infrared radiation (IR)

Infrared radiation (IR), sometimes referred to simply as infrared, is a region of the electromagnetic radiation spectrum where wavelengths range from about 700 nanometers (nm) to 1 millimeter (mm). Infrared waves are longer than those of visible light, but shorter than those of radio waves. Correspondingly, the frequencies of IR are higher than those of microwaves, but lower than those of visible light, ranging from about 300 Ghz to 400 THz.

Infrared light is invisible to the human eye, although longer infrared waves can be sensed as heat. It does, however, share some characteristics with visible light -- namely, infrared light can be focused, reflected and polarized.

Wavelength and frequency

Infrared can be subdivided into multiple spectral regions, or bands, based on wavelength; however, there is no uniform definition of each band's exact boundaries. Infrared is commonly separated into near-, mid- and far-infrared. It can also be divided into five categories: near-, short-wavelength, mid-, long-wavelength and far-infrared.

The near-IR band contains the range of wavelengths closest to the red end of the visible light spectrum. It is generally considered to consist of wavelengths measuring from 750 nm to 1,300 nm -- or 0.75 to 1.3 microns. Its frequency ranges from about 215 THz to 400 THz. This group consists of the longest wavelengths and shortest frequencies, and it produces the least heat.

Visible and invisible light illustration
Visible and invisible light

The intermediate IR band, also called the mid-IR band, covers wavelengths ranging from 1,300 nm to 3,000 nm -- or 1.3 to 3 microns. Frequencies range from 20 THz to 215 THz.

Wavelengths in the far-IR band, which are closest to microwaves, extend from 3,000 nm to 1 mm -- or 3 to 1,000 microns.  Frequencies range from 0.3 THz to 20 THz. This group consists of the shortest wavelengths and longest frequencies, and it produces the most heat.

Infrared radiation uses

Infrared is used in a variety of applications. Among the most well-known are heat sensors, thermal imaging and night vision equipment.

In communications and networking, infrared light is used in wired and wireless operations. Remote controls use near-infrared light, transmitted with light-emitting diodes (LEDs), to send focused signals to home-entertainment devices, such as televisions. Infrared light is also used in fiber optic cables to transmit data.

Electromagnetic spectrum and visible light illustration
Electromagnetic spectrum and visible light

In addition, infrared is used extensively in astronomy to observe objects in space that can't be detected in whole or part by the human eye, including molecular clouds, stars, planets and active galaxies.

History of infrared radiation technology

Infrared was discovered by British astronomer Sir William Herschel in 1800. Herschel knew sunlight could be separated into separate components, a step accomplished by refracting the light through a glass prism. He then measured the temperatures of the different colors that were created. He found the temperature increased as the colors progressed from violet, blue, green, yellow, orange and red light. Herschel then went a step further, measuring the temperature in the portion beyond the red area. There, in the infrared area, he found the temperature to be the highest of all.

This was last updated in May 2017

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