Thermography

Thermography, also thermography is an imaging technique used to display the surface temperature of objects. Here, the intensity of the infrared radiation emitted from one point, interpreted as a measure of its temperature.

A thermal imaging camera converts invisible to the human eye, infrared radiation into electrical signals. From this, the camera produces an image in false color or for thermographic purposes rather rare a monochrome grayscale image.

Historical

The astronomer and musician William Herschel discovered in 1800, the thermal radiation by directing sunlight through a prism and examined the area behind the red end of the visible spectrum with a thermometer. The temperature rose in this area, and Herschel concluded that there must be effective an invisible form of energy. His name radiant heat is still common and was about 100 years later by "Infrared" - in German speaking countries was for a time the term " infra-red " familiar - replaced.

Other researchers doubted his discovery at first because it was not known that the transparency of IR depends strongly on the type of glass of the prism. Looking for a better material discovered in 1830 the Italian physicist Macedonio Melloni that prisms barely damp crystalline rock salt IR radiation and that heat radiation with lenses made ​​from this material can be concentrated. Just a year ago Melloni could significantly increase the accuracy by replacing the relatively inaccurate mercury thermometer by his invention of the thermopile. Both - the lenses of rock salt and arrays of thermopile - were the main components of the first thermal cameras.

The temperature distribution on surfaces (so called " thermal imaging " ) were visualized by Herschel in 1840 by different rates of evaporation of a thin oil film. Later they determined the temperature by direct contact with thermal paper, expressed that changes color when in contact with enough warm surfaces. All of these procedures have lost a lot of importance because they work only in a narrow temperature range, show no changes over time even small temperature differences and are difficult to handle with curved surfaces. Compared to the now commonly used contactless technology but they were considerably cheaper.

The breakthrough in the development of non-contact temperature measurement succeeded Samuel Pierpont Langley in 1880 with the invention of the bolometer. Application areas have included detection of icebergs and hidden people. The further development especially in the field of imaging was performed mostly in secret, and research reports could be published only after 1950 because of military secrecy regulations. Since about 1960, the devices are also available for non-military purposes.

The technique of imaging has changed dramatically now in common usage. A thermal imaging camera converts today to the human eye invisible heat radiation ( infrared light) an object or body from a distance with the help of special sensors into electrical signals that can be easily processed by computers. Thus, the temperature measuring range ( dynamic range) has been significantly expanded, in addition to tiny temperature differences can be observed. Today, thermography is mostly used as a synonym for infrared thermography.

Principle

Every body with a temperature above absolute zero emits thermal radiation. Ideally ( emissivity ) corresponds to the spectrum of the emitted radiation to that of a black body radiator at real surfaces it deviates from. With polished metal surfaces decreases in the IR range to values ​​below 0.1. In conventional building materials is considered.

With increasing temperature, the emitted spectrum shifts to shorter wavelengths ( Wien's displacement law ).

Thermography is used preferably in the infrared range, ie at object temperatures around 300 K, which will be around 20 ° C in the range of ordinary ambient temperatures. Ensure that the data is corrupted on more distant objects only slightly by lying between object and camera atmosphere, the cameras usually work in restricted wavelength ranges, in which the atmosphere hardly emits characteristic radiation ( and absorbed). Such a " window " is, for example, in the range of about 8 to 14 microns ( see atmospheric counterradiation / atmospheric window).

Three heat outputs contribute to the result:

• The main share PObjekt radiates the measurement object itself, whose surface is said to have the highest possible emissivity.
• The objects of the environment, but also the sun radiate energy from PUmgebung, the proportion is scattered on the measurement object and is added to the result. This disturbing additive is particularly pronounced in smooth metal surfaces.
• The intervening air in turn provides PLuft.

All three components are attenuated as it passes through the air for distances of about two meters, you can expect a transmittance of.

The total received power is calculated as

Scattered radiation from sunlight and hot, lateral radiators are to be avoided most easily with careful measurement. However, a problem is the radiation power of the mass of air between the object and the sensor, as the distance increases. Therefore, ground-based infrared telescopes only be used for the observation of the relatively nearby sun. More distant objects can only be identified, when the thickness of the air layer is greatly reduced (as in the Stratospheric Observatory for Infrared Astronomy ) or (as in Wide - Field Infrared Survey Explorer and the Spitzer Space Telescope) completely.

Possible measurement errors

Real surfaces emit less radiation than a black body radiator. The ratio is always between zero and one and is called emissivity. It depends on the material, the surface condition, but hardly depends on the temperature, and especially for small polished metal surfaces. An example illustrates the problem associated: A heavily rusted iron plate of uniform temperature 30 ° C = 303 K is alternating bands of polished, which results due to the very different emissivities of a " picket fence effect" of strong and weak IR radiation. From the Stefan- Boltzmann law

It follows for the power radiated per unit area

The thermal imager only evaluates differences of the received power, which is why giving an apparent temperature difference of

Calculated. If the Imager is set so that the rusted surface 303 K is assigned, it should assign the polished strip is the absolute temperature 149 K, which corresponds to -124 ° C. In fact, probably is a much higher temperature can be displayed, because unwanted IR radiation from the environment on the reflecting surface " appears " is.

At each camera you can select the suspected emission factor. If one were to adjust this so that the temperature of the polished surfaces coincide with reality, this meter would register the rusted bodies so much more radiant power that it would calculate a temperature of 342 ° C = 615 K. Radiation measurements are therefore to be regarded with caution. Does the temperature of bare metal surfaces can be determined measuring equipment are recommended to paint a large enough area dark or covered with dark tape.

The influence of temperature on the emissivity can be neglected for measurements in the temperature range from 0 ° C to 100 ° C in most cases. Many substances have a mid-infrared wavelength of the nearly independent emissivity close to unity. Examples are glass, mineral materials, paints and coatings of any color, any color anodised finishes, plastic materials except polyethylene ( see adjacent photo ), wood and other building materials, water and ice.

The temperature of surfaces with low emissivity can not be reliably determined with thermography.

Technical details

Built is a thermal imaging camera in principle like a regular electronic camera for visible light, the sensors but differ in structure and function, depending on the wavelength to be detected. It is not possible to take very long wave radiation with conventional film, because the photo -sensitive emulsion would be " exposed " even when packaged by the thermal self-radiation.

Through a lens with a lens (es ), an image is projected onto an electronic image sensor. Cameras for the wavelength range 8 to 14 microns using an optical system from moisture-sensitive salts such as sodium chloride (salt ), silver salts, or of the single-crystal semiconductor materials such as germanium or zinc selenide.

For the electronic image sensors suitable semiconductor materials are used; for example, is used for a wavelength of 1 to 2 microns ( SWIR ), indium -gallium- arsenide - sensors ( InGaAs) or lead sulfide sensors.

Thermographic cameras may have cooled or uncooled infrared image detectors. According to the photoelectric effect working detectors are often cooled to temperatures in the range of 70K, so that the characteristic radiation of the camera and the detector does not influence the measurement. This thermal sensitivity (temperature resolution) can be of thermography system over the uncooled systems dramatically increases.

Uncooled infrared sensors, are held in place by thermo-electric cooler, the Peltier elements at a constant temperature in order to reduce the drift signal of the receiver elements. Such systems do not require costly, possibly bulky coolers. In order for these Thermography systems are significantly smaller and cheaper than cooled systems. However, they provide a relatively worse outcome. Uncooled pyroelectric detectors use or microbolometer arrays.

Theoretical works

The detector cell of a microbolometer consists of a few micrometer thick, radiation-sensitive plate which is supported by two curved contacts to the actual detector ( so-called micro bridges ). The discs consist of a material having a strongly temperature -dependent resistance (for example, vanadium oxide ). The incident infrared radiation is absorbed, resulting in a temperature increase of the Scheibchens, which in turn changes the resistance. The measured voltage drop is output as a measurement signal.

Contrast Pyroelectric sensors provide only when the temperature changes a voltage with a very high source impedance.

Both microbolometer and pyrometric sensors require a mechanical chopper or at least a periodic shading of the image sensor. The reason for pyrometric sensors that they can respond only to changes in temperature. In the bolometer arrays chopper or shutter is used to gain a dark image, which ( each pixel has an individually different resistance! ) As sensor-specific reference from the captured image, pixel by pixel is subtracted.

Standards for Thermographic examination

• DIN 54162, Non-destructive testing - Qualification and certification of personnel for thermographic testing - General and specific bases for level 1, 2 and 3
• DIN 54190-1, Non-destructive testing - Thermographic testing - Part 1: General principles
• DIN 54190-2, Non-destructive testing - Thermographic testing - Part 2: Equipment
• DIN 54190-3, Non-destructive testing - Thermographic testing - Part 3: Terms and definitions
• DIN 54191, Non-destructive testing - Thermographic testing of electrical equipment
• E DIN 54192, Non-destructive testing - Active Thermography
• DIN EN 13187, Thermal performance of buildings - Detection of thermal bridges in building envelopes - Infrared method

• ISO 6781, Thermal insulation - Qualitative detection of thermal Irregularities in building envelopes - Infrared method
• ISO 18434-1, Condition monitoring and diagnostics of machines - Thermography - Part 1: General procedures
• ISO 18436-7, Condition monitoring and diagnostics of machines - Requirements for qualification and assessment of personnel - Part 7: Thermography