The spectrum of electromagnetic radiation is divided into different parts. The high-energy gamma radiation with approx. 0.1 Angstrom wavelength is followed by X-ray, UV radiation, visible radiation, infrared radiation, microwaves and radio waves.
The infrared radiation (IR radiation) is further divided. The spectral range between 0.78 µm and 1.4 µm is called near infrared (NIR) and the range between 1.4 and 3 µm short-wavelength infrared (SWIR). The range between 3 µm and 5 µm is referred to as mid-wavelength infrared (MWIR) and the range between 8 µm and 14 µm as long-wavelength infrared (LWIR).
Every body emits electromagnetic radiation, the thermal radiation. The intensity of this radiation is a function of temperature and wavelength and is described by Planck's law. At low temperatures (< 1000 °C) most of the emitted radiation lies in the infrared range.
Planck's law is true for an ideal radiator, a "black body". For real bodies, the emissivity must be taken into account. The emissivity is a factor between 0 and 1. When the IR intensity which is measured by an IR camera is converted to temperature information, one speaks of "thermography" (thermal imaging).