the sun goes down and the other sources of illumination are removed, there is
no light to be reflected and most mammals, especially, cannot see anything.The
unaided human eye cannot see infrared
radiation, but the radiation is always present. It is heat or thermal radiation
in a portion of the
Electromagnetic (EM) spectrum to which our eyes do not respond. Our bodies
respond to infrared, if it is intense enough, by feeling warmed, or sometimes,
illustration above, courtesy of
The University of Tennessee, depicts the EM spectrum ordered in terms of wavelength
given in centimeters. There are 100 centimeters in one meter and 10,000 microns
in one centimeter. (Most of the terminology used in infrared imaging discusses
wavelength in units of micrometers, also called microns). Also shown are the common
names given to the different portions of the spectrum.
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).
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.
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.
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.
familiar object in false color thermal image-Courtesy of NASA
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
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.
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.
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
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
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.
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