Emissivity, in one paragraph
Every surface radiates a fraction of the energy a perfect emitter would at the same temperature. That fraction is its emissivity. Matt paint, plaster, wood, plastic and skin are all close to 0.95. Polished aluminum, stainless steel, copper and foil can be below 0.1 — and instead of radiating, they reflect whatever is around them.
An infrared thermometer set to 0.95 and pointed at bright metal reads far too low, and nothing on the instrument warns you.
The workaround that always works
Stick a patch of matt insulating tape on the shiny surface. Give it a moment to reach the surface temperature, then measure the tape. Tape radiates like tape — close to 0.95 — so the reading is honest whatever the metal underneath is doing.
This works with every thermometer, including the fixed-emissivity ones.
The spot you actually measured
The distance-to-spot ratio tells you how big an area the reading averages. At 12:1 — which is every thermometer we compare — the spot is one twelfth of the distance:
- at 12 cm (4.7 in), a 1 cm (0.4 in) spot
- at 60 cm (23.6 in), a 5 cm (2 in) spot
- at 1.2 m (4 ft), a 10 cm (3.9 in) spot
Aim at a 2 cm (0.8 in) terminal from a meter away and you are measuring the terminal plus most of the enclosure, and reporting a number that belongs to neither.
Get closer. It is the single most effective thing you can do with these instruments.
Reading a difference rather than a value
Most useful infrared measurements are comparisons: this radiator against that one, this terminal against its neighbors, this wall against the one opposite. Comparisons survive an emissivity error because it applies equally to both, which absolute readings do not.
What gets asked here
What emissivity should I set?
Matt, non-metallic surfaces sit close to 0.95, which is why fixed-emissivity thermometers use that value. Bare and polished metals are far lower and reflect their surroundings — set the value if the instrument allows it, or measure a patch of matt tape instead.
What does 12:1 mean?
At twelve units of distance the measured spot is one unit across. At 1.2 m (4 ft) the reading is the average over roughly a 10 cm (3.9 in) circle, so anything smaller than that circle is diluted by whatever surrounds it.
Can I take someone's temperature with it?
No. These instruments measure surfaces. Devices intended for body temperature are a separate, regulated category and are outside what this site covers.
Last reviewed 31 August 2026
