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About Thermography

General information about Thermal Infrared Imaging / Thermography


Infrared energy is part of the electromagnetic spectrum and behaves similarly to visible light. It travels through space at the speed of light and can be reflected, refracted, absorbed, and emitted. The wavelength of IR energy is about an order of magnitude longer than visible light, between 0.7 and 1000 µm (millionths of a meter). Other common forms of electromagnetic radiation include radio, ultraviolet, and x-ray.



Infrared Thermography is the technique for producing an image of invisible (to our eyes) infrared light emitted by objects due to their thermal condition. The most typical type of thermography camera resembles a typical camcorder and produces a live TV picture of heat radiation. More sophisticated cameras can actually measure the temperatures of any object or surface in the image and produce false color images that make interpretation of thermal patterns easier. An image produced by an infrared camera is called a thermogram or sometimes a thermograph.


Night vision goggles amplify small amounts of visible light (and sometimes near infrared light) thousands of times so objects can be seen at night. These only work if some light is present ie. moonlight or starlight. Thermal imaging works by detecting the heat energy being radiated by objects and requires absolutely no light. One advantage of thermography over night vision technologies is that night vision goggles can be easily blinded just by shining a flashlight at them. Since thermal imager only look at the heat they are totally unaffected by light sources.



All objects emit infrared radiation as a function of their temperature.  This means all objects emit infrared radiation.  Infrared energy is generated by the vibration and rotation of atoms and molecules.  The higher the temperature of an object, the more the motion and hence the more infrared energy is emitted. This is the energy detected by infrared cameras. The cameras do not see temperatures, they detect thermal radiation.

At absolute zero (-273.16°C, -459.67°F), material is at its lowest energy state so infrared radiation is at its lowest level.


We know that infrared radiation is a form of electromagnetic radiation, which is longer in wavelength than visible light.  Other types of electromagnetic radiation include x-rays, ultraviolet rays, radio waves, etc.  Electromagnetic radiation is categorized by wavelength or frequency.  Broadcast radio stations are identified by their frequency, usually in kilohertz (kHz) or megahertz (MHz).  Infrared detectors or systems are categorized by their wavelength.  The unit of measurement used is the micrometer, or micron, (mm, where m is the Greek letter mu) which is one millionth of a meter.  A system that can detect radiation in the 8 to 12 mm band we usually call “longwave.”  One that detects radiation between 3 to 5 mm is termed “shortwave.”  (A 3 to 5 mm system can also be classified as “midband,” because there are systems, which can detect radiation shorter than 3 mm.)  The visible part of the electromagnetic spectrum falls between 0.4 and 0.75 mm.  We can see colors because we can discriminate between different wavelengths.  If you have a laser pointer you may have noticed that the radiation is specified in nanometers; usually about 650nm.  If you examine a chart of the electromagnetic spectrum at 650nm (.65 mm) you will see that it is the radiation of red light.


Infrared-sensitive photographic emulsions can be used to study the distribution of objects that are hot enough to emit infrared energy just below red heat levels such as stoves, engine parts, high-pressure boilers, etc. The range of temperatures that can be recorded is from approximately 250°C to 500°C (482°F to 932°F). In comparison, electronic thermography can be used on objects with temperatures ranging from -40 °C to more than 1500 °C (-40 °F to > 2730 °F). So if you wanted to see useful images of your boat with infrared film, it would have to be on fire! If you want to capture and image of your boat in infrared under ambient, non-fire, conditions, infrared film will not work. You must use a thermal infrared camera.



If IR cameras don’t see temperature, what am I seeing on an IR image?  The IR camera captures the radiosity of the target it is viewing. Radiosity is defined as the infrared energy coming from a target modulated by the intervening atmosphere, and consists of emitted, reflected and sometimes transmitted IR energy. An opaque target has a transmittance of zero. The colors on an IR image vary due to variations in radiosity. The radiosity of an opaque target can vary due to the target temperature, target emissivity and reflected radiant energy variations. Thermographers see targets exhibiting this emissivity contrast behavior every day. It could be an insulated electric cable with a bare metal bolted connection. It could be a bare metal nameplate on a painted surface such as an oil filled circuit breaker or load tap changer. It could be a piece of electrical tape placed by the thermographer on  a bus bar to enable a decent reading. The list is long.