What is Infrared?

 

Infrared (IR), sometimes called Infrared Light, is electromagnetic radiation (EMR) with wavelengths longer than those of visible light. It is therefore invisible to the huhttps://madpcb.com/glossary/electromagnetic-radiation/man eye. IR is generally understood to encompass wavelengths from around 1 millimeter (300GHz) to the nominal red edge of the visible spectrum, around 700 nanometers (frequency 430THz) (although the longer IR wavelengths are often designated rather as terahertz radiation. Black-body radiation from objects near room temperature is almost all at IR wavelengths. As a form of electromagnetic radiation, IR propagates energy and momentum, with properties corresponding to both those of a wave and of a particle, the photon.

 

Infrared (IR)

Infrared (IR)

 

Infrared radiation was discovered in 1800 by astronomer Sir William Herschel, who discovered a type of invisible radiation in the spectrum lower in energy than red light, by means of its effect on a thermometer. Slightly more than half of the energy from the Sun was eventually found, through Herschel’s studies, to arrive on Earth in the form of IR. The balance between absorbed and emitted IR radiation has an important effect on Earth’s climate.

IR radiation is emitted or absorbed by molecules when changing rotational-vibrational movements. It excites 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. IR spectroscopy examines absorption and transmission of photons in the IR range.

Infrared radiation is used in industrial, scientific, military, commercial, and medical applications. Night-vision devices using active near-IR illumination allow people or animals to be observed without the observer being detected. IR astronomy uses sensor-equipped telescopes to penetrate dusty regions of space such as molecular clouds, to detect objects such as planets, and to view highly red-shifted objects from the early days of the universe. IR thermal-imaging cameras are used to detect heat loss in insulated systems, to observe changing blood flow in the skin, and to detect the overheating of electrical components.

Military and civilian applications include target acquisition, surveillance, night vision, homing, and tracking. Humans at normal body temperature radiate chiefly at wavelengths around 10μm (micrometers). Non-military uses include thermal efficiency analysis, environmental monitoring, industrial facility inspections, detection of grow-ops, remote temperature sensing, short-range wireless communication, spectroscopy, and weather forecasting.

 

Definition and Relationship to the Electromagnetic Spectrum

 

Infrared radiation extends from the nominal red edge of the visible spectrum at 700 nanometers (nm) to 1 millimeter (mm). This range of wavelengths corresponds to a frequency range of approximately 430 THz down to 300 GHz. Beyond IR is the microwave portion of the electromagnetic spectrum.

 

Name Wavelength Frequency (Hz) Photon Energy (eV)
Gamma ray less than 0.01nm more than 30EHz more than 124keV
X-ray 0.01nm – 10 m 30PHz – 30EHz 124keV – 124eV
Ultraviolet 10nm – 400nm 750THz – 30 Hz 124eV – 3.3eV
Visible 400nm – 700nm 430THz – 750THz 3.3eV – 1.7eV
Infrared 700nm – 1mm 300GHz – 430THz 1.7eV – 1.24meV
Microwave 1mm – 1meter 300MHz – 300GHz 1.24meV – 1.24μeV
Radio 1meter and more 300MHz and below 1.24μeV and below

 

  • Natural Infrared: Sunlight, at an effective temperature of 5,780 kelvins (5,510 °C, 9,940 °F), is composed of near-thermal-spectrum radiation that is slightly more than half IR. At zenith, sunlight provides an irradiance of just over 1 kilowatt per square meter at sea level. Of this energy, 527 watts is IR radiation, 445 watts is visible light, and 32 watts is ultraviolet radiation. Nearly all the IR radiation in sunlight is near IR, shorter than 4 micrometers. On the surface of Earth, at far lower temperatures than the surface of the Sun, some thermal radiation consists of IF in the mid-IR region, much longer than in sunlight. However, black-body, or thermal, radiation is continuous: it gives off radiation at all wavelengths. 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.
  • Regions within the Infrared: In general, objects emit IR 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. The infrared band is often subdivided into smaller sections, although how the IR spectrum is thereby divided varies between different areas in which IR is employed.
  • Visible Limit: Infrared radiation is generally considered to begin with wavelengths longer than visible by the human eye. However, there is no hard wavelength limit to what is visible, as the eye’s sensitivity decreases rapidly but smoothly, for wavelengths exceeding about 700 nm. Therefore, wavelengths just longer than that can be seen if they are sufficiently bright, though they may still be classified as infrared according to usual definitions. Light from a near-IR laser may thus appear dim red and can present a hazard since it may actually be quite bright. And even IR at wavelengths up to 1,050 nm from pulsed lasers can be seen by humans under certain conditions.

 

Commonly Used Sub-Division Scheme

 

Division name Abbreviation Wavelength Frequency Photon Energy Temperature
Near infrared NIR, IR-A DIN 0.75–1.4μm 214–400THz 886–1,653meV 3,864–2,070 K
(3,591–1,797 °C)
Goes up to the wavelength of the first water absorption band, 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. Near-infrared spectroscopy is another common application.
Short-wavelength infrared SWIR, IR-B DIN 1.4–3μm 100–214THz 413–886meV 2,070–966 K
(1,797–693 °C)
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;
MidIR.
intermediate infrared (IIR)		 3–8μm 37–100THz 155–413meV 966–362 K
(693–89 °C)
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 Infrared signature of the target aircraft, typically the jet engine exhaust plume. This region is also known as thermal infrared.
Long-wavelength infrared LWIR, IR-C DIN 8–15μm 20–37THz 83–155meV 362–193 K
(89–80 °C)
The “thermal imaging” region, in which sensors can obtain a completely passive image of objects only slightly higher in temperature than room temperature – for example, the human body – based on thermal emissions only and requiring no illumination such as the sun, moon, or infrared illuminator. This region is also called the “thermal infrared”.
Far infrared FIR 15–1,000μm 0.3–20THz 1.2–83meV 193–3 K
(80.15–270.15 °C)
/

 

NIR and SWIR together is sometimes called “reflected infrared”, whereas MWIR and LWIR is sometimes referred to as “thermal infrared”.

 

CIE Division Scheme

 

The International Commission on Illumination (CIE) recommended the division of infrared radiation into the following three bands:

 

Abbreviation Wavelength Frequency
IR-A 700nm – 1,400nm
(0.7μm – 1.4μm)		 215THz – 430THz
IR-B 1,400nm – 3,000nm
(1.4μm – 3μm)		 100THz – 215THz
IR-C 3,000nm – 1mm
(3μm – 1,000μm)		 300GHz – 100THz

 

ISO 20473 Scheme

 

Designation Abbreviation Wavelength
Near-Infrared NIR 0.78–3μm
Mid-Infrared MIR 3–50μm
Far-Infrared FIR 50–1,000μm

 

Astronomy Division Scheme

 

Designation Abbreviation Wavelength
Near-Infrared NIR 0.7 to 2.5 μm
Mid-Infrared MIR 3 to 25 μm
Far-Infrared FIR above 25 μm.

 

These divisions are not precise and can vary depending on the publication. The three regions are used for observation of different temperature ranges, and hence different environments in space.

The most common photometric system used in astronomy allocates capital letters to different spectral regions according to filters used; I, J, H, and K cover the near-infrared wavelengths; L, M, N, and Q refer to the mid-infrared region. These letters are commonly understood in reference to atmospheric windows and appear, for instance, in the titles of many papers.

 

Telecommunication Bands in the Infrared

 

In optical communications, the part of the IR spectrum that is used is divided into seven bands based on availability of light sources transmitting/absorbing materials (fibers) and detectors:

 

Band Descriptor Wavelength Range
O band Original 1,260–1,360nm
E band Extended 1,360–1,460nm
S band Short wavelength 1,460–1,530nm
C band Conventional 1,530–1,565nm
L band Long wavelength 1,565–1,625nm
U band Ultralong wavelength 1,625–1,675nm

 

The C-band is the dominant band for long-distance telecommunication networks. The S and L bands are based on less well-established technology, and are not as widely deployed.

 

Infrared Applications

 

  • Night vision
  • Thermography
  • Hyperspectral imaging
  • Other imaging
  • Tracking
  • Heating
  • Cooling
  • Communications
  • Spectroscopy
  • Thin film metrology
  • Meteorology
  • Climatology
  • Astronomy
  • Infrared cleaning
  • Art conservation and analysis
  • Biological systems
  • Photobiomodulation
  • Health hazards