Infrared thermometers: what they measure and when to trust them
An IR thermometer measures radiance, not temperature. Emissivity, distance and reflected background all move the reading, which makes it a screening tool.
Infrared thermometers infer surface temperature from thermal radiation. Every surface radiates according to its temperature and emissivity; the instrument assumes an emissivity (often 0.95) and calculates a temperature. Painted, matte or organic surfaces are near 0.95; bare metals may be 0.1–0.3, and the reading is then dominated by reflected surroundings.
Use in storage and distribution
- Screening a pallet's surface on receipt: fast, non-contact, indicative only.
- Not a substitute for a logger inside the load: surface temperature differs from core.
- Not suitable for air temperature at all.
- Calibration: against a blackbody source or flat-plate calibrator, with the emissivity set as in use.
Three things that change the reading before temperature does
An infrared thermometer measures radiance and converts it to a temperature using assumptions. Each assumption is a way to be wrong by several degrees while the instrument itself is working perfectly.
| Source | Typical effect | Worked |
|---|---|---|
| Emissivity assumption | The dominant term for shiny surfaces | A surface at 5 °C with ε = 0.6 read as ε = 0.95 in a 22 °C room can read several degrees high |
| Distance-to-spot ratio | Averages in whatever else is in view | At 12:1, a reading from 3 m averages a 250 mm circle — likely including the shelf behind the box |
| Reflected background | Adds the room to the target | Worst on polished metal and foil packaging, which reflect more than they emit |
| Instrument at a different temperature from the room | Internal reference not settled | Carry it in from a cold van and allow 20–30 min before trusting it |
Emissivity is the one that catches people. Cardboard and matt plastic are around 0.9 to 0.95 and behave well. Bare metal, foil wrap and shiny film can be 0.1 to 0.3, and on those surfaces an IR thermometer is measuring the reflection of the room more than the object. A strip of matt tape on the surface, allowed to equilibrate, is the standard field fix.
Surface is not core, and neither is air
Even with every assumption right, the instrument reports the temperature of a surface. A pallet delivered from a cold vehicle into a 20 °C dock has a surface warming within minutes while the core is still at 5 °C; the reverse happens on a hot day. The surface reading is a lower bound on how long the load has been exposed and no more.
And it cannot measure air at all. Air is effectively transparent in the instrument's band, so pointing one into a room measures whatever wall it eventually hits. For GDP records the instrument that matters is a calibrated logger in the load.
Where it earns its place
- Screening on receipt: fast, non-contact, and good for spotting the pallet that is obviously wrong. Indicative only, and the record should say so.
- Finding a hot or cold surface during a mapping investigation — a warm wall, an uninsulated pipe, a failing door seal. This is the best use it has.
- Checking equipment surfaces where contact is impractical or unsafe.
- Not for GDP or GMP temperature records, not as a substitute for a logger inside a load, and not for air temperature under any circumstances.
- Calibrate against a blackbody source of known emissivity, not by comparing it against a contact thermometer on a random surface. The second method calibrates the surface, not the instrument.
Frequently asked questions
- Can an IR thermometer verify a cold room is at 2–8 °C?
- No. It reads surface temperature of whatever it is pointed at, affected by emissivity and reflection; a calibrated air sensor is required for the space.
References
- [1]ISO/IEC 17025:2017 — General requirements for the competence of testing and calibration laboratories
General technical guidance written against the cited sources. It is not regulatory or legal advice and does not replace the applicable standard, guideline or a qualified reviewer's judgement.
Have a question on this topic?
Ask ValiTrac AI and see the evidence and calculation behind the answer.
Ask ValiTrac AIFree calculator: Pt100 converter
Convert between resistance and temperature for Pt100 / Pt1000 sensors, with class AA, A, B and C tolerances at that temperature.
Open the calculatorCalibration on ValiTrac
Support calibration workflows, uncertainty budgets and certificate generation.
Open the calibration worksheetRelated articles
Selecting a reference thermometer
Pick a reference for stability first and certificate uncertainty second: one that drifts between its own calibrations is worse than a larger honest number.
Ten VIM terms every calibration user should know
Measurand, indication, error, correction, accuracy, precision, trueness, resolution, drift and traceability, as the international vocabulary defines them.