About Thermal Imaging (Infrared Thermography)

The two principle properties of this radiation are:

1. All types travel at the speed of light in a vacuum, although they may differ in speed or not travel at all through or in some materials and,
2. Their physical interactions with various materials can be described mathematically in terms of transverse (electromagnetic) waves (TEM) or, in many cases as uncharged particles each particle having an energy of h*(nu), where (nu) is the frequency and h is a constant number known as Planck's constant (its value differs slightly with the unit system used, but in the International System of Units (Systeme Internationale or SI units) it is: 6.6260693 x E-34 Joule seconds (E-34 is 10 to the -34th power or 1/1,000,000,000,000,000,000,000,000,000,000,000).

Every object at temperatures above Absolute Zero ( 0 K or -273.15 °C) emits thermal radiation, much in the infrared portion of the EM spectrum. Many objects that are very hot emit thermal radiation that is in the visible and even the ultraviolet portion of the EM spectrum as well as the infrared, e.g. an incandescent light bulb or our local star that we call the Sun. See the hand-sketched graph below of thermal radiation intensity versus wavelength from several objects including the Sun, courtesy of an Ohio University web page; note the range of visible wavelengths. Note, too, that here the units of wavelength are in nanometers; 1000 nanometers = 1 micrometer (micron). That is a common variant found in physics courses.

What is invisible to humans, particularly when only thermal infrared is present, can be "seen" by a thermal imager, or more precisely, a thermal imaging camera, especially at night. It works in daylight, too, and one can easily see the surprising differences in appearance of any object from emitted thermal "light" to reflected visible light. The shape will be the same but the brightness distribution and shadows look very different even in black and white and more pronounced when viewed in false colors.

The Snell Infrared , The Infrared Training Center and The Infraspection Institute Web sites and many of the sites of the makers and distributors of infrared thermal imaging cameras have extensive sets of infrared images. Some of the images show comparative visible views as illustrations of the differences in appearance between infrared and normal images.

Another familiar object in false color thermal image-Courtesy of NASA

Thermal imaging devices provide the observer with instruments that can collect (just like a video or still camera) and convert the thermal infrared radiation emitted (and also reflected) by objects into images that can be seen on a view screen or computer display.

If viewed in a gray scale in a thermal imager, hotter things appear whiter, cooler things appear blacker, although that can be reversed in many devices. The imaging devices often have adjustments that enable the user to set the level of sensitivity so that light grays and white occur at higher or lower temperatures, according to the temperature and optical properties of the obects of interest. Some things appear very different. Using a modern "Long Wave" or LW waveband instrument, the faces of people appear strange and unusually marked. If they wear glasses, the lenses appear black. That's because glass and most thick optical plastics cannot transmit infrared and both materials are poor reflectors of LW Thermal Radiation. If the image is colored by the electronics in the camera or computer to represent different "temperatures" the face will exhibit strange color or brightness temperature bands in seemingly random patterns.

Despite the differences in how things appear in the thermal infrared, or more correctly, because of the differences and what they signify, these devices have found many uses that well justify the relatively high prices of these cameras compared, say, with an ordinary video camera. For instance, in early 2004, a good quality video camera with reasonable bells & whistles is priced about US$500. A leading, low-priced LW-IR Thermal Imager calibrated in terms of temperature and having a 30 frame/second video output costs in the neighborhood of 25 to 30 times that without a zoom lens. A good IR zoom lens can cost more than the entire camera. Recently, a new, lower cost thermal imager came on the market from a company in the UK called IRISYS, Ltd. A picture of the new device that also uses a Compaq PDA computer as viewscreen is shown below. Its specifications are also less than the more expensive devices, but there may be a market niche for such a device.

Image courtesy IRISYS, Ltd.

Due to the historically high cost of entry into the thermography business, thermal images are widely available through "Thermography Services" organizations particularly in the areas of predictive and preventive maintenance. Such images are used to detect, for example, the presence of trapped water between layers of materials making up flat roofs on commercial and public buildings. There are many uses described on the web and our links will take you to some of them.

Stockton Infrared in North Carolina, USA is a good example of such an organization. Many willing engineers and entrapeneurs have been sufficiently convinced to make their own investment and begin a service company. The technique is such a successful method of Non-Destructive Testing (NDT) , that the American Society for Non Destructive Testing (ASNT) has recognised it formally and established levels of skills and training requirements, for practicioneers. Many private training companies teach the basics and more for equipment users. Many electric utility and large manufacturing companies have established departments to conduct planned and organized use in identifying potential breakdowns of production and test equipment before they occur.

More robust and compact cameras without cryogenic cooling requirements have been adapted to several continuous processes as a tool for use in controlling product temperatures or locating defects or undesired conditions. It is expected that the coupling of the IR cameras with accurate and rapid temperature measuring capabilities will replace spot IR thermometers and bring new control capabilities to many industrial processes.

Newer variations of the technology especially the technique known as Thermal Wave Imaging, have shown the remarkable ability to locate subsurface corrosion and other problems in aging aircraft structures with great precision and confidence.

Thermal Imaging practiced in the context of medical dignostics is also growing, but there it is known as "Thermology" to perhaps distinguish it from the practices not involving a person's health. However the use of Thermal Imaging in animal health areas seems to have not required a change in name for some clearly analogous measurements for horses and small animals.

Then, not to be left in the dark, so to speak, the fire rescue and law enforcement communities have found there are numerous ways in which the practice of their professions can be made safer and more effective with IR imaging equipment. The new, more capable and compact cameras seem to be almost ideal for these types of uses.