How to adjust the emissivity of an infrared thermometer

Infrared (IR) Radiation

Infrared radiation is ubiquitous and never-ending, and the greater the temperature difference between objects, the more obvious the radiation phenomenon. A vacuum can transmit the infrared radiation energy emitted by the sun through 93 million miles of space and time to the earth, where it is absorbed by us and warms us. When we stand in front of the food refrigerator in the mall, the infrared radiation heat emitted by our bodies is absorbed by the refrigerated food, making us feel very cool. In both cases, the radiation effect is very obvious, and we can clearly feel the change and feel its presence.

When we need to quantify the effect of infrared radiation, we need to measure the temperature of infrared radiation, and this is when we use an infrared thermometer. Different materials have different infrared radiation characteristics. Before using an infrared thermometer to read the temperature, we must first understand the basic principles of infrared radiation measurement and the infrared radiation characteristics of the specific material being measured.

Infrared emissivity = absorptivity + reflectivity + transmittance

No matter what kind of infrared radiation is emitted, it will be absorbed, so absorptivity = emissivity. The infrared thermometer reads the infrared radiation energy emitted by the surface of the object. The infrared radiometer cannot read the infrared radiation energy lost in the air. Therefore, in actual measurement work, we can ignore the transmittance. In this way, we get a basic infrared radiation measurement formula:

infrared radiation rate = emissivity - reflectivity.

The reflectivity is inversely proportional to the emissivity. The stronger the ability of an object to reflect infrared radiation, the weaker its own infrared radiation ability. Usually, the reflectivity of an object can be roughly judged by visual inspection. The reflectivity of new copper is higher and the emissivity is lower (0.07-0.2), the reflectivity of oxidized copper is lower and the emissivity is higher (0.6-0.7), and the reflectivity of copper that has turned black due to severe oxidation is even lower, while the emissivity is correspondingly higher (0.88). The emissivity of most painted surfaces is very high (0.9-0.95), while the reflectivity can be ignored.

For most infrared thermometers, the only thing that needs to be set is the rated emissivity of the material being measured. This value is usually preset to 0.95, which is sufficient for measuring organic materials or painted surfaces.

By adjusting the emissivity of the thermometer, the problem of insufficient infrared radiation energy on the surface of some materials can be compensated, especially metal materials. The effect of reflectivity on the measurement needs to be considered only when there is a high-temperature infrared radiation source near the surface of the object being measured and it reflects.

Appendix: Emissivity of some materials (for reference only)