What is emissivity and why is it important?

What is emissivity and why is it important?

Emissivity is a measure of how effectively a surface can emit energy as thermal radiation. Surfaces can have an emissivity value ranging from 0 to 1.0 with 0 being a perfect reflector and 1.0 being a perfect black body radiator (a black body radiator is an object that absorbs all incoming radiation and does not reflect any, and then radiates out energy at the same rate that it receives it). As a general rule, as an object becomes more reflective, the emissivity value goes down. For example, shiny aluminium has an emissivity value of 0.05 whereas paint has a typical emissivity value of 0.95. In summary, emissivity can be thought of as the ratio of the effectiveness of the surface to emit thermal radiation compared to a perfect black body radiator. 

When using an infrared thermometer, it is important to know the approximate emissivity of the surface you are measuring. As a general rule, most infrared thermometers will be set to a default emissivity of 0.95. This means that the infrared thermometer is assuming that the surface it is measuring has an emissivity value of 0.95. For a lot of applications, this default reading will be suitable and you may not need to change the settings on your infrared thermometer. Indeed, some thermometers will not even allow you to make changes to the emissivity setting.

Below is a chart of some emissivity coefficients of commonly measured items.



Polished aluminium


Asbestos board


Black electrical tape


Glass/Frosted glass


White paper


Black paper


Black plastic


Clear plastic










Typical foods


As you can see, a majority of commonly measured items have emissivity values close to 0.95 which is why this is often set as the default. However to get the highest accuracy results, it is important to adjust the emissivity setting on your infrared thermometer to match what you are measuring.

What if you need to measure something like aluminium but you don’t have an adjustable infrared thermometer?

If you need to measure something with a low emissivity coefficient, then you can stick some black electrical tape on what you wish to measure and then point your infrared thermometer at the tape. The tape should have a fairly similar temperature to the surface it’s attached to but the advantage is that the tape has a high emissivity (0.95). 

If you would like to discuss this article further please feel free to contact one of our friendly Scientists via email or phone on 1300 737 871.

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