The word infrared (IR) is derived from the Latin word infra – meaning below, thereby infrared means below the red colour of the visible spectrum. Infrared radiation has wavelengths between 750 nm and 1mm ie., electromagnetic radiation longer than that of visible light, but shorter than that of radio waves. All objects emit infrared radiation as a function of their temperature. The higher the temperature of an object, the more the motion and hence the more the infrared energy emitted. Cameras do not see temperatures, they detect thermal radiation.

There are different ways that infrared region is divided into, and the most typical is the one delineated by the astronomers: Near Infrared (NIR), Middle Infrared (MIR) and Far Infrared (FIR) (Table 1).

Another scheme is with five subdivisions with first ranging between 0.75 -1.4 µm (IR-A) referred as near-infrared (NIR, IR-A), short-wavelength infrared (SWIR, IR-B) mid-wavelength infrared (MWIR, IR-C )or intermediate Infrared (IIR), long-wavelength infrared (LWIR, IR-C) or far infrared (FIR)(Table 2). NIR and SWIR are sometimes referred to as reflected infrared while MWIR and LWIR is sometimes referred to as thermal infrared. Due to the nature of the blackbody radiation curves, typical 'hot' objects like exhaust pipes often appear brighter in the MW compared to the same object viewed in the LW.

Another important aspect is that of infrared windows in the atmosphere. Table 3 depicts that only a few of the infrared ‘windows’ have both high sky transparency and low sky emission. These infrared windows are mainly at infrared wavelengths below 4 microns.