Fundamentals of Thermal Imaging

Thermal imaging is considered one of the simplest yet most effective methods among predictive maintenance applications.

While thermal imaging is generally thought to be the simplest method, I would like to state that I disagree with this notion.

I believe you will understand the reason for this better after reading the series of articles I have prepared on this method.

In this article, I will try to provide basic information about thermal imaging, specifically the industry-known definition of "Thermal Camera Usage."

Every object with a temperature above absolute zero emits infrared radiation, and this radiation is invisible to the human eye.

In 1884, physicists Josef Stefan and Ludwig Boltzmann proved that there is a correlation between the temperature of an object and the intensity of the infrared radiation it emits.

Thermal imagers measure the long-wave infrared radiation they receive within their field of view and thus calculate the object's temperature.

This calculation takes into account the surface emissivity (ε) and the reflected temperature compensation (RTC) factor of the object. These values ​​can be adjusted manually on the device.

Each pixel of the detector represents a temperature point displayed on the screen.

Emission, Reflection and Transmission

Emissivity (ε)

ε varies depending on surface properties, the material, and for some materials, the temperature of the measurement object and the spectral range of the thermal imager used.

• Maximum emissivity: ε=1.

• Real objects: ε < 1, because real objects also reflect and possibly transmit radiation.

• Many non-metallic materials (PVC, concrete, etc.) have high emissivity in the long-wave infrared range, independent of temperature (ε = 0.8 – 0.95).

• Metals, especially those with shiny surfaces, have low emissivity that fluctuates with temperature.

Reflectivity (ρ)

This is a measure of a material's ability to reflect infrared radiation.

• ρ depends on surface properties, temperature, and material type.

• Smooth and polished surfaces reflect more strongly than rough and matte surfaces made of the same material.

• The temperature of the reflected radiation can be manually adjusted in the thermal imager (RTC).

• RTC corresponds to ambient temperature in many measurement applications, especially in indoor thermography.

Transmittance (τ)

Transmittance τ is a measure of a material's ability to transmit (pass) infrared radiation.

• τ depends on the type and thickness of the material.

• Most materials are not permeable, i.e., cannot transmit, long-wave infrared radiation.

  
  

The sum of these parts is always considered to be 1.

                              ε + ρ + τ = 1

Since conduction rarely plays a role in practice, conduction τ is neglected.

                              ε + ρ = 1

The lower the emissivity;

• The higher the reflected infrared radiation rate

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• The more difficult it is to measure the temperature accurately

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• The more important RTC - reflected temperature compensation - is.

Correlation between emission and reflection

1. Measuring objects with high emissivity (ε ≥ 0.8)

• They have a low reflectance rate ρ.

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• Their temperatures are measured very accurately with the device.

2. Measuring objects with medium emissivity (0.6 < ε < 0.8)

• They have a medium reflectance rate.

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• Their temperatures can be measured accurately with the device.

3. Measuring objects with low emissivity (ε ≤ 0.6)

• They have a high reflectance rate

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• Measurement is possible with the device, but the values ​​must be verified.

Ensuring that the emissivity setting is correct is especially important when there are large temperature differences between the object being measured and the measurement environment.

1. When the temperature of the measurement object is higher than the ambient temperature (consider the radiator):

• Excessively high emissivity settings will result in excessively low temperature readings (camera 2)

• Excessively low emissivity settings will result in excessively high temperature readings (camera 1)

2. When the temperature of the measurement object is lower than the ambient temperature (consider the door):

• Excessively high emissivity settings will result in excessively high temperature readings (camera 2)

• Excessively low emissivity settings will result in excessively low temperature readings (camera 1)

Important Notes

The greater the difference between the temperature of the measured object and the ambient temperature, and the lower the emissivity, the greater the measurement errors. These errors increase if the emissivity setting is incorrect.

For example, you want to determine the temperature of liquid flowing through a PVC pipe outdoors in winter. You need to determine the emissivity value very accurately.

Thermal imaging devices can only measure the temperatures of surfaces. You cannot look inside or through something.

When measuring electrical panels, especially those with high voltage, the polycarbonate protection must be removed.

Many materials that are transparent to the human eye, such as glass, are not permeable to long-wave infrared radiation.

If necessary, all coatings on the measurement object must be removed. Otherwise, the thermal imager will only measure the surface temperature of the covering.

Thermal imaging devices are considered for use in panel rooms for fire early warning purposes. However, this method is not suitable considering that panel covers are kept closed and the ambient temperature varies seasonally.

If elements located below the surface affect the temperature distribution of the measurement object's surface through conduction, the internal structure of the measurement object can usually be identified in the thermal image. However, a thermal imager only measures surface temperature. It is not possible to make a precise statement about the temperature values ​​of the elements inside the measured object.

Source: Testo