In general, objects emit infrared radiation across a spectrum of wavelengths, but sometimes only a limited region of the spectrum is of interest because sensors usually collect radiation only within a specific bandwidth.
Thermal infrared radiation also has a maximum emission wavelength, which is inversely proportional to the absolute temperature of object, in accordance with Wien's displacement law.
These letters are commonly understood in reference to atmospheric windows and appear, for instance, in the titles of many papers.
Near-infrared is the region closest in wavelength to the radiation detectable by the human eye.
Infrared spectroscopy examines absorption and transmission of photons in the infrared range.
Infrared radiation is used in industrial, scientific, and medical applications.
Sunlight, at an effective temperature of 5,780 kelvins, is composed of near thermal-spectrum radiation that is slightly more than half infrared.
At zenith, sunlight provides an irradiance of just over 1 kilowatt per square meter at sea level.
Of these natural thermal radiation processes only lightning and natural fires are hot enough to produce much visible energy, and fires produce far more infrared than visible-light energy.
Infrared thermal-imaging cameras are used to detect heat loss in insulated systems, to observe changing blood flow in the skin, and to detect overheating of electrical apparatus.
Thermal-infrared imaging is used extensively for military and civilian purposes.
Slightly more than half of the total energy from the Sun was eventually found to arrive on Earth in the form of infrared.
The balance between absorbed and emitted infrared radiation has a critical effect on Earth's climate.
The three regions are used for observation of different temperature ranges, and hence different environments in space.