Thermographic Measurement Techniques; Introduction; Emissivity; Finding The Emissivity Of A Sample - FLIR T420 User Manual

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 tempera-
ture 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 re-
flected radiation will also be influenced by the absorption of the atmosphere.
To measure temperature accurately, it is therefore necessary to compensate for the ef-
fects 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 per-
fect blackbody of the same temperature.
Normally, object materials and surface treatments exhibit emissivity ranging from approx-
imately 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 visi-
ble spectrum, has an emissivity over 0.9 in the infrared. Human skin exhibits an emissiv-
ity 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.

32.2.1 Finding the emissivity of a sample

32.2.1.1 Step 1: Determining reflected apparent temperature
Use one of the following two methods to determine reflected apparent temperature:
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