Image of a small dog taken in mid-infrared ("thermal") light (false color)

Infrared (IR) radiation is electromagnetic radiation of a wavelength longer than visible light, but shorter than microwave radiation. The name means "below red" (from the Latin infra, "below"), red being the color of visible light of longest wavelength. Infrared radiation spans three orders of magnitude and has wavelengths between 700 nm and 1 mm.

Different regions in the infrared

IR is often subdivided into:

• near infrared NIR, IR-A DIN, 0.7–1.4 µm in wavelength, defined by the water absorption, and commonly used in fiber optic telecommunication because of the low attenuation losses in the SiO₂ glass medium.
• short wavelength IR SWIR, IR-B DIN, 1.4–3 µm Water absorption increases significantly at 1450 nm
• mid wavelength IR MWIR, IR-C DIN, also intermediate-IR (IIR), 3–8 µm
• long wavelength IR LWIR, IR-C DIN, 8–15 µm)
• far infrared FIR, 15–1000 µm

However, these terms are not precise, and are used differently in various studies i.e. near (0.7–5 µm) / mid (5–30 µm) / long (30–1000 µm). Especially at the telecom-wavelengths the spectrum is further subdivided into individual bands, due to limitations of detectors, amplifiers and sources. Infrared radiation is often linked to heat, since objects at room temperature or above will emit radiation mostly concentrated in the mid-infrared band (see black body).

The common nomenclature is justified by the different human response to this radiation (near infrared = the red you just cannot see, far IR = thermal radiation), other definitions follow different physical mechanisms (emission peaks, vs. bands, water absorption) and the newest follow technical reasons (The common Si-detectors are sensitive to ~1050 nm, while InGaAs sensitivity starts around 950 nm and ends between 1700 and 2200 nm, depending on the specific configuration). Unfortunately the international standards for this specifications are not freely available.

Telecomunication bands in the infrared

Optical telecommunication in the near infrared is technically often seperated to different frequency bands because of availability of light sources, transmitting /absorbing materials (fibers) and detectors.

• O-band 1260-1360 nm
• E-band 1360-1460 nm
• S-band 1460-1530 nm
• C-band 1530-1565 nm
• L-band 1565-1625 nm
• U-band 1625-1675 nm

History

In 1800 the English astronomer William Herschel held a mercury thermometer in the spectrum produced by a glass prism to measure the heat content of different coloured lights. He found that the thermometer registered an increase in temperature even when held beyond the red end of the spectrum, where there was no visible light. This was the first experiment to show that heat could be transmitted by an invisible form of light.

The Earth as an infrared emitter

The Earth's surface absorbs visible radiation from the sun and re-emits much of the energy as infrared back to the atmosphere. Certain gases in the atmosphere, chiefly water vapor, absorb this infrared, and re-radiate it in all directions including back to Earth. This, the greenhouse effect, keeps the atmosphere and surface much warmer than if the infrared absorbers were absent from the atmosphere.

Applications

Infrared is used in night-vision equipment, when there is insufficient visible light to see an object. The radiation is detected and turned into an image on a screen, hotter objects showing up brighter, enabling the police and military to chase targets.

Smoke is more transparent to infrared than to visible light, so fire fighters use infrared imaging equipment when working in smoke-filled areas. Fire fighters also use this equipment in wood-frame buildings after a fire has been extinguished to look for hot spots behind the walls, where a fire can break out again.

In the fields of building maintenance and property management, IR can be a cost saving tool when used to image various building components and equipment. IR thermography is a type of photography that measures temperature differences in the IR range. This is useful when looking for equipment "hot-spots" or scanning the exterior or roof of a building looking for heat loss.

A more common use of IR is in television remote controls. In this case it is used in preference to radio waves because it does not interfere with other devices in adjoining rooms - this is especially important in areas of high population density (IR does not penetrate walls). IR data transmission is also employed in short-range communication among computer peripherals and personal digital assistants. These devices usually conform to standards published by IrDA, the Infrared Data Association. Remote controls and IrDA devices use infrared light-emitting diodes (LEDs) to emit infrared radiation which is focused by a plastic lens into a narrow beam. The beam is modulated, i.e. switched on and off, to encode the data. The receiver uses a silicon photodiode to convert the infrared radiation to an electric current. It responds only to the rapidly pulsing signal created by the transmitter, and filters out slowly changing infrared radiation from ambient light.

In infrared photography, infrared filters are used to capture only the infrared spectrum. Digital cameras also often use infrared blockers.

IR can also be used to warm muscles and ease tension on tendons during medical procedures