Infrared (IR) radiation is electromagnetic radiation of a wavelength greater than that of visible light but shorter than that of microwaves.

The name means “below red” (from the Latin infra: “below”), red being the longest wavelength colour of visible light. This wavelength is between 700 nm and 1 mm.

Infrared is often subdivided into near IR (0.7-5 μm), medium IR (5-30 μm) and far IR (30-1000 μm). However, this classification is not precise, as each field of use has its own idea of the boundary between the different types.

Infrared is often associated with heat because, at normal temperature, objects spontaneously emit radiation in the infrared range, the exact relationship being given by the law of black body radiation; moreover, infrared radiation vibrates the atoms of the body that absorbs them and thus raises its temperature (heat transfer by radiation). As a first approximation, the wavelength of the maximum emission of a black body brought to an absolute temperature T is 3670/T. This means that at room temperature (T around 300 K), the maximum emission is around 10 micrometers. This poses specific problems in astronomy in particular, because of the thermal background it causes.
Uses and uses

Infrared is used in night vision equipment when the amount of visible light is insufficient to see objects. The radiation is detected and displayed on a screen, with the hottest objects also becoming the brightest. In addition to night vision, the use of infrared thermography for remote and non-contact measurement of the temperature of target objects should also be added.

Infrared is also used in the military field to guide air-to-air or anti-aircraft missiles: an infrared detector then guides the missile to the heat source of the target aircraft’s engine(s). Such missiles can be avoided by special manoeuvres (alignment with the sun) or by the use of thermal decoys.

A more common use is their use in remote controls (remote controls), where they are preferred to radio waves, as they do not interfere with other electromagnetic signals such as television signals. In this field, there are several information codings (RC5 for Philips, SIRSC for Sony, etc.). Infrared is also used for short-range communication between computers and their peripherals. Devices using this type of communication generally comply with the standards published by the Infrared Data Association (IrDA).

The light used in optical fibres is generally infrared. For this application, we use wavelengths where the proper absorption of the material constituting the fibre is minimal: 1.3 µm and 1.55 µm.

They are widely used in robotics or in devices requiring short distance data transmission without obstacles.

In addition, infrared spectroscopy is one of the most effective (and widespread) methods for identifying organic and inorganic molecules based on their vibrational properties. Indeed, infrared radiation excites specific vibration modes (deformation, elongation) of chemical bonds. The comparison between incident and transmitted radiation through the sample is therefore sufficient to determine the main chemical functions present in the sample.

Infrared astronomy is difficult because of the Earth’s atmosphere. This is why astronomers send infrared satellites: IRAS (1983; NASA, United Kingdom, Netherlands), ISO (1995; ESA), Wire (1999; USA, failed launch), Spitzer (2003; ex-SIRTF, USA); ASTRO-F (2006; Japan) and the future Herschel (2007 ?; ESA).

Infrared was discovered in 1800 by William Herschel, an English astronomer of German origin.

Herschel placed a mercury thermometer in the spectrum obtained by a glass prism to measure the heat specific to each color. He found that the heat was strongest on the red side of the spectrum, even where there was no more light. This was the first experiment to show that heat could be transmitted through an invisible form of light.