If a thermal camera shows a temperature that seems wrong, emissivity is often the reason. Understanding it is the biggest single step toward reliable readings.
What emissivity is
Emissivity measures how efficiently a surface gives off infrared radiation, on a scale from 0 to 1. A surface with high emissivity radiates heat well, so the camera reads it accurately. A surface with low emissivity radiates poorly and reflects heat from its surroundings, so the camera can be badly misled.
Typical values
- Most painted, organic and building surfaces (wood, brick, drywall, skin, insulation): around 0.90 to 0.95.
- Polished or bare metals (copper, aluminium, stainless steel): can be as low as 0.05 to 0.2.
Many cameras default to around 0.95, which suits most materials but gives large errors on shiny metal.
Reflections
Low-emissivity surfaces act partly like mirrors. A bright reflection of a heater, lamp or even your own body can show as a hot spot that is not really there. If a spot moves when you change your viewing angle, it is likely a reflection.
How to get better readings
- Check the emissivity setting and match it to the material if your camera allows.
- Use a high-emissivity patch. A piece of matte black electrical tape or a dab of matte paint on shiny metal gives a surface you can measure accurately. Let it reach the same temperature as the metal first.
- Change your angle and re-check to rule out reflections.
- Get close enough that the target fills several pixels.
- Avoid glass. It reflects thermal radiation rather than passing it.
Accuracy specs
Camera accuracy is often quoted as a few degrees or a few percent of the reading, under stated conditions. For finding problems, relative differences between similar items matter more than absolute numbers.
For how this affects electrical work, see electrical inspections. Compare camera specs in the buyer's guide.