Introduction:

      Infrared (IR) radiation is a part of the electromagnetic spectrum that includes wavelengths longer than about 1 micrometer (1 x 10[sup -6] meters).  This section of the electromagnetic spectrum consists of wavelengths that are too long for the human eye to detect.  From a physical standpoint, the longer the electromagnetic wavelength, the lower the energy of the radiation.  Thus, IR radiation is of lower energy than visible light and most other parts of the electromagnetic spectrum.  Microwaves tend to be considered of lower energy than IR (even though they were originally considered just a part of the IR spectrum), and radio waves consist of even longer wavelenghts (lower energies).

      More importantly, most IR wavelengths correspond to the vibrational frequencies of water.   This is one of the defining characteristics of IR radiation not only because it can be used to heat water (and subsequently most forms of living tissue) but also to detect sources of heat.  Because IR is just a form of light, both of these endeavors can be done without the hindrance of physical thermal contact which has proved to be important in many medical applications.  There are many experimental methods of implementing IR light sources.   Doctors have recently found a very promising technique of heating venous ulcers with monochromatic IR energy that leads to healing in cases where other methods have failed.

      Here's a representation that shows how the IR range can be found at wavelengths longer than about 1 micrometer (1 x 10[sup -6] meters) -- which is "below" what the human eye can see: