Ensuring Measurement Accuracy of Infrared Thermometers
A thorough understanding of infrared technology and its principles is essential for accurate temperature measurement. When an infrared thermometer takes a temperature reading, the infrared energy emitted by the measured object passes through the thermometer's optical system and is converted into an electrical signal by the detector. This signal is then displayed as a temperature reading. Several key factors determine accurate temperature measurement; the most important ones are emissivity, field of view, distance-to-spot ratio, and spot positioning.
Emissivity All objects reflect, transmit and emit energy. Only the emitted energy indicates the object's temperature. When an infrared thermometer measures surface temperature, the instrument receives all three forms of energy. Therefore, all infrared thermometers must be adjusted to read only the emitted energy. Measurement errors are usually caused by infrared energy reflected from other light sources.
Some infrared thermometers support adjustable emissivity. Emissivity values for various materials can be found in published emissivity tables. Other instruments have a fixed preset emissivity of 0.95. This value is suitable for surface temperature measurement of most organic materials, painted surfaces or oxidized surfaces. For other surfaces, compensation can be applied by attaching tape or applying flat black paint to the measured surface. Once the tape or paint reaches the same temperature as the base material, measuring the surface temperature of the tape or paint yields the true temperature of the object.
Distance-to-Spot Ratio The optical system of an infrared thermometer collects energy from a circular measuring spot and focuses it onto the detector. Optical resolution is defined as the ratio of the distance from the infrared thermometer to the object to the measured spot size (D:S). The higher the ratio, the better the resolution of the infrared thermometer and the smaller the measurable spot size. Laser aiming is only used to assist alignment with the measurement point. The latest advances in infrared optics include close-focus capability, which enables accurate measurement of small target areas and minimizes the influence of background temperature.
Field of View Ensure the target is larger than the measuring spot size of the infrared thermometer. The smaller the target, the closer the thermometer should be positioned. When high accuracy is critical, make sure the target is at least twice the size of the measuring spot.
