-established.
- Category 2 — a further 32-hour course covering quantitative interpretation, advanced equipment configuration, emissivity correction, and report writing to a recognised standard. Required for any survey where the output supports an engineering or insurance decision.
For PV array work, IEC 62446-3 requires the surveyor to hold Cat 2 ITC certification. For commercial building envelope work to BS EN 13187, Cat 2 is the recognised standard of care. A Cat 1 survey is not wrong — it’s appropriately scoped for simpler tasks — but the deliverable should declare the surveyor’s category alongside their name.
Every Angell Surveys thermal deliverable carries the surveyor’s Cat 2 ITC certification number on the report cover.
Five applications that justify the survey
1. Building envelope and heat loss
Mapping thermal bridges, missing or compressed insulation, air-leakage paths and moisture ingress in commercial roofs and façades. Performed in heating season with an internal-to-external Δt ≥10 K. The deliverable identifies anomalies, classifies them by severity, and provides plan-located thermograms with paired visible-light photos.
Cost-payback is usually clear when the heating bill exceeds £30k/year — a 5–10% reduction from targeted remediation pays the survey back in one heating season.
2. Solar PV array fault finding
Drone-mounted thermal of operating PV arrays under load. Common findings include bypass-diode failures (a single substring shows as a hot triangle on a module), full-module failures (a module reads ~14 K above its neighbours), and string-level fuse failures (a whole string reads warm). To IEC 62446-3, with the report including module IDs and recommended remediation per fault.
Standard for any PV array over ~1 MWp and an annual requirement on most O&M contracts. For ground-mounted PV at NSIP scale — see our 250 ha solar farm + BESS topographic case study for the pre-planning capture phase — thermal flights become a recurring O&M deliverable from year-one energisation onwards.
3. Substation and switchgear inspection
Thermal imaging of energised electrical apparatus to find loose connections, failing breakers, unbalanced loads, and oil-cooled transformers running hot. A hot joint is the leading indicator of a switchgear failure — and switchgear failure is high-consequence.
Surveys are typically done annually for HV substations, six-monthly for distribution boards in critical-process environments (data centres, hospitals, food production).
4. Roof leak diagnosis
Flat roofs (membrane, asphalt, single-ply) hold water in saturated insulation that you can’t see from above. After sunset on a clear evening, the saturated zones cool more slowly than the dry zones and show as bright warm patches on a drone thermal. A short flight identifies the wet-zone extent without core sampling.
Particularly useful before a re-roof tender — surveying before asking for prices means you can scope replacement to actual wet area rather than over-quote the whole roof.
5. District heating network leak detection
Buried pre-insulated district heating mains lose heat at faulty joints and corroded sections. A drone thermal flown along the route on a cold morning highlights the leak as a warm linear feature on the ground above the pipe. Used by district heating operators to triage repair priority across hundreds of metres of network in a single mobilisation.
What a thermogram tells you (and what it doesn’t)
A thermogram is a snapshot of apparent temperature, not a diagnosis. A trained Cat 2 thermographer reads it in the context of:
- The driving conditions at survey time (Δt, irradiance, wind, time since last rain)
- The expected thermal pattern of a healthy target (a healthy PV module’s edge-to-centre profile, a healthy roof’s uniform low-emissivity reflection, a healthy busbar’s symmetric joint temperature)
- The deviation from expected — quantified in K, located in plan, classified by likely cause
A raw thermogram with no interpretation is worth very little. What you’re paying for is the analysis layer that turns the image into an actionable remediation list.
Drone thermal vs handheld thermal
Both have a place:
| Factor | Drone (UAV) | Handheld |
|---|---|---|
| Coverage rate | Fast — 5 MWp PV in a day | Slow — limited to walkable area |
| Resolution | Limited by altitude (typ. 5–10 cm/pixel) | Excellent — millimetre-scale detail |
| Access | Roofs, masts, large arrays | Inside, switchgear, panel-level work |
| Permissions | CAA authorisation, OSC documentation | None beyond site induction |
| Cost per area | Low | Higher per m² but higher detail per find |
Most engagements use both: a drone overview to triage where to look, then handheld detail at the anomalies that warrant it.
Standards reference
- BS EN 13187 — Qualitative detection of thermal irregularities in building envelopes (the building survey standard)
- IEC 62446-3 — Outdoor infrared thermography of PV modules and plants
- BS EN ISO 9712 — General qualification of non-destructive testing personnel (the umbrella standard ITC sits under)
- BS EN ISO 18436-7 — Condition monitoring — thermography specific Cat 1 / Cat 2 syllabus
Asking for compliance with the relevant standard from the above list is a useful way to filter surveyors at procurement.
What to put in a brief
- The target type (envelope, PV array, substation, roof, district heating)
- The required standard (BS EN 13187, IEC 62446-3, etc.)
- The minimum thermographer category (Cat 2 for any decision-supporting work)
- The deliverable format (PDF report with thermograms paired to visible-light images, classified anomaly list, plan locations)
- Constraints — when the site is accessible, when it’ll be under load (for PV / electrical), expected weather window
Frequently asked questions
Can a thermal survey be done in summer? Yes, but only for targets where summer conditions create a usable Δt — PV arrays under irradiance, energised electrical apparatus, district heating networks. Building envelope work needs heating season (October–April in the UK).
Does the survey work through rain or fog? No. Water absorbs LWIR strongly. Surfaces must be dry and atmospheric humidity reasonable. Foggy mornings are a no-fly day.
How long is a typical PV array survey? A 5 MWp array takes about a day on site, weather permitting, with the report issued within 5 working days. Larger sites scale linearly.
What’s the difference between LWIR and shortwave thermal? LWIR (7.5–14 µm) sees emitted radiation from objects at terrestrial temperatures. Shortwave (3–5 µm) sees emitted radiation from hotter objects and reflected solar. For building, PV and substation work in the UK, LWIR is the right tool.
Do you provide a sample report? Yes — request one from the contact page and we’ll send a recent anonymised sample relevant to your application.
For drone and handheld thermal imaging surveys across the UK to BS EN 13187 and IEC 62446-3, see our thermal imaging survey service. All work is delivered by a Cat 2 ITC-certified thermographer with full reporting to the appropriate standard.