IR wavelengths are longer than that of visible light, but shorter than that of terahertz radiation microwaves. Bright sunlight provides an irradiance of just over 1 kilowatt per square meter at sea level. Of this energy, 527 watts is infrared radiation, 445 watts is visible light, and 32 watts is ultraviolet radiation.
Overview
Humans at normal body temperature radiate chiefly at wavelengths around 12μm (micrometers), as shown by Wien's displacement law.
At the atomic level, infrared energy elicits vibrational modes in a molecule through a change in the dipole moment, making it a useful frequency range for study of these energy states for molecules of the proper symmetry. Infrared spectroscopy examines absorption and transmission of photons in the infrared energy range, based on their frequency and intensity.
Origins of the term
The name means below red, the Latin infra meaning "below". Red is the color of the longest wavelengths of visible light. Infrared light has a longer wavelength (and so a lower frequency) than that of red light visible to humans, hence the literal meaning of below red.
Different regions in the infrared
Objects generally emit infrared radiation across a spectrum of wavelengths, but only a specific region of the spectrum is of interest because sensors are usually designed only to collect radiation within a specific bandwidth. As a result, the infrared band is often subdivided into smaller sections.
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CIE division scheme
The International Commission on Illumination (CIE) recommended the division of infrared radiation into the following three bands:
- IR-A: 700 nm–1400 nm (0.7 µm – 1.4 µm)
- IR-B: 1400 nm–3000 nm (1.4 µm – 3 µm)
- IR-C: 3000 nm–1 mm (3 µm – 1000 µm)
A commonly used sub-division scheme is:
Near-infrared (NIR, IR-A DIN): 0.75-1.4 µm in wavelength, defined by the water absorption, and commonly used in fiber optic telecommunication because of low attenuation losses in the SiO2 glass (silica) medium. Image intensifiers are sensitive to this area of the spectrum. Examples include night vision devices such as night vision goggles.
Short-wavelength infrared (SWIR, IR-B DIN): 1.4-3 µm, water absorption increases significantly at 1,450 nm. The 1,530 to 1,560 nm range is the dominant spectral region for long-distance telecommunications.
Mid-wavelength infrared (MWIR, IR-C DIN) also called intermediate infrared (IIR): 3-8 µm. In guided missile technology the 3-5 µm portion of this band is the atmospheric window in which the homing heads of passive IR 'heat seeking' missiles are designed to work, homing on to the IR signature of the target aircraft, typically the jet engine exhaust plume.
Long-wavelength infrared (LWIR, IR-C DIN): 8–15 µm. This is the "thermal imaging" region, in which sensors can obtain a completely passive picture of the outside world based on thermal emissions only and requiring no external light or thermal source such as the sun, moon or infrared illuminator. Forward-looking infrared (FLIR) systems use this area of the spectrum. Sometimes also called the "far infrared."
Far infrared (FIR): 15 - 1,000 µm (see also far infrared laser).
NIR and SWIR is sometimes called "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, such as exhaust pipes, often appear brighter in the MW compared to the same object viewed in the LW.
