Range within the Electromagnetic Spectrum

Infrared radiation is that part of the electromagnetic spectrum that is immediately adjacent to the red light of approx. 760 nm on the long-wave side of the visible spectrum and extends to a wavelength of approx. 1 mm.

In this respect, the wavelength range of up to approx. 20 µm is of importance to technical temperature measuring.

In the second half of the 19th century, it became known that heat radiation and other electromagnetic waves, such as visible light or radio waves, were similar in nature. This was followed by the discovery of the laws of radiation by KIRCHHOFF, STEFAN, BOLTZMANN, WIEN and PLANCK. By the mid-20th century, intensive and successful work on the military use of infrared technology facilitated the building of first infrared viewers. With some distance in time and technology, also the first thermographic devices for non-military application available in the 60s. Parallel to this, however, in considerably larger diversification of available devices, pyrometry developed to become a wide-spread approach in industrial temperature measuring.

Radiation Laws of the Black Body

The bodies occurring in real life show very diverse radiation properties. Therefore, it has proved worthwhile to initially consider the simplified laws of a model body of ideal radiation properties to be then applied to actually occurring objects. This model body is known in radiation physics as the “black body“. It distinguishes itself by the fact that, of all bodies of equal temperature, it shows the largest possible emitted radiation.

The spectral spread of radiation emitted by a black body is described by PLANCK’s radiation law:

Formula for the Planck Radiation Law
Planck Radiation Law

This representation shows that the spectral composition varies with the object temperature. Bodies of a temperature of beyond 500 °C, for example, also emit radiation in the visible range. Furthermore, it must be noted that, at each wavelength, radiation intensity increases with rising temperature.

PLANCK’s radiation law represents the principal correlation regarding non-contact temperature measuring. Due to its abstract nature, however, it is not directly applicable in this form to many practical calculations. But a variety of further correlations can be derived from it, two of which shall briefly be mentioned in the following. So by means of integrating, for example, the spectral radiation intensity across all wavelengths, the value of the entire radiation emitted by the body is obtained. This correlation is called the STEFAN BOLTZMANN’s law.

Formula for the Boltzmann Law
Boltzmann Law

Due to its simple mathematical correlation, it is well suited for rough estimates, particularly when calculating the heat balance of objects as well as interrelations of total radiation pyrometers. However, the spectral measuring range of most measuring devices is usually strongly limited and, therefore, this equation is inapplicable to this purpose.

The graphic representation of PLANCK’s radiation law shows that the wavelength, at which radiation emitted by a black body has a maximum, shifts with changing temperature. WIEN’s displacement law can be derived from PLANCK’s equation by differentiation.

Formula for the Planck Radiation Law

The lower the temperature of the object to be measured the further its radiation maximum shifts towards larger wavelengths. It is at about 10 µm when close to room temperature.

Spectral transmittance of air

The level of transmittance of air is strongly dependent on wavelength. Ranges of high attenuation alternate with ranges of high transmittance (shaded), the so-called "atmospheric windows". While transmittance in the range of (8 ... 14) µm, i.e., the long-wave atmospheric window, maintains to be equally high over longer distances, measurable attenuation caused by the atmosphere already occurs in the range of (3 ... 5) µm, i.e., the short-wave atmospheric window, at measuring distances of some ten meters.

Influence by the Measured Object

The black body as a radiometric model is indispensable when considering principal correlations. Since real objects that are to be measured deviate more or less strongly from that model, it may become necessary to take this influence into account in measurements. Especially suited for this purpose is the parameter of emittance which is the measure for a body’s capability of emitting infrared radiation. Having a value of 1, the black body has the highest possible emittance, which is additionally dependent on wavelength.

Contrary to this, the emittance of real objects to be measured may show more or less strong dependence on wavelength. The following parameters may also be of some influence:

 

  • Material composition
  • Oxide film on the surface
  • Surface roughness
  • Angle to the surface normal
  • Temperature
  • Polarisation degree

 

A multitude of non-metallic materials – at least within the long-wave spectral range – shows high and relatively constant emittance, regardless of its surface structure. These include the human skin in the same as way as most mineral building materials and coating paints.

Spectral emittance of non-metallic materials

In contrast, metals generally have low emissivity that greatly depends on the surface properties and drops as wavelengths increase.

Spectral emittance of metallic materials

Relevant Indus­tries & Applic­a­tions for Thermography

  • Active Thermography - Picture Credits: © Rainer / Fotolia.com
    Active Thermography

    Active Thermography

    Make use of active thermography for non-destructive and contact-free material testing, for both automated inline and offline solutions.

  • InfraTec - Additive manufacturing for process control
    Additive Manufacturing

    Additive Manufacturing

    By in-line monitoring of thermal process parameters, infrared cameras from InfraTec support the optimisation of additive manufacturing processes.

  • thermography for aerial photography
    Aerial Thermography

    Aerial Thermography

    Detect persons and objects in the field or monitor wide-area geologic properties or environmental damages.

  • thermal imaging in aviation
    Aerospace Industry

    Aerospace Industry

    Comply with highest security demands by applying state-of-the-art infrared camera systems.

  • thermal imaging for automotive applications
    Automotive Industry

    Automotive Industry

    Infrared camera systems help you to secure a high product quality in production and at your suppliers.

  • InfraTec Bondung and Sealing - Hot gas
    Bonding and Sealing

    Bonding and Sealing

    Users can utilise infrared cameras during bonding and sealing to monitor and control various technological parameters. This includes the geometry of the adhesive application, adhesive temperature and adherend temperature, curing temperature as well as the adhesive bond.

  • building thermography
    Building Thermography

    Building Thermography

    Save money doing thermographic building inspections by using a high performance infrared camera and expert software.

  • thermal imaging in chemical industry
    Chemical Industry

    Chemical Industry

    Systematically control reactive process heat and display with infrared camera systems of InfraTec.

  • thermal imaging in electronics
    Electronics & Electrical Engineering

    Electronics & Electrical Engineering

    Measure temperature distributions of smallest electronic components with infrared cameras.

  • thermal imaging for environmental studies
    Environmental Studies

    Environmental Studies

    Look for smallest temperature differences in landscapes, biotopes and labs using infrared camera models of highest measurement precision.

  • thermal imaging in glass industry
    Glass Industry

    Glass Industry

    Measure correctly temperature distributions behind and on glass by using selected spectral filters.

  • high-speed thermography
    High-Speed Thermography

    High-Speed Thermography

    Get even faster and analyze temperatures at highest process speeds.

  • thermal imaging to inspect electrical installations
    Inspection of Electrical Installations

    Inspection of Electrical Installations

    Search electrical installations or high-voltage nets with infrared camera systems for dangerous hotspots.

  • thermal imaging for predictive maintenance
    Inspection of Mechanical Assemblies

    Inspection of Mechanical Assemblies

    Thermography reveals irregularities in heating of mechanical components in time.

  • Laser Industry

    Laser Industry

    High-end thermography in heat-generating laser applications in materials processing.

  • thermal imaging for material testing
    Material Testing

    Material Testing

    Save time and costs by non-destructively testing components with infrared camera systems by InfraTec.

  • Thermal imaging of a horse
    Medicine

    Medicine

    Infrared camera systems help you to efficiently diagnose multiple illnesses with high certainty.

  • thermal imaging in metallurgy
    Metallurgy

    Metallurgy

    Efficiently reduce the energy consumption of your metallurgic processes using infrared camera systems.

  • microthermography
    Micro-Thermography

    Micro-Thermography

    Micro-thermography allows for the thermal analysis of extremely small structures in the micrometer range, providing a detailed representation of the temperature distribution on complex electronic assemblies and components.

  • thermal imaging to inspect phovoltaic installations
    Photovoltaic Inspection

    Photovoltaic Inspection

    Take advantage of thermography in order to visualise defects on existing and running photovoltaic installations.

  • thermal imaging for asset inspections
    Plant Inspection

    Plant Inspection

    Using infrared camera systems and thermographic software by InfraTec will make you more efficient and more productive.

  • thermal imaging in plastic industry
    Plastics Industry

    Plastics Industry

    Cost-efficiently control die-casting and extrusion processes by means of infrared camera systems.

  • thermal imaging for security tasks
    Security Applications

    Security Applications

    Observation tasks for applications like investigation, remote surveillance, border guard and the protection of critical infrastructure are great challenges for the related authorities. Powerful infrared cameras are getting more and more indispensable nowadays.

  • spectral ir thermography
    Spectral Thermography

    Spectral Thermography

    Using precision filters for specific applications you will measure exactly those spectral ranges being of interest to you.

  • thermal imaging for thermal optimisation
    Thermal Optimisation

    Thermal Optimisation

    Identify possible problems already in the development phase well before serial production.

Get in contact with InfraTec thermography division

Would You Like to Know More?

It is not unusual for tasks to be associated with special requirements. Discuss your specific application needs with our specialists, receive further technical information or learn more about our additional services.

Europe & Rest of World

InfraTec GmbH
Infrarotsensorik und Messtechnik
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01217 DresdenGERMANY

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Infrarotthermographie
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Infrarottechnik
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Erfassung und Messung von Wärmestrahlung
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/5/ Walther, L.; Gerber, D.
Infrarotmeßtechnik
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The infrared handbook
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The infrared and electro-optical systems handbook
SPIE Optical Engineering Press, Washington 1993

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Handbuch der technischen Temperaturmessung
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Touloukian, Y. S.; DeWitt, D. P.
Thermophysical properties of matter
Vol.8: Thermal radiative properties - Metallic elements and alloys
Vol.9: Thermal radiative properties - Nonmetallic solids
IFI / Plenum, New York and Washington 1972

Technische Temperaturmessungen - Strahlungsthermometrie
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in VDI / VDE-Handbuch Meßtechnik I, Juni 1993

Gaussorgues, G.
Infrared Thermography (Microwave Technology Series 5)
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ISBN 0412479001