Thermographic Measurement Techniques; Introduction; Emissivity - FLIR ThermaCAM PM575 Operator's Manual

8

Thermographic measurement techniques

8.1

Introduction

The ThermaCAM™ PM575/595 measures and images the emitted infrared radia-
tion from an object. The fact that radiation is a function of object surface tempera-
ture 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 objecta but is also a function of the emissivity. Radiation also origi-
nates 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 automat-
ically by the ThermaCAM™ PM575/595. The following object parameters must
however, be supplied for the camera:
• The emissivity of the object.
• The ambient temperature.
• The distance between the object and the camera.
• The relative humidity.
8.2

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 emissivities ranging from
approximately 0.1 to 0.95. A highly polished (mirror) surface falls below 0.1, while
an oxidised or painted surface has much higher emissivity. Oil-based paint, regard-
less of colour in the visible spectrum, has an emissivity over 0.9 in the infrared.
Human skin exhibits an emissivity close to 1.
Non-oxidised metals represent an extreme case of almost perfect opacity and high
specular reflectivity, 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.
© FLIR Systems AB – Publ. No. 557 369 – Ed. A
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