Infrared

Infrared, often called infrared light, is a type of electromagnetic radiation. Its wavelengths are longer than visible light but shorter than microwaves. Because it is just beyond red light, our eyes cannot see it. It ranges from about 780 nm up to 1 mm.

This kind of radiation was first discovered in 1800 by astronomer Sir William Herschel. He 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 is infrared.

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

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. Infrared usually starts at about 780 nm and goes up to 1 mm. This range matches frequencies from around 430 THz down to 300 GHz. After infrared comes the microwave part of the electromagnetic spectrum.

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 from the sun has a lot of infrared energy. More than half of the energy in sunlight is infrared. When the sun is high in the sky, it gives about 1 kilowatt of energy for every square meter. About half of this is infrared. The rest is visible light and a small amount of ultraviolet light.

Here on Earth, we also give off infrared energy. Everything around us naturally gives off energy at many different wavelengths. Even fires give off more infrared energy than visible light.

Regions

Objects give off infrared radiation over a range of wavelengths, but 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. Our eyes become less sensitive to longer wavelengths. 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 can warm surfaces that absorb it. Infrared light from the Sun helps warm the Earth a lot. Objects at normal room temperatures give off radiation mostly in the 8 to 25 micrometer range.

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. Objects can give off thermal radiation at any wavelength, and at very high temperatures, this radiation can become visible light.

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.

Applications

Night vision

Main article: Night vision

Infrared is used in night vision equipment when there is very little light. Night vision devices change light into something we can see. Infrared light helps these 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.

Thermography

Main article: Thermography

Infrared radiation can tell us how hot objects are from far away. This is called thermography. It is used in many places, including cars.

Thermographic cameras use infrared radiation to make images. All objects give off infrared radiation, so thermography can show things even when there is no visible light.

Hyperspectral imaging

Main article: Hyperspectral imaging

A hyperspectral image shows many colors at each point. It is used in biology, minerals, defense, and industry.

Thermal infrared hyperspectral imaging uses a thermographic camera to show a full spectrum at each point. This helps identify chemicals without needing extra light. It is used for measuring the ground, watching areas from above, and with drones.

Other imaging

In infrared photography, infrared filters capture near-infrared light. Some 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 difficult but new developments make it more useful.

Tracking

Infrared tracking, or infrared homing, guides missiles using the infrared light from a target. Missiles that use this are called “heat-seekers” because hot objects give off strong infrared light.

Heating

Infrared radiation can 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 drying and shaping materials.

Infrared heating is often used in factories instead of ovens.

Cooling

Main article: Passive daytime radiative cooling

Some technologies use infrared to cool buildings or other systems. The LWIR range can send radiation into space, which helps cool things without using energy or causing pollution. This method has been suggested to help slow down global warming.

Communications

Further information: Consumer IR

Infrared is used to send data between devices. 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 to help people who cannot see, and it is used for assistive audio instead of wires.

Spectroscopy

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

Infrared vibrational spectroscopy identifies 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.

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 learn 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 all the time.

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 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. Research is also looking into its effects on the brain and fighting viruses.

Health hazards

Strong infrared radiation in very hot places can be dangerous to the eyes and may cause damage. 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 told his findings to the Royal Society of London in 1800. Herschel used a prism to split sunlight and felt that the area just beyond the red light was warmer. He called this "Calorific Rays." The term "infrared" came later, with "infra-" meaning below, as it is light beyond red.

Many scientists helped us understand infrared radiation. In 1830, Leopoldo Nobili made the first device to detect infrared, and in 1840, John Herschel made the first thermal image. Over time, new devices were made to measure infrared, and theories explained how objects give off heat. Important inventions include the bolometer by Samuel Pierpont Langley in 1878. In the 20th century, infrared technology grew fast and found uses in many areas.