The Latin prefix "infra" means "below" or "beneath." Thus "infrared" refers to the region beyond or beneath the red end of the visible color spectrum. The infrared region is located between the visible and microwave regions of the electromagnetic spectrum. Because heated objects radiate energy in the infrared, it is often referred to as the heat region of the spectrum. All objects radiate some energy in the infrared, even objects at room temperature and frozen objects such as ice.
The higher the temperature of an object, the higher the spectral radiant energy, or emittance, at all wavelengths and the shorter the predominant or peak wavelength of the emissions. Peak emissions from objects at room temperature occur at 10 µm. The sun has an equivalent temperature of 5900 K and a peak wavelength of 0.53 µm (green light). It emits copious amounts of energy from the ultraviolet to beyond the far IR region.
Much of the IR emission spectrum is unusable for detection systems because the radiation is absorbed by water or carbon dioxide in the atmosphere. There are several wavelength bands, however, with good transmission.The long wavelength IR (LWIR) band spans roughly 8-14 µm, with nearly 100% transmission on the 9-12 µm band. The LWIR band offers excellent visibility of most terrestrial objects. The medium wavelength IR (MWIR or MIR) band (3.3-5.0 µm) also offers nearly 100% transmission, with the added benefit of lower, ambient, background noise. Visible and short wavelength IR (SWIR or near IR, NIR) light (0.35-2.5 µm) corresponds to a band of high atmospheric transmission and peak solar illumination, yielding detectors with the best clarity and resolution of the three bands. Without moonlight or artificial illumination, however, SWIR imagers provide poor or no imagery of objects at 300K.