Infrared

Infrared, often called infrared light, is a type of electromagnetic radiation with wavelengths longer than visible light but shorter than microwaves. Because it's just beyond the red light that our eyes can see, we can't see infrared with our eyes. It ranges from about 780 nm up to 1 mm.

This kind of radiation was first discovered in 1800 by the astronomer Sir William Herschel, who noticed that objects could be warmed by a type of invisible light beyond red light, using a thermometer. He found that more than half of the Sun's energy that reaches Earth comes in the form of infrared.

Infrared radiation has many important uses. It helps scientists study molecules and their movements. Special cameras can see heat and are used to find problems in buildings, help firefighters, and even watch for overheating electrical parts. Infrared is also used in night-vision devices, telescopes that look through space dust, and many other technologies that help us see in the dark or measure heat from a distance.

Definition and relationship to the electromagnetic spectrum

Infrared radiation, often just called infrared, is a type of energy that our eyes cannot see. It has wavelengths longer than red light but shorter than microwaves. Usually, infrared starts at about 780 nm and goes up to 1 mm. This range of wavelengths matches frequencies from around 430 THz down to 300 GHz. After infrared comes the microwave part of the electromagnetic spectrum. Some people now include terahertz radiation with microwaves instead of infrared, which would move the infrared range to start at 0.1 mm (3 THz).

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

Nature

Sunlight, coming from a very hot source, contains a lot of infrared energy. In fact, more than half of the energy in sunlight is infrared. When the sun is directly overhead, it provides about 1 kilowatt of energy for every square meter. Of this, about half is infrared, with the rest being visible light and a small amount of ultraviolet light.

Here on Earth, which is much cooler than the Sun, we also give off infrared energy. Everything around us naturally radiates energy at many different wavelengths. Only very hot things like lightning or fires can produce a lot of visible light, but even fires give off more infrared energy than visible light.

Regions

Objects give off infrared radiation over a range of wavelengths, but sometimes only a certain part of this range is important because sensors usually pick up radiation within a specific area. Infrared radiation is often split into smaller parts, but how this is done can vary depending on what it's used for.

Infrared radiation starts just beyond what our eyes can see. There isn't a strict limit to what is visible, as our eyes become less sensitive to longer wavelengths. However, very bright light just beyond the visible range can sometimes be seen as a dim red glow. Different fields use different ways to divide the infrared spectrum into parts, often based on how sensors or detectors respond or what they are used to observe.

Division name	Abbreviation	Wavelength	Frequency	Photon energy	Temperature	Characteristics
Near infrared	NIR, IR-A DIN	0.75–1.4 μm	214–400 THz	886–1,653 meV	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	1.4–3 μm	100–214 THz	413–886 meV	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 (see transmission windows).
Mid-wavelength infrared	MWIR, IR-C DIN; MidIR. Also called intermediate infrared (IIR)	3–8 μm	37–100 THz	155–413 meV	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 seekers 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	8–15 μm	20–37 THz	83–155 meV	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 or moon or an infrared illuminator. This region is also called the "thermal infrared".
Far infrared	FIR	15–1,000 μm	0.3–20 THz	1.2–83 meV	193–3 K (−80.15 – −270.15 °C)	(see also far-infrared laser and far infrared)

Abbreviation	Wavelength	Frequency
IR-A	780–1400 nm	215–384 THz
IR-B	1400–3000 nm	100–215 THz
IR-C	3–1000 μm	0.3–100 THz

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

Designation	Abbreviation	Wavelength
Near infrared	NIR	0.7–2.5 μm
Mid infrared	MIR	3–25 μm
Far infrared	FIR	above 25 μm

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

Heat

Main article: Thermal radiation

Materials with higher emissivity appear closer to their true temperature than materials that reflect more of their different-temperature surroundings. In this thermal image, the more reflective ceramic cylinder, reflecting the cooler surroundings, appears to be colder than its cubic container (made of more emissive silicon carbide), while in fact, they have the same temperature.

Infrared radiation is often called "heat radiation," but any kind of light or electromagnetic waves can heat surfaces that absorb them. Infrared light from the Sun helps warm the Earth by about 49%, with the rest coming from visible light that is absorbed and then re-released at longer wavelengths. Objects at normal room temperatures give off radiation mostly in the 8 to 25 micrometer range, but this is not different from the visible light given off by very hot objects.

Heat is energy that moves from a warmer place to a cooler one. Unlike heat that moves through touch or air, thermal radiation can travel through empty space. This type of radiation has a special pattern of wavelengths linked to the vibrations of molecules in an object at a certain temperature. Objects can give off thermal radiation at any wavelength, and at very high temperatures, this radiation can extend into visible light, ultraviolet, and even X-rays, like in the Sun's outer layer. The common idea that infrared radiation is linked to heat is mostly because of the typical temperatures found on Earth's surface.

Understanding how well a surface gives off infrared radiation, called emissivity, is important. Two objects at the same temperature might look different in infrared images if their emissivity is different. Objects with higher emissivity appear hotter, while those with lower emissivity appear cooler. If an object does not give off radiation perfectly, it can also reflect or let through light from its surroundings. This means that the temperature of the environment can affect how hot or cold the object appears in infrared images. Not choosing the right emissivity or ignoring the temperature of the surroundings can lead to wrong measurements when using infrared cameras.

Applications

Night vision

Main article: Night vision

Infrared is used in night vision equipment when there is not enough visible light to see. Night vision devices change ambient light into electrons, which are then made into visible light. Infrared light can help night vision devices work better in the dark.

The use of infrared light for night vision is different from thermal imaging, which shows images based on temperature differences.

Thermography

Main article: Thermography

Infrared radiation can be used to find out how hot objects are from a distance. This is called thermography. It is used in military and industrial work but is now also used in cars because it costs less to make.

Thermographic cameras show images using infrared radiation. Since all objects give off infrared radiation based on their temperature, thermography helps us see things even without visible light.

Hyperspectral imaging

Main article: Hyperspectral imaging

A hyperspectral image is a picture that shows a wide range of colors at each point. It is used in biology, minerals, defense, and industry.

Thermal infrared hyperspectral imaging uses a thermographic camera where each point shows a full spectrum. This helps identify chemicals without needing outside light. It is used for geological measurements, outdoor watching, and UAV applications.

Other imaging

In infrared photography, infrared filters are used to capture near-infrared light. Some cheaper cameras and phone cameras can see near infrared, which looks purple-white. There is also T-ray imaging, which uses far-infrared or terahertz radiation. T-ray imaging is hard because there are not many bright sources, but new developments have made it more interesting.

Tracking

Infrared tracking, or infrared homing, is a way to guide missiles using the infrared light from a target. Missiles that use this are called “heat-seekers” because hot objects like people, car engines, and planes give off strong infrared light.

Heating

Thermography helped to determine the temperature profile of the Space Shuttle thermal protection system during re-entry.

Infrared radiation can be used to heat things. It is used in infrared saunas to heat people. It is also used to remove ice from airplane wings and in factories for curing coatings, shaping plastics, and drying prints.

Infrared heating is used more in factories where it replaces ovens and direct heating.

Cooling

Main article: Passive daytime radiative cooling

Some technologies use infrared to cool buildings or other systems. The LWIR range is useful because some of this radiation can escape into space. This helps cool things without using energy or causing pollution. This method has been suggested as a way to help slow down global warming.

Communications

Further information: Consumer IR

Infrared is used to send data between computer parts and personal digital assistants. These devices follow standards set by the Infrared Data Association. Remote controls use infrared LEDs to send signals. Infrared works well inside rooms because it does not go through walls.

Free-space optical communication uses infrared lasers for fast connections in cities. Infrared lasers are also used in fiber optic systems. Infrared is used to send audio from signs to help people who cannot see, and it is used for assistive audio instead of loops.

Spectroscopy

Infrared light from the LED of a remote control as recorded by a digital camera

Infrared vibrational spectroscopy is used to identify molecules by looking at their bonds. Each bond vibrates at a certain frequency, and if it changes the molecule’s dipole, it will absorb infrared light at that frequency. This helps study organic compounds and find out what they are made of.

Thin-film metrology

In the semiconductor industry, infrared light is used to study materials like thin films. By measuring how the light reflects off a semiconductor wafer, we can find out important details about the material.

Meteorology

Weather satellites use infrared to make pictures of the Earth. These pictures help us see cloud heights, temperatures over land and water, and ocean features. Infrared pictures can be made at night, which helps us study weather continuously.

Infrared pictures can show ocean currents and help fishermen and farmers. They can also help spot things like El Niño.

Climatology

In climatology, infrared radiation is studied to understand how energy moves between the Earth and the atmosphere. This helps us learn about long-term changes in our climate. It is one of the main things studied in research about global warming, along with solar radiation.

Astronomy

Reflected light photograph in various infrared spectra to illustrate the appearance as the wavelength of light changes

Astronomers use infrared to observe objects in space. Infrared helps us see cold, dark clouds, young stars before they shine, and planets around stars. It is also good for seeing through dust and for observing distant galaxies.

Cleaning

Infrared cleaning is used by some film and flatbed scanners to reduce dust and scratches in pictures. It works by using an extra infrared picture to find and fix these problems.

Art conservation and analysis

Infrared reflectography can be used on paintings to see hidden layers. This helps art experts understand if a painting is the original or a copy, and if it has been changed. It can also show the artist’s early plans. This technique works well on old documents too.

Biological systems

Some animals, like pit vipers and certain bats, can sense infrared light. This helps them find their prey by feeling the heat it gives off. Other animals, like some beetles and butterflies, can also sense infrared. This helps them find forest fires or avoid too much heat.

Photobiomodulation

Near-infrared light is used to help heal wounds and treat mouth sores from chemotherapy. Research is also looking into its effects on the brain and fighting viruses.

Health hazards

Strong infrared radiation in places with high heat can be dangerous to the eyes and may cause damage or blindness. Special IR-proof goggles are needed in these places because the radiation is invisible.

Scientific history

The discovery of infrared radiation is credited to William Herschel, an astronomer, in the early 1800s. He shared his findings with the Royal Society of London in 1800. Herschel used a prism to split sunlight and noticed that the area just beyond the red light felt warmer, calling these "Calorific Rays." The term "infrared" came later, with "infra-" meaning below, as it is light beyond red.

Many scientists contributed to understanding infrared radiation. In 1830, Leopoldo Nobili created the first device to detect infrared, and in 1840, John Herschel made the first thermal image. Over the years, important advances included the development of devices to measure infrared, theories explaining how objects emit heat, and applications in military technology and astronomy. Notable milestones include the invention of the bolometer by Samuel Pierpont Langley in 1878 and the formulation of laws describing how heat radiates. In the 20th century, infrared technology advanced rapidly, leading to applications in missiles, night vision, and even helping rats sense infrared light in 2013.