An infrared thermometer reads the radiation of the surface it looks at, not the product, and it assumes an emissivity when turning that radiation into a temperature. Emissivity is 0.90-0.97 for plastics, 0.93 for paper and about 0.97 for ice, but only about 0.04 for aluminium foil. On shiny packaging the device therefore sees the reflection of its surroundings rather than the heat of the product.
Even a correct surface reading does not show the inside of the product: the outside of a stretch-wrapped pallet, a surface that cooled after the lid was opened, and frosted packaging all drift from the true state. The measured area grows with distance; when a small item is measured from afar, the shelf and air behind it enter the average.
What the acceptance decision rests on. ATP 2024 (Annex 2, Appendix 2) describes temperature measurement with a contact probe: a flat-head probe between packs and, if needed, a sharp-stemmed probe in the centre of the product. A between-pack measurement may deviate from the true product temperature by up to 2 °C.
| Purpose | Method |
|---|---|
| Finding a suspect spot | Infrared scan |
| Acceptance decision | Probe, between packs or inside the product |
Write the measurement the decision rests on into the record, with method, point and device. Scan values can be added as notes.
