In 1970 commercial grade thermal camera arrived for market. These cameras were bulky with several components. They required liquid nitrogen to cool the system. Typical example for this type camera is AGA Thermovision 660 [2]. Within 10 years size of camera reduced considerably. Now uncooled micro bolometer based infrared cameras are available. They look like visible camera i.e. conventional digital camera.
As for as working of thermal camera is concerned, is very similar to visible cameras. Infrared camera optics is focused on an object-of-interest and its heat signals are captured by detector. The detector is read row-wise and outputs are got in the form of voltage. Then A-to-D converters map the voltages into numbers. As infrared is invisible they don’t have inherent colour. So, false colours used and they are mapped to the data range to produce a thermal image. This is shown in LCD screen that is present in the camera. These images can be stored in memory cards.
Dissimilarities
- First is pixel count. A low end camera will contain 4 million pixels as compared to high end thermal camera that may contain 3 lakh (0.3 million) pixels.
- A low end thermal cameras cost around $6000 US dollars [3] or 3 lakhs Indian rupees.
- Primary objective of thermal camera is to measure heat and record from every point of the object. Professionals are required to take a useful thermal image. They are expected to know the physics behind the thermal imaging and should have the ability to interpret the thermal images properly. Put in simpler terms it is a measurement tool. In contrast digital cameras are used to freeze important moments in life. So visible cameras are designed to be operated by everyone and shoot pictures with ease.
- A thermal image is always accompanied with visible picture. As thermal image is radiometric one, it should be always accompanied by a temperature scale (a bar that contains the colours used in the image along with corresponding temperature. This can be seen in Figure 2 thermal image). Thermal fusion images are optional but they are very informative. Please refer figure 1. For more images refer [1, 2].
- Infrared images use, false colour palettes like gray rainbow, iron.
Night vision camera and infrared thermometer are sounds similar to infrared camera. Night vision camera requires very minimum visible light (moon light) and near infrared to capture image. A novice will mistook output of night vision camera as from infrared camera. Figure 2 will help to understand the difference. Infrared thermometer will capture temperature of particular spot i.e. pixel resolution is one pixel only. Any low end infrared thermal cameras will posses 3600 pixel resolution.
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| Figure 2. Difference between visible, night vision and thermal infrared images |
Applications
Few important applications of thermal imaging are electrical installations, machineries, thermal insulation inspections in buildings as well as in heat conducting pipes and flare detection. Thermal images can be used in medical diagnostics also. It is mainly used to detect breast cancer tissues. Thermal images are used to visualize the level of injury in the body. Medical related thermal images will not be discussed in this post. Refer [4] for other areas applications like sports, veterinary, music and dance.
Thermal cameras are primarily used as non-invasive testing equipments. Non-invasive testing is very popular for two reasons. It can be used in preventive maintenance and is very economical. Today all the machineries present in factories run 24x7 and throughout the year almost without break to produce cost effective products. Wear and tear and associated failure is natural in any machinery. Problem of occurrence of failure is avoided with preventive maintenance.
Greatest beneficiary of thermal imaging is inspection of electrical systems. They are classified into high-voltage installations and low-voltage systems. First generation thermal cameras were primarily used to inspect high voltage power lines. Corrosion in electrical connections in high-voltage installations increases resistance. This causes rapid rise in temperature and subsequent meltdown or breakdown of connections. Outcome of this will be unplanned outages and fire hazard. Electrical short-circuit is a major cause of fire in buildings, factories and godowns. Fires destroy materials worth of billion dollars in a year.
It is a common knowledge, when a moving part in equipment is misaligned or malfunctioning then it will produce heat, later it manifest itself in excess vibration and in extreme condition it produces sound. As the first symptom of malfunction i.e. rise of temperature cannot be detected by conventional means, we rely on secondary symptoms like vibration or sound. For example, in mechanical systems like conveyor belts worn out of rollers can be easily detected using thermal imaging. Some of the problems like motor failures due to brush contact-wear and armature shorts produce excess heat only. They go undetected by vibration analysis.
Building inspection: Western countries reel under severe winter and to make life comfortable houses use central heating systems. If walls and windows of the building are properly insulated then heat will seep through them. At summer, places like in Middle East and India will reel under excess heat. To combat this centralized air-conditioning systems are used. If the house is not properly insulated then heat will seep through walls and reduce the efficiency of cooling system. Thus thermal images of house are used to find out insulation efficiency of house. Properly insulated houses require less energy to cool or warm them. Thus it leads to lot of savings. Same holds good for pipes that conduct steam and cold liquids.
Infrared camera fitted with protective filters can see through flames. This helps a lot for fire fighters to look beyond flames and take appropriate rescue operation. In certain industries like petroleum refineries harmful gasses are generated and they should be burned off in flares to avoid environmental degradation. Normally these gasses are invisible to naked eye. Thermal images of chimney will show whether gasses burned or not.
The last but not the least application is to teach physics. The interaction between walking person shoes with floor can be captured in thermal camera in video mode. The footage will help students to see the frictional force and visualize its effects. A basketball that falls on the floor will get deformed for brief moment and its temperature will rise. A high-speed thermal video footage will help student to see rather than to believe teachers’ lecture [4].
Note
o Ref. 1 contains 28 pages. It is very interesting to read and has good amount of illustrations to convey concepts.
o Please search for thermal image videos in youtube. I hope it will really help us.
Source
1. IR Thermography Primer, FLIR Systems Co. Ltd, http://www.termogram.cz/pdf/thermography_primer.pdf, (749 KB, PDF)
2. Thermal imaging guidebook for industrial applications, FLIR Systems AB, http://www.flirmedia.com/MMC/THG/Brochures/T820264/T820264_EN.pdf, (2014 KB, PDF)
3. B.R. Lyon Jr., R.J. Rogers, Techniques, “Tips and Tricks: Operating a Modern Radiometric Thermal Imaging Camera,” Presentation handout of ASNT 2007 Fall Conference, Las Vegas, USA, Nov. 14, 2007.
4. M. Vollmer and Klaus-Peter Möllmann, Infrared Thermal Imaging: Fundamentals, Research and Applications, Wiley-VCH Verlag GmbH & Co., Germany 2010.
