On the surface of Earth, at far lower temperatures than the surface of the Sun, some thermal radiation consists of infrared in the mid-infrared region, much longer than in sunlight. However, black body or thermal radiation is continuous: it gives off radiation at all wavelengths. Of these natural thermal radiation processes, only lightning and natural fires are hot enough to produce much visible energy, and fires produce far more infrared than visible-light energy. In general, objects emit infrared radiation across a spectrum of wavelengths, but sometimes only a limited region of the spectrum is of interest because sensors usually collect radiation only within a specific bandwidth.
Thermal infrared radiation also has a maximum emission wavelength, which is inversely proportional to the absolute temperature of object, in accordance with Wien's displacement law. A commonly used sub-division scheme is: . Due to the nature of the blackbody radiation curves, typical "hot" objects, such as exhaust pipes, often appear brighter in the MW compared to the same object viewed in the LW.
The International Commission on Illumination CIE recommended the division of infrared radiation into the following three bands: . ISO specifies the following scheme: . Astronomers typically divide the infrared spectrum as follows: . These divisions are not precise and can vary depending on the publication. The three regions are used for observation of different temperature ranges, and hence different environments in space. The most common photometric system used in astronomy allocates capital letters to different spectral regions according to filters used; I, J, H, and K cover the near-infrared wavelengths; L, M, N, and Q refer to the mid-infrared region.
These letters are commonly understood in reference to atmospheric windows and appear, for instance, in the titles of many papers. A third scheme divides up the band based on the response of various detectors: . Near-infrared is the region closest in wavelength to the radiation detectable by the human eye.
Other definitions follow different physical mechanisms emission peaks, vs. No international standards for these specifications are currently available. However, particularly intense near-IR light e. Leaves are particularly bright in the near IR, and if all visible light leaks from around an IR-filter are blocked, and the eye is given a moment to adjust to the extremely dim image coming through a visually opaque IR-passing photographic filter, it is possible to see the Wood effect that consists of IR-glowing foliage.
The C-band is the dominant band for long-distance telecommunication networks. The S and L bands are based on less well established technology, and are not as widely deployed. Infrared radiation is popularly known as "heat radiation",  but light and electromagnetic waves of any frequency will heat surfaces that absorb them. Visible light or ultraviolet -emitting lasers can char paper and incandescently hot objects emit visible radiation.
Heat is energy in transit that flows due to a temperature difference. Unlike heat transmitted by thermal conduction or thermal convection , thermal radiation can propagate through a vacuum. Thermal radiation is characterized by a particular spectrum of many wavelengths that are associated with emission from an object, due to the vibration of its molecules at a given temperature. Thermal radiation can be emitted from objects at any wavelength, and at very high temperatures such radiation is associated with spectra far above the infrared, extending into visible, ultraviolet, and even X-ray regions e.
Thus, the popular association of infrared radiation with thermal radiation is only a coincidence based on typical comparatively low temperatures often found near the surface of planet Earth. The concept of emissivity is important in understanding the infrared emissions of objects. This is a property of a surface that describes how its thermal emissions deviate from the idea of a black body.
Night Vision: Exploring the Infrared Universe
To further explain, two objects at the same physical temperature will not show the same infrared image if they have differing emissivity. For example, for any pre-set emissivity value, objects with higher emissivity will appear hotter, and those with a lower emissivity will appear cooler. For that reason, incorrect selection of emissivity will give inaccurate results when using infrared cameras and pyrometers. Infrared is used in night vision equipment when there is insufficient visible light to see. The use of infrared light and night vision devices should not be confused with thermal imaging , which creates images based on differences in surface temperature by detecting infrared radiation heat that emanates from objects and their surrounding environment.
Infrared radiation can be used to remotely determine the temperature of objects if the emissivity is known.
This is termed thermography, or in the case of very hot objects in the NIR or visible it is termed pyrometry. Thermography thermal imaging is mainly used in military and industrial applications but the technology is reaching the public market in the form of infrared cameras on cars due to the massively reduced production costs. Thermographic cameras detect radiation in the infrared range of the electromagnetic spectrum roughly —14, nanometers or 0. Since infrared radiation is emitted by all objects based on their temperatures, according to the black body radiation law, thermography makes it possible to "see" one's environment with or without visible illumination.
The amount of radiation emitted by an object increases with temperature, therefore thermography allows one to see variations in temperature hence the name. A hyperspectral image is a "picture" containing continuous spectrum through a wide spectral range at each pixel. Typical applications include biological, mineralogical, defence, and industrial measurements.
Thermal infrared hyperspectral imaging can be similarly performed using a Thermographic camera , with the fundamental difference that each pixel contains a full LWIR spectrum.
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Consequently, chemical identification of the object can be performed without a need for an external light source such as the sun or the moon. Such cameras are typically applied for geological measurements, outdoor surveillance and UAV applications.
In infrared photography , infrared filters are used to capture the near-infrared spectrum. Digital cameras often use infrared blockers. Cheaper digital cameras and camera phones have less effective filters and can "see" intense near-infrared, appearing as a bright purple-white color. This is especially pronounced when taking pictures of subjects near IR-bright areas such as near a lamp , where the resulting infrared interference can wash out the image. There is also a technique called ' T-ray ' imaging, which is imaging using far-infrared or terahertz radiation.
Lack of bright sources can make terahertz photography more challenging than most other infrared imaging techniques. Recently T-ray imaging has been of considerable interest due to a number of new developments such as terahertz time-domain spectroscopy.
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Infrared tracking, also known as infrared homing, refers to a passive missile guidance system , which uses the emission from a target of electromagnetic radiation in the infrared part of the spectrum to track it. Missiles that use infrared seeking are often referred to as "heat-seekers" since infrared IR is just below the visible spectrum of light in frequency and is radiated strongly by hot bodies.
Many objects such as people, vehicle engines, and aircraft generate and retain heat, and as such, are especially visible in the infrared wavelengths of light compared to objects in the background.
Infrared radiation can be used as a deliberate heating source. For example, it is used in infrared saunas to heat the occupants. It may also be used in other heating applications, such as to remove ice from the wings of aircraft de-icing. Infrared heating is also becoming more popular in industrial manufacturing processes, e. In these applications, infrared heaters replace convection ovens and contact heating.
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Efficiency is achieved by matching the wavelength of the infrared heater to the absorption characteristics of the material. A variety of technologies or proposed technologies take advantage of infrared emissions to cool buildings or other systems.
IR data transmission is also employed in short-range communication among computer peripherals and personal digital assistants. Remote controls and IrDA devices use infrared light-emitting diodes LEDs to emit infrared radiation that is focused by a plastic lens into a narrow beam. The beam is modulated , i. 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. Infrared communications are useful for indoor use in areas of high population density. IR does not penetrate walls and so does not interfere with other devices in adjoining rooms.
Infrared is the most common way for remote controls to command appliances. Infrared lasers are used to provide the light for optical fiber communications systems. IR data transmission of encoded audio versions of printed signs is being researched as an aid for visually impaired people through the RIAS Remote Infrared Audible Signage project. Transmitting IR data from one device to another is sometimes referred to as beaming. Infrared vibrational spectroscopy see also near-infrared spectroscopy is a technique that can be used to identify molecules by analysis of their constituent bonds.
Each chemical bond in a molecule vibrates at a frequency characteristic of that bond. A group of atoms in a molecule e. If an oscillation leads to a change in dipole in the molecule then it will absorb a photon that has the same frequency. The vibrational frequencies of most molecules correspond to the frequencies of infrared light.
A spectrum of all the frequencies of absorption in a sample is recorded. It is the frequency divided by the speed of light in vacuum. In the semiconductor industry, infrared light can be used to characterize materials such as thin films and periodic trench structures.