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Infrared Ther­mo­graphy – Phys­ical Basics

The principle of infrared thermography is based on the physical phenomenon that any body of a temperature above absolute zero (-273.15 °C) emits electromagnetic radiation. There is clear correlation between the surface of a body and the intensity and spectral composition of its emitted radiation. By determining its radiation intensity the temperature of an object can thereby be determined in a non-contact way.

Infrared Thermography – Physical Basics - Picture Credits: © sabdesign85 / Fotolia.com

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
Spectral transmittance of air (10 m, 25 °C, 1013 mbar, 85% rel.hum.) /5/

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
The spectral emissivity of a few non-metals (enamel, gypsum, concrete, chamotte) /5/

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

Spectral emittance of metallic materials
Spectral emittance of metals (Silver, Gold, Platinum, Rhodium, Chromium, Tantalum, Molybdenum) and other pure materials (Graphite, Selenium, Antimony) /5/

Relevant Indus­tries & Applic­a­tions for Thermography

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

Active Ther­mo­graphy

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

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

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

All branches and application areas
Contact to thermography division of InfraTec

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Further Glossary Items relating to Phys­ical Basics

infrared cameras

FLIR - Forward Looking InfraRed | InfraTec

The term FLIR stands for the abbreviation Forward Looking InfraRed. The abbreviation FLIR originated in the course of the further development of the infrared camera at the beginning of the 1960s.

infrared cameras

Thermal Camera, Infrared Camera | Thermography Knowledge

A thermal camera – also called IR camera, thermal imaging camera or infrared camera – is a measuring instrument used for non-contact measurements of the surface temperature of objects.

Circuit board ImageIR® 10300

Thermal Image | InfraTec Thermography Knowledge

A thermal image can be captured with a infrared camera. To acquire the thermal image, the infrared radiation, which is generated by the heat of an object, is recorded by a thermographic camera.

InfraTec Thermography Glossar - Temperature Measurement with Thermography - Picture credits: © iStock.com / nordroden

Temperature Measurement | InfraTec Thermography Knowledge

One method of determining and monitoring temperature without contact is thermography. It can be used to make defects visible without destroying the test object during testing. 

Bibli­o­graphy

Norbert Schuster, Valentin Kolobrodov
Infrarotthermographie
WILEY-VCH Verlag, Berlin 2000, ISBN 3-527-40130-X

Stahl, K.; Miosga, G.
Infrarottechnik
Hüthig Verlag, Heidelberg 1986

Glückert, Udo
Erfassung und Messung von Wärmestrahlung
Franzis Verlag, München 1992
ISBN 3-7723-6292-3

Wissensspeicher Infrarottechnik
Fachbuchverlag, Leipzig 1990, ISBN 3-343-00498-7

/5/ Walther, L.; Gerber, D.
Infrarotmeßtechnik
Verlag Technik, Berlin 1983

Wolfe, W. L.; Zissis, G. J.
The infrared handbook
Office of Naval Research, Washington 1978

The infrared and electro-optical systems handbook
SPIE Optical Engineering Press, Washington 1993

Lieneweg, F.
Handbuch der technischen Temperaturmessung
Vieweg Verlag, Braunschweig 1976

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
VDI / VDE-Richtlinie 3511, Fachausschuß 2.6 Technische Temperaturmessung
in VDI / VDE-Handbuch Meßtechnik I, Juni 1993

Gaussorgues, G.
Infrared Thermography (Microwave Technology Series 5)
Chapman & Hall, 1994, London
ISBN 0412479001