16 Thermographic measurement
techniques
16.1 Introduction
An infrared camera measures and images the emitted infrared radiation from an
object. The fact that radiation is a function of object surface temperature makes
it possible for the camera to calculate and display this temperature.
However, the radiation measured by the camera does not only depend on the
temperature of the object but is also a function of the emissivity. Radiation also
originates from the surroundings and is reflected in the object. The radiation from
the object and the reflected radiation will also be influenced by the absorption
of the atmosphere.
To measure temperature accurately, it is therefore necessary to compensate for
the effects of a number of different radiation sources. This is done on-line auto-
matically by the camera. The following object parameters must, however, be
supplied for the camera:
■
The emissivity of the object
■
The reflected temperature
■
The distance between the object and the camera
■
The relative humidity
16.2 Emissivity
The most important object parameter to set correctly is the emissivity which, in
short, is a measure of how much radiation is emitted from the object, compared
to that from a perfect blackbody.
Normally, object materials and surface treatments exhibit emissivity ranging from
approximately 0.1 to 0.95. A highly polished (mirror) surface falls below 0.1, while
an oxidized or painted surface has much higher emissivity. Oil-based paint, regard-
less of color in the visible spectrum, has an emissivity over 0.9 in the infrared. Hu-
man skin exhibits an emissivity close to 1.
Non-oxidized metals represent an extreme case of almost perfect opacity and
high spectral reflexivity, which does not vary greatly with wavelength. Consequent-
ly, the emissivity of metals is low – only increasing with temperature. For non-
metals, emissivity tends to be high, and decreases with temperature.
80 Publ. No. 1 557 536 Rev. a35 – ENGLISH (EN) – January 20, 2004
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