Learning Library · Thermal imaging · 1
Emitted vs. reflected heat
A thermal camera measures infrared radiation, not temperature. Every surface sends out infrared it emits because of its own temperature, plus infrared it reflects from everything around it. Emissivity decides the mix, and the camera has to be told it to turn radiation back into the right temperature.
A plate of the chosen material with a strip of matte black tape (emissivity 0.95) on it. Both are at the same true temperature. Colours show what the camera reports. The bar shows what the camera is actually seeing from the plate.
Emissivity in one sentence
Emissivity, from 0 to 1, is how much infrared a surface emits compared with a perfect emitter at the same temperature. Whatever a surface does not emit, it reflects, since an opaque surface transmits nothing.
A matte black surface at 0.95 is mostly telling you its own temperature. Polished aluminium at about 0.05 is mostly a mirror: the camera sees the room, the ceiling lights, or you, reflected in it.
| Surface | Typical emissivity |
|---|---|
| Human skin | 0.98 |
| Matte black electrical tape | 0.95 |
| Most paints, wood, plastics | 0.90 to 0.95 |
| Heavily oxidized steel | about 0.80 |
| Oxidized aluminium | 0.2 to 0.4 |
| Polished aluminium, bare copper | 0.02 to 0.10 |
Try this
- Start with oxidized aluminium at 80 °C and the camera set to 0.95. The tape reads correctly. The plate reads tens of degrees too cold, though both are at the same temperature.
- Set the camera to 0.30 to match the plate. The plate now reads 80 °C, and the tape reads too hot instead. One setting cannot be right for both.
- Pick polished aluminium and raise the surroundings temperature. The plate's reading follows the surroundings, not the plate.
Getting a trustworthy reading
- Measure on a high-emissivity spot: paint, a label, or a piece of matte black tape left long enough to reach the surface temperature.
- Set the camera's emissivity to the surface you are measuring, and set its reflected temperature to what the surface is reflecting, often room temperature.
- Change your viewing angle. A reflection moves when you move. Real heat stays put.
This model uses the total radiated power, which rises with the fourth power of absolute temperature. Real thermal cameras measure a band, typically 8 to 14 µm, so the exact numbers differ, but the behaviour is the same: low emissivity means a small, error-prone signal.
Set emissivity and reflected temperature on the camera. Professional thermal imagers.
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