A radiometric thermal camera does not measure temperature — it measures infrared radiation and calculates temperature from it. That calculation assumes an emissivity value, and when the assumed value is wrong, readings can be off by tens of degrees. Bare, shiny metal is the classic offender.
- 0 to 1The emissivity scale
- 0.90–0.98Painted, concrete, vegetation, skin
- <0.2Polished aluminium and steel
- 0.95A sensible default
Key takeaways
- Emissivity describes how efficiently a surface radiates heat. Painted surfaces, concrete, vegetation and skin sit around 0.90–0.98 and read reliably.
- Polished aluminium or stainless steel can sit below 0.2 — they emit little of their own heat and mirror their surroundings instead.
- A cool-looking bare busbar can be dangerously hot. Comparison between two different materials is not a fair comparison.
- Rely on relative deltas between identical components rather than absolute values on shiny parts.
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Emissivity in One Paragraph
Emissivity describes how efficiently a surface radiates heat, on a scale from 0 to 1. A perfect emitter radiates everything its temperature implies; a poor emitter radiates a fraction of it and reflects its surroundings instead.
Painted surfaces, concrete, vegetation and skin sit around 0.90–0.98 and read reliably. This covers most of what a thermal drone looks at — buildings, roofs, crops, people — which is why thermography works as well as it does most of the time.
Polished aluminium or stainless steel can sit below 0.2. They emit little of their own heat and mirror the sky and surroundings instead, which produces the counter-intuitive and dangerous result: a cool-looking bare busbar can be dangerously hot. The camera is not malfunctioning; it is reporting the radiation it received, and most of that radiation came from the sky.
Where It Bites in the Field
Electrical inspection is full of mixed surfaces: oxidised copper next to bright clamps, painted tanks with bare fittings, galvanised steel beside anodised aluminium. Each has a different emissivity, and a single emissivity setting cannot be correct for all of them simultaneously.
A hotspot comparison between two different materials is not a fair comparison. This is the practical trap — an inspector sees one component reading 20 °C hotter than its neighbour and reports a fault, when the difference is that one is painted and one is bare.
On substation surveys, inspectors compare like-for-like phases of the same component instead of trusting absolute numbers on shiny parts. Three phases of the same disconnect are the same material in the same conditions, so a difference between them is a real difference — substation workflow covers the practice.
| Surface | Approximate emissivity | Reads reliably? |
|---|---|---|
| Painted metal | 0.90–0.95 | Yes |
| Concrete and masonry | 0.92–0.95 | Yes |
| Vegetation, full canopy | 0.95–0.98 | Yes |
| Human skin | ~0.98 | Yes |
| Oxidised or weathered metal | 0.6–0.9 | With care |
| Galvanised steel | 0.2–0.3 | Poorly |
| Polished aluminium or stainless | Below 0.2 | No — compare peers only |
Practical Rules
Three rules cover almost every field situation.
Use radiometric mode with emissivity set for the dominant surface. A value of 0.95 is a sensible default for painted and non-metallic targets, and it will be approximately right for most of what fills the frame on a building, roof or vegetation survey.
Rely on relative deltas between identical components rather than absolute values. This single habit neutralises emissivity error, reflected temperature error, atmospheric attenuation and much of the effect of viewing angle in one step, because the reference experiences all of the same conditions.
Remember what the accuracy specification assumes. Figures such as ±2 °C or ±2%, and the ±3 °C or ±3% published for the LX-9A, are defined under controlled conditions — correct emissivity, known distance, stable ambient. Field conditions are not controlled, and the specification is a floor rather than a promise. See the radiometric explainer.
Comparison beats calibration. A perfect emissivity setting on a single component is harder to achieve than a fair comparison between two identical ones. When both are available, trust the comparison.
The Other Half: Reflected Temperature
Emissivity has a companion parameter that gets less attention and matters just as much on low-emissivity surfaces. If a surface only emits 20% of its own radiation, the other 80% of what the camera sees is reflected from its surroundings — and the camera needs to know what temperature those surroundings are to subtract them.
That is the reflected apparent temperature setting. Outdoors, the dominant reflection is often the sky, which in clear conditions is radiatively very cold — which is exactly why bare metal outdoors reads cooler than it is. On a cloudy day the same component reads differently, because the sky it is reflecting has changed.
The practical consequence: bare metal readings taken under different sky conditions are not comparable with each other, even on the same component on the same asset. This is one more reason relative comparison within a single frame is the robust approach — thermal reflections and false hot spots covers the related failure where the reflection is mistaken for the finding entirely.
Getting It Right in Practice
A workable field discipline that does not require a physics degree.
- Set emissivity for the dominant surface in the frame — 0.95 for painted and non-metallic targets.
- Identify the shiny components before you start, and mark them as comparison-only in your notes.
- Grade every finding against an identical neighbour under the same conditions.
- Record the sky state, because it changes what low-emissivity surfaces reflect.
- Capture radiometric so emissivity can be corrected afterwards if it matters.
- Verify critical findings at close range with a handheld before high-cost decisions.
The last two are the safety nets. Radiometric capture means an emissivity error is recoverable in post-processing — managing thermal drone data covers why that argues for capturing radiometric by default. Close-range verification is what the two-stage workflow exists for, and emissivity is the main reason it exists.
This material is also the core of formal thermography training, which is where the discipline becomes reflex rather than checklist — see thermography training and certification.
Related reading
- Radiometric Temperature Measurement on UAV Payloads
- Shutters, Blackbodies and Two-Point Correction
- Radiometric vs Non-Radiometric Thermal
- Thermal Reflections and False Hot Spots
- Seven Thermal Imaging Mistakes Operators Keep Making
- Substation Thermal Inspection by Drone
- From SD Card to Client Report
- Drone Thermography Training and Certification
- LX-9A — thermometry pod
- MV-2P — 640×512 thermal at 130 g
- OP-125A — thermometry optional
- Substation Drone Inspection
FAQ
Can I fix emissivity in post-processing?
Yes, if you recorded radiometric data — per-pixel temperatures can be recalculated with a corrected emissivity value afterwards. Plain video or rendered JPEG snapshots cannot, because the temperature information was never stored. This is one of the strongest arguments for capturing radiometric by default even when the live workflow does not need it, provided the payload is a thermometry configuration.
What emissivity should I set for solar panels?
Panel glass is a high-emissivity surface and behaves well, typically in the same 0.90–0.95 range as painted surfaces, so the default is close to correct. The larger error source on solar surveys is not emissivity but reflection — panels are deliberately smooth and will mirror the sky at oblique angles. Keep the viewing angle close to perpendicular and treat anything that moves with the aircraft as a reflection.
Why does bare metal often read cooler than it is?
Because a low-emissivity surface radiates little of its own heat and reflects its surroundings instead. Outdoors the dominant reflection is usually the sky, which is radiatively very cold in clear conditions, so the camera receives mostly cold sky radiation and calculates a low temperature. A hot bare busbar can genuinely appear cool — which is why shiny components are graded by comparison with identical neighbours rather than by absolute reading.
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