Infrared Thermal Imaging: See The Unseen

       Infrared thermography also called infrared thermal imaging uses thermal imagers or Infrared imaging cameras. This technique is widely used in many application fields, such as analyzing component stress in PCB, surveillance affairs, monitoring body temperature etc. Infrared thermal imaging becomes effective method as monitoring tool since every matter radiates thermal energy. In bioengineering application, this method can be used to monitor the temperature distribution of human body in hyperthermia treatment. By monitoring, the researcher can control heat distribution to specific localization in disease area.

The Spectrum

       When 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, cooled.

 

       The illustration above 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 IR or infrared portion occupies roughly the region between 10 to the minus 4 to 10 to the minus 3 centimeters, or, from about 1 micron to about 100 microns. But most commercial equipment comes designed to operate in portions of the region, for a number of reasons (lower atmospheric absorption of IR radiation -or IR "atmospheric windows", detector availability at reasonable cost). Commercial IR thermography equipment comes in in the following wavelength bands and their filtered sub-bands. Common jargon follows approximately the terminology listed below:

·         The Near IR region and band is from about 0.7 to 1.7 microns,

·         The short wave or SW band is from about 1.8 to 2.4 microns,

·         The medium wave or MW band is from about 2.4 to 5 microns, and the

·         Long wave or LW band is from about 8 to 14 microns.

       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 (System International 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; 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.