32
Thermographic measurement
techniques
32.1
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 tem-
perature 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 automatically
by the camera. The following object parameters must, however, be supplied for the
camera:
■
The emissivity of the object
■
The reflected apparent temperature
■
The distance between the object and the camera
■
The relative humidity
■
Temperature of the atmosphere
32.2
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 of the same temperature.
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 a higher emissivity. Oil-based paint, regardless of
color in the visible spectrum, has an emissivity over 0.9 in the infrared. Human skin
exhibits an emissivity 0.97 to 0.98.
Non-oxidized metals represent an extreme case of perfect opacity and high reflexivity,
which does not vary greatly with wavelength. Consequently, the emissivity of metals
is low – only increasing with temperature. For non-metals, emissivity tends to be high,
and decreases with temperature.
32
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