Infrared thermography is a powerful non-contact temperature measurement technique widely used in industrial condition monitoring, electrical inspections, mechanical diagnostics, building energy audits, and research applications. The foundation of thermography lies in the understanding of radiative heat transfer, particularly the way infrared radiation interacts with surfaces. In thermographic inspections, the surface being observed by the thermal camera is known as the target surface and the radiation leaving or arriving at this surface determines the temperature measurement obtained by the instrument.
To fully understand thermography and interpret thermal images correctly, it is essential to understand how infrared radiation is emitted, reflected, transmitted and absorbed at the target surface. These interactions govern the energy balance at the surface and influence the accuracy of temperature measurements.
Infrared Radiation and Non-Contact Temperature Measurement
All objects with a temperature above absolute zero emit electromagnetic radiation. A portion of this radiation falls within the infrared region of the electromagnetic spectrum, which can be detected by infrared cameras. Thermal imaging devices measure this radiation and convert it into temperature values.
Unlike traditional contact thermometers, thermographic systems do not need to touch the object being measured. Instead, they rely on the infrared energy emitted by the surface of the object. Because of this, the quality of thermographic measurements depends strongly on the characteristics of the surface from which the radiation originates.
The measurement of infrared radiation is therefore the basis of non-contact temperature measurement used in thermography.
The Target Surface in Thermography
In thermographic analysis, the target surface refers to the surface of the object that is being evaluated by the thermal camera. This surface acts as the interface where infrared radiation leaves the object and travels toward the detector.
However, the radiation detected by the thermal camera is not always purely emitted radiation from the object itself. Instead, the radiation leaving the surface can consist of three different components:
- Emitted Radiation
- Reflected Radiation
- Transmitted Radiation
Understanding these three components is critical for interpreting thermographic measurements accurately.
Radiosity or Exitance from a Surface
The total infrared radiation leaving a surface is known as radiosity or exitance.
Radiosity represents the total radiant energy emitted, reflected and transmitted by the surface. In thermography, radiosity is important because the thermal camera receives this combined radiation rather than just the radiation emitted by the object.
The total radiosity is the sum of three components:
- Emitted component (Wₑ) – radiation generated by the surface due to its temperature
- Reflected component (Wᵣ) – radiation from surrounding objects reflected by the surface
- Transmitted component (Wₜ) – radiation passing through the material from behind
The relationship between these components can be expressed as:
- Total Radiosity = Wₑ + Wᵣ + Wₜ
Among these components, only the emitted radiation is directly related to the temperature of the object.
Emitted Radiation and Surface Temperature
The emitted component (Wₑ) is the radiation produced by the object itself due to its temperature. According to the principles of thermal radiation, the intensity of emitted radiation increases with temperature.
In thermography, the surface temperature of the object is determined from the emitted radiation component. However, if reflected or transmitted radiation contributes significantly to the measured signal, it may lead to incorrect temperature readings.
This is why thermographers must consider factors such as:
- Surface emissivity
- Reflected temperature from surroundings
- Atmospheric effects
- Material transparency
Proper correction of these factors ensures accurate temperature measurement.
Interaction of Incoming Radiation with a Surface
When thermal radiation strikes a surface, several processes can occur. The incident radiation may be:
Absorbed
Reflected
Transmitted
These interactions determine how much energy remains within the object and how much leaves the surface.
- Absorption increases the internal energy of the material.
- Reflection redirects radiation away from the surface.
- Transmission allows radiation to pass through the material.
These interactions form the basis of energy balance at the surface.
Kirchhoff’s Law of Thermal Radiation
A fundamental principle governing radiation exchange is Kirchhoff’s Law of Thermal Radiation. This law states that for a surface in thermal equilibrium, the ability of a material to emit radiation is equal to its ability to absorb radiation.
In terms of energy fractions, the sum of the absorbed, reflected and transmitted components of incoming radiation must equal the total incident radiation.
This relationship can be expressed mathematically as:
- E_t = E_alpha + E_rho + E_tau
Where:
- Eₜ = Total incident radiation
- Eα = Absorbed radiation
- Eρ = Reflected radiation
- Eτ = Transmitted radiation
The fractional sum of these components is equal to unity (1) or 100 percent.
- ε + ρ + τ = 1
Where typically in thermography and radiative heat transfer:
- ε (epsilon) = Emissivity
- ρ (rho) = Reflectivity
- τ (tau) = Transmissivity
This equation indicates that the sum of emissivity, reflectivity and transmissivity of a surface is equal to 1 (or 100%), which represents the conservation of radiant energy at a surface.
This means that all incoming radiation must be accounted for by one of these three processes.
Absorptivity, Reflectivity and Transmissivity
The three fractions in the equation correspond to fundamental material properties.
Absorptivity (α)
- This represents the fraction of incident radiation that is absorbed by the surface. Materials with high absorptivity tend to heat up more easily when exposed to radiation.
Reflectivity (ρ)
- Reflectivity represents the portion of radiation that is reflected from the surface. Highly reflective materials such as polished metals often cause measurement errors in thermography because they reflect surrounding radiation.
Transmissivity (τ)
- Transmissivity is the fraction of radiation that passes through the material. Many solids are opaque in the infrared range, meaning their transmissivity is nearly zero. However, some materials such as glass, plastics and thin films may transmit infrared radiation.
Practical Implications in Condition Monitoring
In industrial thermography, understanding radiation exchange at the target surface is critical when inspecting equipment such as:
- Electric motors
- Electrical panels
- Transformers
- Bearings
- Pumps and compressors
- Steam systems
- Furnaces and boilers
For example, polished metallic surfaces may reflect surrounding heat sources, producing false temperature readings. In such cases, thermographers often apply high-emissivity coatings such as electrical tape or paint to obtain more reliable measurements.
Similarly, when inspecting glass or plastic components, the possibility of infrared transmission must be considered.
Role in Advanced Thermographic Analysis
Advanced thermographic training emphasizes the importance of radiation physics. Professionals must understand how surface properties influence infrared measurements and how to compensate for these effects using proper camera settings and measurement techniques.
Modern thermal cameras allow users to adjust parameters such as:
- Emissivity
- Reflected temperature
- Atmospheric attenuation
- Distance to target
These adjustments improve measurement accuracy and help isolate the true temperature of the target surface.
