Thermal Imaging – Theory and Applications

What is Thermal Imagery

We know that our eyes see reflected light, so it is easy for us to understand the principle of forming the image from Visual (daylight and night vision cameras). But if there is not enough light it is impossible for us or the camera to see. This is not the case in the thermal imagery domain. Thermal cameras measure temperature and emissivity of objects in the scene. In the thermal infrared technologies, most of the captured radiation is emitted from the observed objects, in contrast to visual and near infrared, where most of the radiation is reflected. Thus, knowing or assuming material and environmental properties, temperatures can be measured using a thermal camera (i.e., the camera is said to be radiometric).  But, let’s not forget: “Thermal cameras detect more than just heat though; they detect tiny differences in heat – as small as 0.01°C – and display them as shades of grey or with different colors.” [1]

Thermal image is different from visual camera image and cannot be treated as a grayscale visual image. In thermal infrared there are no shadows, and noise characteristics are different then in visual tracking. There are also no color patterns like in visual domain, but patterns come out from variations in material or temperature of objects.

The infrared wavelength band is usually divided into different sub-bands, according to their different properties: near infrared (NIR, wavelengths 0.7–1 µm), shortwave infrared (SWIR, 1–3 µm), midwave infrared (MWIR, 3–5 µm), and longwave infrared (LWIR, 7.5–12 µm). These bands are separated by regions where the atmospheric transmission is very low (i.e., the air is opaque) or where sensor technologies have their limits. LWIR, and sometimes MWIR, is commonly referred to as thermal infrared (TIR). TIR cameras should not be confused with NIR cameras that are dependent on illumination and in general behave in a similar way as visual cameras. Thermal cameras are either cooled or uncooled. Images are typically stored as 16 bits per pixel to allow a large dynamic range. Uncooled cameras give noisier images at a lower frame rate, but are smaller, silent, and less expensive. [2,3] 

Q&A

1. What is the biggest difference between a high and low cost thermal camera?

   The biggest difference is typically resolution. The higher the resolution, the better the picture clarity. This translates to a better picture at a greater distance as well, similar to the megapixels of a regular digital camera.

2. Can thermal imaging cameras see through objects?

   No. Thermal imaging cameras only detect heat; they will not “see” through solid objects, clothing, brick walls, etc. They see the heat coming off the surface of the object.

3. Is there a difference between night vision and thermal imaging?

    Yes. Night vision relies on at least a very low level of light (less than the human eye can detect) in order to amplify it so that it can produce a picture. Night vision will not work in complete darkness whereas thermal imaging will  

    because it only “sees” heat.

4. Can rain and heavy fog limit the range of thermal imaging cameras?

    Yes. Rain and heavy fog can severely limit the range of thermal imaging cameras because light scatters off of droplets of water.

[4]

Applications

Applications of  thermal vision are numerous, in civil as well as in military sector, but here we will focus on applications in civil sector that can be of help in every day life. So, this technology can be used to observe and analyze human activities from a distance in a noninvasive manner, for example. Traditional computer vision utilizes RGB cameras, but problems with this sensor include its light dependency. Thermal cameras operate independently of light and measure the radiated infrared waves representing the temperature of the scene. In order to showcase the possibilities, both indoor and outdoor scenarios applications which use thermal imaging only are presented.

Surveillance: People counting in urban environments

Human movement can be automatically registered and analyzed. For both real-time and long-term perspectives, this knowledge can be beneficial in relation to urban planning and for shopkeepers in the city. Information in real-time can be used for analyzing the current flow and occupancy of the city, while long-term analysis can reveal trends and patterns related to specific days, time or events in the city.

Security: Analyzing the use of sports arenas

The interest in analyzing and optimizing the use of public facilities in cities has a large variety of applications in both indoor and outdoor spaces. Here, the focus is on sports arenas, but other possible applications could be libraries, museums, shopping malls, etc. The aim is to estimate the occupancy of sports arenas in terms of the number of people and their positions in real time. Potential use of this information is both online booking systems, and post-processing of data for analyzing the general use of the facilities. For the purpose of analyzing the use of the facilities, we also try to estimate the type of sport observed based on people’s positions.

In indoor spaces, the temperature is often kept constant and cooler than the human temperature. Foreground segmentation can therefore be accomplished by automatic thresholding the image. In some cases, unwanted hot objects, such as hot water pipes and heaters, can appear in the scene. In these situations, background subtraction can be utilized.

Health and safety: Gas leaking location and event alert

Some public buildings of interest can be monitored with thermal cameras, while gas or water leakage can be discovered before a hazardous situation happens.

Localizing a suspected leak in a building can turn to be delicate, sometimes requiring stopping the operations, if not probe walls or floors. Whatever the mix of construction materials, thermal imaging can be the right answer: in most cases, a leakage translates into an abnormal temperature pattern. Thermal imaging is de facto a non-contact operation, increasing inspector safety, capable of visualizing fluid leakage as well as electrical dysfunction. Thermal imaging can of course also detect thermal bridges and, as such, is a key tool to generate property investigation report.

Water leakage can be both hot and cold, and thermal imagers can catch them both. It can sometimes be close to impossible to spot a water leak on your own, especially when they are behind walls. That is why thermal cameras prevent dangerous situations.

Traffic control: Traffic monitoring and specific event alert

As for monitoring heterogenous traffic, thermal imaging can be a precious camera type reducing overall system costs and increasing reliability. On contrary to Visible and NIR-based detectors, LWIR cameras are not affected by the lighting conditions of the scene: e.g. night vs day, and sun orientation. This remains true over long distances, enabling the detection of a child, a biker, a car or a truck. Once coupled with relevant processing, LWIR cameras turn to be a key asset of ITS, reducing the number of cameras while increasing alarms reliability. This helps the manager on duty to take quickly the right decision in case of e.g. obstacle detection, reverse direction vehicle, abnormal traffic jam, etc. to ensure road-users security as well as optimal commuting time.

Energy saving: Building occupancy

Monitoring building occupancy turns to be highly relevant for management of commercial complex or public infrastructure: optimal adjustment of energy supply, scheduling of maintenance services, as well as comfort and health of occupants.
It is also useful for sizing security services, and of crucial importance in case of event requiring building evacuation. Advanced solutions, relying on thermal sensors, integrate thermal imaging: low resolution detectors (detecting presence / human activity) and/or a high-resolution thermal camera spotting relevant doorways (for people counting / human activity characterization).

This time, our goal was to explain more the science behind thermal cameras and its applications. If there are some additional questions or anything else you would like to know about this topic, feel free to ask via mail or comments.