Temperature Measurement

Accuracy of infrared temperature measurement

March 2021 Temperature Measurement

Infrared (IR) thermometers have the ability to measure temperature without coming into physical contact with a particular product. This ability is based on the fact that every object emits radiant energy, and the intensity of this radiation is a function of that object’s temperature. The infrared thermometer simply measures the intensity of radiation, thereby measuring an object’s temperature. Over the years, IR thermometers improved in accuracy and speed and have become essential instruments in today’s fast moving production lines. To assure measurement, however, the following have to be kept in mind: spot ratio and field of view; environmental conditions; ambient temperature; and emissivity.

Spot ratio and field of view

The optical system of an infrared sensor collects energy from a circular measurement spot and focuses it on the detector. Optical resolution is defined by the ratio of the distance from the instrument to the object, compared to the size of the spot being measured: the larger the number, the better the instrument’s resolution and the smaller the spot size that can be measured from a greater distance. A recent innovation in infrared optics is the addition of a close focus feature, which provides accurate measurement of small target areas without including unwanted background temperatures.

Make sure that the target is larger than the spot size the unit is measuring. The smaller the target, the closer you should be to it. When accuracy is critical, make sure that the target is at least twice as large as the spot size.

Environmental conditions

Steam, dust, smoke, etc. can prevent accurate measurement by obstructing the unit’s optics. Noise, electromagnetic fields, or vibration are other conditions that should also be considered before installation begins. A protective housing, air purging and/or water cooling can protect the sensor and ensure accurate measurements.

Ambient temperature

If the thermometer is exposed to abrupt ambient temperature differences of 20°C or more, allow it to adjust to the new ambient temperature for at least 20 minutes. For excessive ambient temperatures, air-cooling and water-cooling options such as ThermoJackets are available.


Emissivity is the measure of an object’s ability to emit infrared energy. Emitted energy indicates the temperature of the object. Emissivity can have a value from 0 (shiny mirror) to 1,0 (blackbody).


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