In physics, infrared (IR) or infrared radiation (first discovered in 1800 by astronomer William Herschel) is the electromagnetic radiation with a frequency band of the electromagnetic spectrum lower than that of visible light but greater than that of radio waves, i.e. wavelength between 700 nm and 1 mm (infrared band). The range of Infrared region is 12800 ~ 10 cm-1 and can be divided into near-infrared region (12800 ~ 4000 cm-1), mid-infrared region (4000 ~ 200 cm-1) and far-infrared region (50 ~ 1000 cm-1). This range of wavelengths corresponds to a frequency range of approximately 430 THz down to 300 GHz and includes most of the thermal radiation emitted by objects near room temperature. The term means “under the red” (from the Latin infra, “under”), because red is the visible color with the lowest frequency.
It is often associated with the concepts of “heat” and “thermal radiation”, since every object with a temperature above absolute zero spontaneously emits radiation in this band (according to Wien’s law increasing the temperature the peak of emission moves more and more towards the visible until the object becomes incandescent).
Infrared light is emitted or absorbed by molecules when they change their rotational-vibrational movements. Infrared energy elicits vibrational modes in a molecule through a change in the dipole moment, making it a useful frequency range for the study of these energy states for molecules of the proper symmetry. Slightly more than half of the energy from the Sun arrives on Earth in the form of infrared radiation. The balance between absorbed and emitted infrared radiation has a critical effect on the Earth’s climate. Infrared light is used in industrial, scientific, and medical applications. Night-vision devices using active near-infrared illumination allow people or animals to be observed without the observer being detected. Incandescent bulbs convert only about 10% of their electrical energy input into visible light energy, while the other 90% is converted to infrared radiation, according to the Environmental Protection Agency.
Infrared rays, invisible to human eye, are emitted by all bodies, at any temperature, with emission that increases with temperature: it is therefore ray energy; they have also the characteristic property to produce considerable heat development when they are absorbed by bodies.
Infrared radiations are diffused by fog, smog, gaseous molecules and are selectively absorbed by many gases in the atmosphere, such as ozone, carbon dioxide and water vapor, to a negligible extent by oxygen and nitrogen. Since infrared radiation is of the same nature as light, the formation of images is altered by the phenomena of diffraction and aberration; in addition, optical systems can be used for them as for visible radiation: lenses, mirrors, prisms, sometimes diffraction gratings.
For the detection of infrared radiation are used different devices and apparatuses: receivers or detectors are transducers that convert the ray energy into electric current or voltage, exploiting the variation of certain physical properties of the detector, or the property of the radiation to impress photographic film. Thermal detectors exploit the variation of physical properties of the detector due to the heating produced by the radiation, such as bolometers, bismuth-silver or copper-constantan thermocouples, thermopiles (succession of several thermocouples with thermoelectric joints in series).
The detectors called photoelectric effect, more sensitive, can be photoelectronic cells, photomultipliers, photovoltaic cells, photoluminescent elements, ie based on a photoluminescent semiconductor crystal, on which the radiation, depending on the material, can have both the effect of stimulating the light emission, resulting in a bright image of the source on a dark background, and to accelerate the decay, resulting in a dark image of the source on a light background. Infrared rays are produced in nature by many sources, including warm-blooded animals, in the technique are obtained by lamps called infrared rays, consisting of an electrical resistance to a temperature of about 600 º C, and equipped with parabolic mirror reflection.