What Is Infrared?
An image of Earth in infrared wavelengths shows relative temperatures around the world. The photo includes a plume of carbon monoxide pollution near the Rim Fire that burned near Yosemite National Park in California on Aug. 26, 2013.
Credit: NASA/JPL-Caltech/Space Science Institute

Infrared radiation is a type of electromagnetic radiation, as are radio waves, ultraviolet radiation, X-rays and microwaves. Infrared (IR) light is the part of the EM spectrum that people encounter most in everyday life, although much of it goes unnoticed. It is invisible to human eyes, but people can feel it as heat.

IR radiation is one of the three ways heat is transferred from one place to another, the other two being convection and conduction. Everything with a temperature above about 5 degrees Kelvin (minus 450 degrees Fahrenheit or minus 268 degrees Celsius) emits IR radiation. The sun gives off half of its total energy as IR, and much of its visible light is absorbed and re-emitted as IR, according to the University of Tennessee. 

According to the Environmental Protection Agency, incandescent bulbs convert only about 10 percent of their electrical energy input into visible light energy; about 90 percent is converted to infrared radiation. Household appliances such heat lamps and toasters use IR radiation to transmit heat, as do industrial heaters such as those used for drying and curing materials. These appliances generally emit blackbody radiation with a peak energy output below the wavelength of visible, though some energy is emitted as visible red light.

A TV remote control uses IR waves to change channels. In the remote, an IR light-emitting diode (LED) or laser sends out binary coded signals as rapid on/off pulses, according to NASA. A detector in the TV converts these light pulses to electrical signals that instruct a microprocessor to change the channel, adjust the volume or perform other actions. IR lasers can be used for point-to-point communications over distances of a few hundred meters or yards. 

Electromagnetic (EM) radiation is transmitted in waves or particles at different wavelengths and frequencies. This broad range of wavelengths is known as the electromagnetic spectrum. The spectrum is generally divided into seven regions in order of decreasing wavelength and increasing energy and frequency. The common designations are radio waves, microwaves, infrared (IR), visible light, ultraviolet (UV), X-rays and gamma-rays. 

Infrared waves are longer than those of visible light, just beyond the red end of the visible spectrum. Infrared (IR) falls in the range of the (EM) spectrum between microwaves and visible light. It has frequencies from about 3 GHz up to about 400 THz and wavelengths of about 30 centimeters (12 inches) to 740 nanometers (0.00003 inches), although these values are not definitive. 

British astronomer William Herschel discovered infrared light in 1800, according to NASA. In an experiment to measure the difference in temperature between the colors in the visible spectrum, he placed thermometers in the path of light within each color of the visible spectrum. He observed an increase in temperature from blue to red, including an even warmer temperature measurement just beyond the red end of the visible spectrum. 

One of the most useful applications of the IR spectrum is in sensing and detection. All objects on Earth emit IR radiation, or heat, which can be detected by electronic sensors, such as those used in night-vision goggles and infrared cameras. A simple example of such a sensor is the bolometer, which consists of a telescope with a temperature-sensitive resistor, or thermistor, at its focal point. If a warm body comes into this instrument's field of view, the heat causes a detectable change in the voltage across the thermistor. Night-vision cameras use a more sophisticated version of a bolometer. These cameras typically contain charge-coupled-device (CCD) imaging chips that are sensitive to IR light. The image formed by the CCD can then be reproduced in visible light. These systems can be made small enough to be used in hand-held devices or wearable night-vision goggles. They can also be used for gun sights with or without the addition of an IR laser for targeting. 

Infrared spectroscopy measures IR emissions from materials at specific wavelengths. The IR spectrum of a substance will show characteristic dips and peaks when photons are absorbed or emitted by electrons in molecules as they transition between orbits, or energy levels. This information can then be used to identify substances and monitor chemical reactions. According to Robert Mayanovic, professor of physics at Missouri State University, infrared spectroscopy, such as Fourier transform infrared (FTIR) spectroscopy, is highly useful for numerous scientific applications. These include the study of molecular systems and 2D materials such as graphene.

The California Institute of Technology (CalTech) describes infrared astronomy as "the detection and study of the infrared radiation (heat energy) emitted from objects in the universe." Advances in IR CCD imaging systems have allowed for detailed observation of the distribution of IR sources, revealing complex structures in nebulae, galaxies and the large-scale structure of the universe. 

One of the advantages of IR observation is that it can detect objects that are too cool to emit visible light. This has led to the discovery of previously unknown objects such as comets, asteroids and wispy interstellar dust clouds that seem to be prevalent throughout the entire galaxy. IR astronomy is particularly useful for observing cold molecules of gas and also dust particles in the interstellar medium to determine their chemical makeup, said Robert Patterson, professor of astronomy at Missouri State University. These observations are conducted using specialized charged-coupled device (CCD) detectors that are sensitive to IR photons. 

Another advantage of IR radiation is that because of its longer wavelength, it is less subject to scattering than is visible light. Whereas visible light can be absorbed or reflected by gas and dust particles, the longer IR waves simply go around these small obstructions. Because of this property, IR can be used to observe objects whose light is obscured by gas and dust. Such objects include newly forming stars imbedded in nebulae or the center of Earth's galaxy. 

Most IR observations are conducted by satellites to avoid atmospheric interference. One of the more prominent of these satellites was the Infrared Astronomical Satellite (IRAS), which produced images of the sky at wavelengths of 12, 25, 60 and 100 micrometers (µm). Its imaging sensor had to be cooled to 2 K (minus 456 F) using 161 lbs. (73 kilograms) of superfluid liquid helium, which limited the satellite's mission to only 10 months. However, over that period it completed a survey covering 96 percent of the sky. It identified several asteroids, comets and interstellar dust clouds, and it produced the first images of the galactic core. The data produced is still being used today to guide observations at other wavelengths. 

Since then, a number of satellites have been launched to make more detailed observations of specific objects and limited areas of the sky. However, there was not another complete sky survey conducted until 2006 when the Japanese space agency JAXA launched the satellite AKIRI, which is Japanese for "light." This satellite's improved cryogenic system allowed its mission to reach 18 months, about 50 percent more than IRAS. AKIRI also had more sensitive and higher-resolution sensors than IRAS, resulting in some truly remarkable images containing a wealth of new data. 

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