Every object around us whether it is a human body, a machine, a building wall, or even ice continuously emits energy. This energy is known as radiant energy and it exists primarily in the infrared spectrum for most everyday temperatures. Understanding how this energy behaves is the foundation of infrared thermography, a powerful non-contact technique used in industries such as electrical inspection, predictive maintenance, building diagnostics and research.
A key concept in thermography is that all objects above absolute zero (-273.15°C or 0 K) emit radiation. The characteristics of this radiation depend mainly on two factors:
- Temperature (T)
- Wavelength (λ)
The relationship between these two factors defines how energy is emitted, distributed and detected.
Radiant Energy and Temperature Relationship
As temperature increases, the amount of emitted radiation increases dramatically. At low temperatures, objects emit energy only in the infrared region, which is invisible to the human eye. However, as temperature rises:
- Objects begin to glow dull red
- Then bright red
- Then orange/yellow
- Finally white-hot
For example:
The Sun (~6000 K) emits radiation across visible and infrared wavelengths, appearing bright white.
A stove heating element (~800 K) glows red when hot but continues emitting infrared radiation even after the visible glow disappears.
The human body (~300 K) emits radiation purely in the infrared region, which is why thermal cameras can detect body heat.
Even when radiation is invisible, it can still be felt as heat, demonstrating that radiation does not need to be visible to exist.
Blackbody Concept in Thermography
To understand radiation behavior, scientists use the concept of a blackbody.
A blackbody is an ideal object that:
- Absorbs 100% of incoming radiation
- Emits the maximum possible radiation at a given temperature
Real-world objects are not perfect blackbodies, but many materials approximate this behavior closely.
Blackbody radiation curves help us visualize:
- How energy is distributed across wavelengths
- How peak emission shifts with temperature
- Why hotter objects emit more energy and at shorter wavelengths
This concept is crucial in thermography because it provides a reference model for comparing real materials.
Stefan–Boltzmann Law: Total Radiated Energy
One of the most important laws governing radiation is the Stefan–Boltzmann Law, which states that the total radiant energy emitted from a surface is proportional to the fourth power of its absolute temperature.
- W=σεT4
where,
- W (Radiant Flux)
- The total energy emitted per unit area (W/m²)
- σ (Stefan–Boltzmann Constant),
- ε (Emissivity) : A measure of how efficiently a surface emits radiation
- Blackbody → ε = 1, Real surfaces → ε < 1
- T (Temperature in Kelvin) : Absolute temperature of the object
The most critical takeaway from this law is:
- Radiated energy increases exponentially with temperature
If temperature doubles, emitted energy increases by 16 times (2⁴). This explains why:
- Hot machinery stands out clearly in thermographic images
- Small temperature changes at high temperatures produce large radiation differences
- Thermal cameras are highly sensitive to temperature variations
Wien’s Displacement Law: Peak Wavelength
Another fundamental principle is Wien’s Displacement Law, which explains how the wavelength of maximum emission shifts with temperature.
Where:
- λmax = peak wavelength (µm)
- b = 2897 µm·K
- T = temperature in Kelvin
Key Insight:
- As temperature increases, peak wavelength decreases
This means:
- Hot objects emit radiation at shorter wavelengths
- Cooler objects emit radiation at longer wavelengths
Practical Examples
The Sun, with an approximate surface temperature of 6000 Kelvin (K), is an excellent example of a very hot radiating body.
- Peak Wavelength: ~0.5 micrometers (µm)
- Region: Visible spectrum
At this temperature, the Sun emits radiation strongly in the visible range, which is why it appears bright to the human eye. The peak wavelength of 0.5 µm corresponds roughly to green-yellow light, where human vision is most sensitive.
Human Body: Moderate Temperature, Infrared Radiation
The human body operates at an average temperature of about 300 K (27°C).
- Peak Wavelength: ~9–10 µm
- Region: Infrared
Unlike the Sun, the human body emits radiation primarily in the infrared region, which is invisible to the human eye. However, this radiation can be detected using thermal imaging cameras.
Ice: Low Temperature, Longer Infrared Wavelength
Ice, at approximately **273 K (0°C)**, represents a relatively low-temperature object.
- Peak Wavelength: ~10 µm
- Region: Infrared
At this lower temperature, the peak wavelength shifts further into the long-wave infrared region.
Key Observation:
- The radiation emitted is weaker compared to hotter objects
- The wavelength is longer, meaning lower energy radiation
This is why cold objects like ice appear darker or cooler in thermal images.
