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What is Thermography in Engineering?


Thermography, including infrared thermography (IRT), thermal imaging, and thermal video, is a thermal imaging science used for multiple applications. The technique uses thermographic cameras to detect radiation in the long-infrared range of the electromagnetic spectrum (around 9,000–14,000 nanometers or 9–14 µm) and produces images of this thermal radiation, known as thermograms.

The blackbody radiation law states that all objects with a temperature above absolute zero emit infrared radiation, meaning that it is possible to ‘see’ with thermography without visible illumination by reading temperature variations. As the temperature of an object increases, so does the amount of radiation it emits, which means that thermography can read variations in temperature range.

When using a thermal imaging camera, warm objects show up against cooler backgrounds regardless of time of day, making thermography useful to the military and for surveillance cameras. Thermography also has lots of uses in the medical industry, including items such as infrared thermometers.

Thermographic Inspection

Thermographic inspection of a wind turbine blade sample performed at the TWI Technology Centre (Wales).

Thermography can be split into two distinct types, passive and active. Passive thermography uses the natural temperature of a part to create and image, while active thermography involves heating the surface of an object and then observing the heat decay as it cools. Active thermography shows flaws in the material by variations in the temperature decay rate. Various testing procedure in active thermography are deployed to bring energy to the specimen inspected: pulsed thermography, step heating, lock-in thermography and vibrothermography.

In engineering, thermography is used as a non-destructive testing (NDT) method in aerospace, defence and many other industries to detect flaws in structures. Able to inspect large areas, this NDT technique doesn’t require contact with the object being tested and can highlight defects including corrosion; cracking; delamination; disbonds; diffusivity; impact damage; moisture ingress; porosity; systemic wall thinning and voids. It can also be used to inspect electrical and mechanical equipment based on the fact that most malfunctioning components will show an increase in temperature, which can be read by the camera.

How does it Work?

Thermography involves the use of thermal imagers, which are sophisticated devices that measure the natural emission of infrared radiation from a heated object to produce a thermal picture or video. Modern infrared cameras are portable and easy to operate, meaning that they can be used for many different applications.

Objects emit electromagnetic energy when heated, with more energy being released as the temperature increases. This energy is emitted as a wave which travels at the speed of light.

The human eye responds to visible light in the range of 0.4 to 0.75 microns, but the majority of infrared temperature measurements range from 0.2 to 20 microns. A thermal imager is able to focus this energy on to a detector much like a regular camera, except it responds to the infrared radiation rather than visible light. This image is represented in different colours to convey the temperature information.

Because infrared energy is emitted, transmitted and reflected by an object, infrared cameras use algorithms to interpret the data to create an accurate interpretation of the operating temperature.

This is explained in the formula:

Incident Radiant Power = Emitted Radiant Power + Transmitted Radiant Power + Reflected Radiant Power

The incident radiant power is the profile as viewed through a thermal imaging camera, emitted radiant power is the energy that is intended to be measured, transmitted radiant power is that which passes through the object from a remote thermal source, and reflected radiant power is the energy that is reflected from the object from a remote thermal source. This is known as radiant heat exchange.


Thermographic inspection of a solar panel component (roll to roll (R2R) printed conducting Ag grid lines on PET substrate*) performed for the OLEDSOLAR collaborative project.

* Substrate supplied by VTT Technical Research Centre of Finland Ltd

The ability of an object to emit is known as emissivity and the absorption of radiation is called absorptivity. Specular surfaces, such as metallic ones, reflect infrared radiation. This means that thermal cameras will also pick up on energy from the surroundings and reflected by the specular surface. A camera will also need to account for energy that passes through and is transmitted by transparent objects. Emissivity controls take account of these reflections and transmissions to create an accurate thermogram.  

A thermal imager will also take account of environmental factors such as cooling from wind to build a thermogram that is usually displayed as a JPEG.

Case Study: OLEDSOLAR Project

TWI is part of the collaborative, EC-funded OLEDSOLAR project to develop innovative manufacturing processes and in-line monitoring techniques for solar panels. The project aims include improving the quality and yield of fabricated devices as well as improving the processing efficiency and sustainability. Automated processing software will control roll-to-roll and sheet-to-sheet manufacture, while new recycling strategies will reduce product waste costs. The process is monitored by sensors that provide quality control, inspection and functional testing.

TWI is using  dark lock-in thermography (DLIT) to run laboratory-based in-line inspection of OLEDs and solar cells. A square wave modulated voltage is applied to solar film samples, which are then imaged with an infrared camera. The camera records thermal images as an in-house designed relay circuit switches the power supply on and off according to the generated square wave. The camera also records the signal generator’s synchronised square wave reference signal. The DLIT system uses thermography to show hotspots to locate defective scratched solar panel prints.

You can find out more about the project and the novel use of thermography to monitor solar panel manufacture here

The OLEDSOLAR project received funding from the European Union's Horizon 2020 research and innovation programme under grant agreement No 820789


There are a number of advantages for thermography:

1. Monitoring Large Areas

Because thermography uses visual pictures, it is able to capture and compare temperatures over a large area.

2. Monitoring Moving Objects

The technique can also be used to catch moving tagets in real time.

3. Detection of Faults Before Failure

Thermography is able to detect deterioration of a component before it fails by picking up on higher temperature areas that are indicative of a problem.

4. Can be Conducted during Operation

Since thermography doesn’t require physical contact with a systems, inspections can be conducted under full operational conditions, resulting in no downtime and loss of production.

5. Ideal for Hard-to-Reach, Hazardous and Poorly Lit Areas

This non-destructive method can also be used to observe and measure inaccessible or hazardous areas, as well as being able to detect objects in dark areas.

6. Wide Variety of Applications

Thermography is usable for a wide range of applications, from monitoring pipes, shafts and other metal and plastic parts, through to military use and medical applications, such as in physiotherapy.


Despite the many advantages offered by thermography, there are still a few potential drawbacks:

1. Equipment can be Expensive

Although there are affordable options available, the highest quality cameras can be expensive. This is due to the cost of larger pixel arrays (1280 x 1024 as compared to between 40 x 40 and 160 x 120). Fewer pixels mean a reduced image quality, which can make it difficult to distinguish between targets in the same field of view. Less expensive cameras may also have a much lower refresh rate (5-15 Hz as compared to 180 Hz or more). Finally, there can be marked differences in irradiance measurements, with cheaper cameras not being able to effectively take account of emissivity, distance, ambient temperature and relative humidity, resulting in less accurate thermograms.

2. Difficulty of Interpretation

Items with erratic temperatures can be difficult to interpret, although this limitation is lessened with active thermal imaging.

3. Less Accurate than Contact Methods

Thermography is not as accurate as contact methods due to most cameras having a ±2% accuracy or worse in the measurement of temperature.

4. Limited Detection Capabilities

Thermographic methods and instruments are limited to directly detecting surface temperatures.


Thermographic inspection has an array of applications across industry, including engineering uses like plant condition monitoring, preventative or predictive maintenance and process monitoring. Thermography is a good method for maintaining electrical and mechanical systems, such as with the location of thermal leaks or the higher temperatures in overheated regions. It can also be used to inspect refractory lined structures and locate overheating joints and sections of power lines, which are a sign of impending failure.

Common engineering applications include:

Building Inspection

Thermography can be used to find faulty thermal insulation by locating heat leaks in order to improve the efficiency of heating and air conditioning units. An inspection of the building envelope can also find air leaks in window and doorframes. Thermography can also be used to find waterlogged sections of roofing where the membrane has admitted rainwater that then becomes trapped between the layers of the roof.

Plant Maintenance

Thermography can be used for plant maintenance procedures, collecting thermal images of relevant machine parts to detect potential faults for repair.

Electrical Wiring Maintenance

Electrical wiring uses physical connection between cables, connectors and mounting studs. High quality electrical connections involve low electrical resistance between the various parts. As the quality of a connection degrades, the amount of electrical power that dissipates increases, in a process called ohmic heating, which can be picked up as increased heat by a thermal imager.

Locating Energy Loss

Lost energy can be a drain on any facility, and thermal imaging can help eliminate this. For example, excessive steam consumption and defective steam traps that heat downstream condensate return piping can easily be located by an infrared imager. Thermography can also find other wasteful energy losses, such as defects in refractory blocks in kilns, boilers or furnaces.

Bridge and Paved Surface Inspection

Thermography can also be used to inspect concrete bridges and other paved surfaces, locating voids and delaminations between the layers of material. Any air or water in these interlaminar spaces affects thermal conductivity and can therefore be found with a thermal imager. This can even extend to finding hidden rust, cracking, blistering and other defects between paint layers.


Thermography has a wide range of applications across industry – from non-destructive testing to military night-fighting applications. As with many techniques, there are some drawbacks, but these are outweighed by the versatility and many advantages of this NDT method.

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