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What is Emissivity?

Emissivity(ε) is an important technical index in the purchase of infrared thermometer.

First of all, we should know the principle of infrared thermometer: Every object has thermal radiation emitted outward. The optical system gathers the infrared energy radiated by the object, focuses it on the photodetector, converts it into electrical signal, and then into temperature signal value after calculation. Please refer to the following figure.

Figure 1: The principle of infrared thermometer

What is the definition of emissivity?

Emissivity is the ratio of the energy radiated by the surface of an object to that radiated by a black body of the same temperature. ( A black body is an idealized radiator that absorbs all wavelengths of radiant energy without energy reflection or transmission, and has an emissivity of 1 on its surface. )

Thermal Image
Figure 2: Thermal image

For most infrared thermometers, the rated emissivity of the measured material needs to be set. Most organic materials, coatings, or oxidized surfaces have an emissivity of about 0.95. Emissivity is usually set to this value.

By adjusting the emissivity of the thermometer, the problem of insufficient infrared radiation energy on the surface of some materials, especially metals, can be solved. So the emissivity of the thermometer should be adjusted to correspond to the emissivity of the object, so that the temperature measured is closest to the absolute value.

When purchasing a thermometer, it is best to choose one with adjustable emissivity. Most UNI-T’s professional infrared thermometers can adjust emissivity from 0.1 to 1.

UT301 Series emissivity setting interface
Figure 3: UT301+ Series emissivity setting interface

The principle of adjusting emissivity

Adjust the emissivity to the value of the measured object, and the internal amplifier will adjust the amplification accordingly, so that the actual radiation from an object with a certain temperature produces the same signal in the system as that from a black body with the same temperature. For example, if the emissivity of an object is 0.8, increase the magnification times to 1/0.8=1.25 times, which will make the measured value closer to the actual value.

How to determine emissivity?

Here are some common methods:

① Visual inspection: The stronger the object’s ability to reflect infrared radiation, the weaker its own infrared radiation ability. The visual method can generally estimate the reflectivity of an object.

The new copper has bright surface, high reflectivity and low emissivity (0.07~0.2). Copper oxide has low reflectivity and high emissivity (0.4~0.8).

② Refer to the table: The following table lists the emissivity of some metals and non-metals.

Figure 4: Emissivity of some metals and non-metals.

③ Paste a piece of black insulating tape (known emissivity) on the surface of the measured object, and then adjust the infrared thermometer emissivity, so that the temperature of the measured material surface and insulating tape surface temperature is the same or close. The emissivity at this time is the emissivity of the material to be measured.

④ Use a contact thermometer, such as thermocouple, thermal resistance, etc., to measure the surface temperature of an object, and then adjust the infrared thermometer emissivity, until the surface temperature measured by the infrared thermometer is the same or close to the surface temperature measured by the contact thermometer. The emissivity at this time is the emissivity of the material to be measured.

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