Infrared radiation

from Wikipedia, the free encyclopedia

Infrared radiation , also called IR radiation , infrared , rarely ultrared radiation , is in physics electromagnetic radiation in the spectral range between visible light and the longer-wave terahertz radiation . Usually this means light with a wavelength between 780 nm and 1 mm. This corresponds to a frequency range from 300 GHz to 400 THz or a wave number range from 10 cm −1 to 12,800 cm −1 .

A tree in front of the Lehmbruck Museum in Duisburg
Duisburg Lehmbruck IR normal.JPG
Recording of the visible spectral range
Duisburg Lehmbruck IR IR.JPG
Photographed using an IR transmission filter, only infrared in the range of 700-1,000 nm is recorded (subject without a person)

An infrared photograph (MIR, around 10 µm) of a small dog (
false color display )

Classification of the spectral range

A classification of the infrared spectral range is based on the types of molecular vibrations that affect the applications. The terms and limits are not clearly defined as in the visible area and are partly determined by applications or special physical phenomena, which is why there are several different definitions. The International Commission on Illumination (CIE) and DIN propose the division into three bands: IR-A, IR-B and IR-C. The definition with the designations NIR, MIR and FIR follows ISO 20473.

Classification of infrared radiation according to DIN 5031
designation Abbreviation Wavelength
in μm
to Vienna in K
Areas of application / comments
near infrared NIR IR-A 0.78 ... 1.40 3700 ... 970
  • Short-wave part of the NIR range, 780 nm limit due to the human sense of sight adapted to the solar spectrum .
  • Photographic infrared (ColorInfraRed, CIR) is 0.7 to 1.0 µm: photographic film can record this wave range.
IR-B 1.4 ... 3.0
  • long wave part of the NIR range
  • the limitation is due to the water absorption at 1450 nm.
mid infrared ME IR-C 03… 50 970 ... 600
  • Range of thermal radiation at terrestrial temperatures
far infrared FIR 0050 ... 1000 60… 30
  • The atmosphere absorbs strongly here, at the border to the microwave range the cosmic 3-Kelvin radiation is just visible.

In addition, other stipulations are also common, for example a subdivision that is often used in the Anglo-American region and that is applied in the specification of earth exploration cameras:

  • near infrared (English: near infrared , NIR) is short-wave IR radiation, which connects directly to the visible (red) range of 780 nm to 1400 nm.
  • Short wave infrared (English: short wavelength , SWIR) 1.4 to 3.0 µm
  • medium infrared (English: mid wavelength , MWIR) with wavelengths from 3.0 µm to 8 µm.
  • long-wavelength infrared ( LWIR) 8 to 15 µm
  • far infrared (English: far infrared , FIR) long-wave IR radiation of from 15 microns to 1 mm, and extends into the region of the terahertz radiation .


The IR radiation was discovered around 1800 by the German-British astronomer , technician and musician Friedrich Wilhelm Herschel while trying to measure the temperature of the different colors of sunlight. To do this, he let sunlight fall through a prism and placed a thermometer in the individual color areas. He noticed that beyond the red end of the visible spectrum, the thermometer was showing the highest temperature. From the observed increase in temperature , he concluded that the solar spectrum continues beyond the red.


Infrared light lamp for medical purposes

Colloquially, infrared radiation is often equated with thermal radiation , even if both microwaves and visible light, like the entire electromagnetic spectral range, contribute to the increase in temperature. Broadband IR sources are thermal emitters such as incandescent lamps and radiant heaters. Different infrared emitters were developed in order to cover the variety of applications of infrared radiation more specifically, for example Globar and Nernst-Stift .


Thermal detectors ( thermocouples or bolometers ) are suitable for detecting IR radiation of all wavelengths . In the short-wave range, semiconductor-based detectors are used (with an internal photoelectric effect ), although digital cameras are also suitable if their IR cut filter is not too strong. Special photographic films are also suitable for recording IR images in the near infrared range, and cooled semiconductor detectors, pyroelectric sensors (PIR sensors) or thermopiles are used for longer wavelengths (medium infrared) .



A major application is radiation heating. Every radiator also emits infrared radiation, especially at temperatures well above 100 ° C. Most of these are mainly due to the release of heat into the air; however, the comfort increases due to the proportion of radiation. Complete house heating or transitional heating in the bathroom are implemented using infrared panels; the Mir space station has been heated in this way since 1986.

Chemical analysis and process engineering

Transmission infrared spectrum of a styrene-acrylonitrile copolymer (SAN)

Infrared radiation stimulates molecules to oscillate and rotate. Infrared spectroscopy is a physico-chemical analysis method. The absorption of infrared light of defined wavelengths is used to determine the structure of unknown substances. The purity of known substances can be determined by quantitative determination. Infrared spectroscopy is used to identify and separate plastics in waste separation .

The absorption centers of the molecular vibrations are directly linked to the refractive index of the materials and thus to their reflection behavior . In the infrared range, this is used in infrared reflectography, among other things .

Art history

Infrared reflectography is an examination method mainly used in art studies, with which drawing elements made of more reflective materials can be made visible through the different reflection properties of the colorants applied to an image carrier . With this non-contact and non-destructive technique, it is possible to penetrate the upper layer of a painting and to document the otherwise invisible signature .


The Andromeda Nebula in infrared light at 24 μm

In infrared astronomy, one observes “cool” objects (colder than 1100 K), which can hardly be seen in other spectral ranges, or objects that are in or behind an interstellar cloud. In addition, the IR spectroscopy helps with the analysis of the viewed objects. As in chemistry, bands of certain substances are detected by means of infrared spectroscopy, for example the methane gas on the exoplanet near the fixed star HD 189733 .

Electronics and computer technology

USB infrared port for PC
Infrared sensors on an iPhone 3GS that are used as proximity sensors

Infrared remote controls , optocouplers and most light barriers work in the near infrared at 880 to 950 nm wavelength, since this is where silicon photodiodes and phototransistors have their highest sensitivity. Infrared interfaces of computers also work in this wavelength range and enable wireless communication with peripheral devices. The optical data transmission through the atmosphere by means of an IR laser is characterized by the optical free space transmission .

One of the first companies to combine infrared technology with IT was Hewlett-Packard . In 1979 an IR interface was integrated into a pocket calculator for the first time in order to establish a connection to a printer . In 1990 an IR interface was integrated into a personal computer for the first time . This interface became a standard. Because it worked serially, it was named Serial Infrared , or SIR for short . For speed reasons, this standard has now been replaced by the downward-compatible Fast-IR, which supports every desktop mainboard in personal computers built around 2002 or later. PDAs and notebooks (built around 2006) have such an infrared device built in, as do some cell phones. The infrared interfaces are increasingly being replaced by Bluetooth .

In telecommunications , IR-A is preferred in optical waveguides due to its low absorption and dispersion . The standard wavelength is 1550 nm.

The motion detection of an infrared radiation source can take place by means of thermal image sensors. This is used, for example, to control the Nintendo Wii game console .

The project of a light beam telephone based on infrared did not succeed.


In the near infrared, the green vegetation has a reflection about six times higher than in the visible spectral range, since fresh leaf tissue has a good reflectivity and the remaining wavelengths are absorbed by chlorophyll and the accompanying carotenoids . This effect is used to identify areas of vegetation. Two images are taken of a scene, one in the visible, the other in the near infrared. Multispectral cameras are often used. By dividing both images, the vegetation becomes clearly visible and can be easily distinguished.

The vegetation recognized in this way is measured by a vehicle or airplane. The comparative measurement of indoor vegetation observes a plant over a longer period of time. The measurement of the vegetation from vehicles gives a statement about the locally prevailing conditions. The determination of the area proportion of the vegetation in relation to the total area taken from the air is a common case and the vegetation volume within a predefined space is determined. This volume measurement of vegetation is important for motorway and road maintenance companies as well as operators of rail networks. Vegetation that protrudes into the clearance profile of vehicles is automatically recognized and the pruning can be initiated.

By means of the spectral reflection, especially in the near to far infrared of green vegetation, vegetation types are differentiated and the respective health status of the vegetation is recognized. The health of the plants depends primarily on their water supply. The drought is measured, and fungal and insect infestation can be seen.


Infrared photography

Infrared image on Kodak HIE film (Botanical Garden, Bern)

In analog photography , sensitized, so-called infrared films can be used in the near infrared range up to 820 nm . The visible light is completely or for the most part switched off ( red filter ) by filters installed in front . The typical result is the wood effect : a dark sky and light chlorophyll-containing parts of plants (leaves, grass). Infrared cameras can penetrate light haze and fog better because of the lower scattering due to the greater wavelength.

IR recordings are used by the military in aerial photography for espionage and military (air and terrain) reconnaissance, in astronomy and in driver assistance systems.

Color films with “ false color reproduction ” are also used. These false color films depict different infrared wavelengths as visible colors that can be distinguished. Such materials are used in aerial photography, for example, for mapping forest damage and in aerial archeology and for examining paintings and colored surfaces.

Distance measurement

Canon AF35M with autofocus via infrared distance measurement (1979)

Distance measurement using infrared transit time method - The time covered by the light emitted by the built-in infrared transmitter is evaluated in the receiver sensor and automatically transferred to the lens and, if necessary, to the built-in flash unit. Some versions partly work in the visible light spectrum and allow the use of optical autofocus systems in poor lighting conditions.


Heating lamps radiate in the infrared and have been in use for medical purposes for a long time.

Thermal radiation from radiant heaters, such as ceramic infrared emitters with long-wave IR radiation or red light lamps emitting primarily near infrared , are used for the local treatment of inflammations (for example of the paranasal sinuses). Infrared heat cabins are used for full-body treatment . Infrared radiation is often used in medicine in the form of lasers . The areas of application include in particular skin , ophthalmology and dentistry ( measuring , sclerosing , cutting , coagulation , light therapy ). In addition, infrared is used to search for the (warmer) sources of inflammation in order to be able to treat them more effectively. Thermography is used to find local foci of inflammation .

Near infrared penetrates deep into and under the skin, while MIR in particular is already absorbed on the surface of the skin and the cornea of ​​the eye. High-intensity near infrared (laser radiation) is therefore particularly dangerous for the eyes and skin, as it reaches the retina unnoticed in the eye, is focused there and can cause damage. In the body, it is absorbed in regions where there are no temperature sensors and can therefore often cause damage without being noticed.

For measuring temperature are pyrometer used that measure the temperature in the ear by means of thermal radiation in the mid infrared. Finally, pulse oximetry is used to measure the oxygen saturation of red blood cells.

Police and military

Night vision device attached to a soldier's helmet

Police and military use portable night vision devices and residual light intensifiers in the near infrared, the central components of which are image intensifiers , in order to be able to recognize objects that are otherwise invisible in the dark. In addition, infrared lighting can be used. Helicopter pilots fly at night with the help of night vision goggles attached to their helmets, in which a monochrome image of the near-infrared radiation of objects on the ground is generated in front of each eye. A movable camera system can be mounted on the outside of the helicopter, which delivers videos or thermal images in the visible as well as in the mid-infrared. These are used, for example, to search for missing or fugitive people, even in the dark.

Many types of autonomously target-seeking guided missiles find their target via thermal radiation that is emitted, for example, by aircraft engines. For defense, newer combat aircraft and military ships have devices that emit decoys ( flares ) in order to divert these weapons away from the target object.


Building thermography : uninsulated external wall

With the help of thermography "thermal images" can be generated, for which the infrared radiation of the heat from objects is used. A well-known application is building thermography for quality assurance and visualization of thermal bridges and heat losses in buildings. As a result, thermal insulation measures can then be used in a targeted manner. The fire brigade uses portable thermal imaging cameras to detect sources of fire and embers or people to be rescued in smoky interiors.

In the diagnosis and maintenance of electrical, electronic and mechanical assemblies, systems or machines, thermography is used as a supplementary measurement method for preventive defect and damage detection. This means that critical states ("hot spots") of machines, systems and installations can be determined without contact during operation in order to take measures to limit the effects at an early stage and to avoid failures and damage.

Thermography is used for vibration analysis and strength testing. Cracks and loose connections are revealed by their heat development. With infrared pyrometers, process temperatures and temperatures of components and heat sinks are measured and controlled without contact.

Material processing

Many thermal processes in industry are performed using infrared radiation. In addition to being used for drying, materials can be cured, plastics can be softened and deformed. Materials are welded, labeled and cut with infrared lasers , metals are also hardened. Material webs are dried with infrared heaters that are heated with gas or electrically. Such additional devices are available on paper machines, for example.

Security documents

Banknotes: visible on the left, infrared illuminated on the right and photographed through a night vision device

Infrared-sensitive security features in passports and banknotes are checked with infrared emitters. In the case of euro bills , for example, the infrared absorption of certain materials at defined wavelengths can be checked, along with other features. On the other hand, the infrared fluorescence of methylene blue in the British passport is a feature that is used for testing equipment.


Infrared radiation is also used in gastronomy. They are often found in the form of radiant heaters and patio heaters for outside and smoking areas. They are also used for culinary enjoyment, in the form of infrared grills (as a large gastro grill or small table grill). Compared to classic wood, electric or gas grills, they have the advantage of radiant heat that no fat hits the heating elements, as the radiators are attached above the food to be grilled. This also prevents smoke development.


Web links

Commons : Infrared Radiation  - Collection of pictures, videos and audio files
Wiktionary: Infrared radiation  - explanations of meanings, word origins, synonyms, translations

Individual evidence

  1. a b radiation physics in the optical field and lighting technology; Designation of the wavelength ranges . In: German Institute for Standardization (Ed.): DIN . 5031 part 7, January 1984.
  2. ISO 20473: 2007 Optics and photonics - Spectral bands. International Organization for Standardization (ISO), accessed January 11, 2019 .
  3. Peter Kosack (TU Kaiserslautern): Report on the research project "Exemplary comparative measurement between infrared radiant heating and gas heating in old buildings" , October 2009.
  4. AT-Mega Project: On the trail of hotspots with the infrared camera. In: ComputerClub². September 18, 2008, accessed on December 16, 2009 (TV broadcast; episode 23).
  5. The police FLIR helicopter in detail ., accessed on June 22, 2016 (video example).
  6. Differences between IR and classic grill. Retrieved January 21, 2018 .