How IR Thermometers Measure Temperature

The advent of low-cost handheld infrared (IR) thermometers has led to a proliferation of non-contact temperature measurement in the food, building, and lowtemperature processing industries. However, these instruments are not as simple to use as they first appear due to systematic effects that are present in almost all measurements. This technical guide provides information relating to the calibration of “low-temperature” IR thermometers, which typically measure temperatures in the range –50 °C to 500 °C (see Figure 1). These thermometers commonly use uncooled thermopile detectors that detect radiation in the 8 μm to 14 μm spectral range (or similar). Because these detectors are uncooled, radiation emitted by the detector itself must be considered in the calibration process. The emissivity setting on the thermometer, which is often fixed at a value of 0.95, and any radiation reflected from the surroundings, must also be taken into account. As a consequence of these systematic effects, calibration methods are more complicated than for contact thermometers or high-temperature IR thermometers. The expected reading, even on a perfect IR thermometer, does not necessarily match the reading of the reference thermometer. Procedures for determining the required corrections are given in this guide.