Determining the Emissivity Value

In the previous article, I tried to provide basic information about the use of thermal imaging (thermal cameras), which is the most commonly used method in predictive maintenance applications.

In this article, I will try to provide more details about the emissivity value, which is crucial for obtaining the most accurate measurements in thermal imaging devices.

The solution to the problem of inaccurate or variable temperature measurements, which those of us who actively use thermal cameras inevitably encounter, lies in determining the correct emissivity coefficient.

Accurate measurement with a contact thermometer.

First, we need to measure the surface temperature of the object using a contact thermometer.

Next, we must measure the surface temperature of the object using a thermal imaging device with a pre-set emissivity value.

The difference between the temperature readings obtained with a contact thermometer and a thermal imaging device is due to the emissivity being set too high.

By gradually decreasing the emissivity setting, you can adjust the measured temperature until it corresponds to the value obtained by contact measurement.

In this case, the set emissivity will correspond to the emissivity of the surface of the object being measured.

Accurate measurement using emissivity tape.

First, you need to apply a piece of emission tape to your measurement object.

After waiting a short time, you can measure the surface temperature of the area covered by the tape using your thermal imaging device with the emissivity value set for the adhesive tape.

This temperature will be your reference temperature.

Next, you need to adjust the emissivity setting until the thermal imaging device measures the same temperature in the unbanded area as the reference temperature just measured.

The emissivity you have set is the emissivity of the surface of the object being measured.

As an alternative to emission tape, you can coat the measurement object with a coating or paint of known emissivity.

Determining the temperature of reflected radiation.

After eliminating all potential sources of interference that could affect your measurement, the temperature of the reflected infrared radiation is the same as the ambient temperature.

You can measure the ambient temperature with an air thermometer and enter the RTC value accordingly in your thermal imager. However, if there are infrared radiation sources in the measurement environment, you must determine the temperature of the reflected radiation to obtain an accurate measurement result.

Measurement on Glass Surfaces

We can see through glass with our eyes, but glass is impermeable to infrared radiation. Therefore, a thermal imager only measures the surface temperature of the glass, not the temperature of the materials behind it.

However, we must remember that since glass is permeable to short-wave radiation such as solar radiation, sunlight entering through the window could heat the object being measured.

Glass is also a reflective material. Therefore, when taking measurements on glass, it is necessary to pay attention to mirror reflections.

Measurement on Metal Surfaces

Metals, especially those with shiny surfaces, reflect long-wave infrared radiation strongly.

They have very low emissivity and can become temperature-dependent at high temperatures.

Therefore, it is difficult to measure their temperature with a thermal imaging device.

Therefore, in addition to regulating emissivity, correctly adjusting the reflected heat is particularly important.

In addition, attention should be paid to advice given regarding reflection, such as in mirrors.

If the metals are painted, the measurement will not pose a problem because paint usually has high emissivity. However, you should still pay attention to the reflections of ambient radiation.

As you can see, using a thermal camera is based on very different criteria than using a regular camera.

Furthermore, this information becomes even more important if you also want to calculate the cost of the footage you've captured.

Source: Testo