Infrared radiation (IR) refers to the electromagnetic energy that is emitted from all matter above absolute zero. This energy is a result of the acceleration of charged particles within matter due to molecular vibration or collisions between particles. Accelerating electric charge is the source of all electromagnetic waves. Electromagnetic (EM) waves at higher frequencies are referred to as radios waves and are produced by accelerating charged particles within an antenna using electronic methods. Because all molecular motion ceases at absolute zero, it follows that charged particles are not accelerated so EM waves are not generated.
An electromagnetic (EM) wave is composed of an electric field and a magnetic field that are oscillating together. The fields are oriented perpendicular to each other and the wave travels at the speed of light and in a direction (propagates) perpendicular to both of the fields (see image below). The distance a wave travels over one oscillation is called the wavelength, denoted by the Greek letter λ.
The complete electromagnetic (EM) spectrum is shown below. Note that the only difference between classifications of EM waves is the frequency and wavelength (λ) of the oscillation. As the frequency of oscillation increases, λ decreases, keeping the velocity of the wave constant at the speed of light. The wavelengths of the entire infrared spectrum ranges from .75 µm – 1000 µm but the portion suitable for thermal infrared includes wavelengths between 1.3 µm – 14 µm.
The detector in the infrared imager functions like a radio tuner for infrared wavelengths. The spectrum of wavelengths used for thermal infrared are divided into three categories; short, mid, and long-wave. The three categories are divided by two bands that are unusable because atmospheric conditions absorb the EM energy at these wavelengths. Infrared Imagers are typically designed to detect either, short, mid, or long-wave infrared radiation. Because the infrared wavelength of EM radiation to be detected is dependent on the temperature and atomic structure of the material, the correct wavelength imager must be chosen for the application.