Every object above absolute zero emits infrared (thermal) radiation, and the hotter it is, the more it emits. A thermal imaging camera contains a detector (a microbolometer) sensitive to long-wave infrared; it measures the radiation arriving from each point in the scene and, applying some physics and a few corrections, converts that into a map of surface temperature displayed as a false-colour image. Crucially, it measures surface temperature only — it cannot see inside or through a wall.
Emissivity — the number that makes or breaks a reading
The radiation a surface emits depends not only on its temperature but on its emissivity (ε) — how efficiently that material radiates compared to a perfect 'black body' (ε = 1.0). Most ordinary building surfaces — plaster, brick, render, painted wood, paper — are good emitters, with ε around 0.90–0.95, which is why thermography works well on buildings. But low-emissivity surfaces wreck the measurement:
| Surface | Approx. emissivity (ε) | Thermography reliability |
|---|---|---|
| Plaster, brick, render, paint | 0.90–0.95 | Reliable |
| Wood, paper, most matt finishes | 0.90–0.95 | Reliable |
| Glass | ~0.85–0.90 (but reflective at angles) | Usable with care |
| Bare/shiny metal, foil | 0.05–0.30 | Unreliable — mostly reflects |
| Gloss / polished surfaces | Variable, often low | Unreliable |
Reflected apparent temperature
Because no real surface is a perfect black body, some of the radiation the camera receives is reflected from other objects, not emitted by the target. To measure accurately, the surveyor sets the camera's reflected apparent temperature (the temperature of the surroundings being reflected) as well as the emissivity. On a typical interior survey of high-emissivity surfaces the correction is small, but on cooler or more reflective surfaces it matters — and getting it wrong shifts the measured temperatures.
Spatial vs thermal resolution
Two different 'resolutions' govern what a thermal camera can reveal:
- Spatial resolution (detector pixels, e.g. 160×120 up to 640×480) — how much fine detail the image shows. Too few pixels and a narrow thermal bridge or a small leak averages out with its warmer surroundings and disappears. Higher resolution resolves smaller features and lets you survey larger areas from a usable distance.
- Thermal resolution / sensitivity (NETD, measured in milli-kelvin) — the smallest temperature difference the camera can distinguish. A sensitive camera (low NETD) reveals subtle thermal patterns; a poor one only shows gross differences.
A cheap clip-on phone camera with low spatial and thermal resolution will show only the most obvious patterns and miss the subtle, important ones — which is why building thermography needs a proper radiometric instrument, not a gadget.
Qualitative vs quantitative thermography
There are two levels of survey, and conflating them is a common error:
- Qualitative — reading the pattern. Comparing relative temperatures across a surface to spot anomalies (a cold streak, a warm patch). Most building-defect surveys are primarily qualitative: the pattern tells you where the problem is.
- Quantitative — measuring actual temperatures. Reporting real surface temperatures (e.g. to compare against the dew point, or to calculate an fRsi temperature factor) requires correct emissivity, reflected-temperature compensation, known distance and a properly calibrated radiometric camera.