User Manual

Thermographic measurement techniques30
30.2.1.2 Step 2: Determining the emissivity
Follow this procedure:
1. Select a place to put the sample.
2. Determine and set reflected apparent temperature according to the previous
procedure.
3. Put a piece of electrical tape with known high emissivity on the sample.
4. Heat the sample at least 20 K above room temperature. Heating must be reasonably
even.
5. Focus and auto-adjust the camera, and freeze the image.
6. Adjust Level and Span for best image brightness and contrast.
7. Set emissivity to that of the tape (usually 0.97).
8. Measure the temperature of the tape using one of the following measurement
functions:
• Isotherm (helps you to determine both the temperature and how evenly you have
heated the sample)
• Spot (simpler)
• Box Avg (good for surfaces with varying emissivity).
9. Write down the temperature.
10. Move your measurement function to the sample surface.
11. Change the emissivity setting until you read the same temperature as your previous
measurement.
12. Write down the emissivity.
Note
• Avoid forced convection
• Look for a thermally stable surrounding that will not generate spot reflections
• Use high quality tape that you know is not transparent, and has a high emissivity you are certain of
• This method assumes that the temperature of your tape and the sample surface are the same. If they
are not, your emissivity measurement will be wrong.
30.3 Reflected apparent temperature
This parameter is used to compensate for the radiation reflected in the object. If the emis-
sivity is low and the object temperature relatively far from that of the reflected it will be im-
portant to set and compensate for the reflected apparent temperature correctly.
30.4 Distance
The distance is the distance between the object and the front lens of the camera. This pa-
rameter is used to compensate for the following two facts:
• That radiation from the target is absorbed by the atmosphere between the object and
the camera.
• That radiation from the atmosphere itself is detected by the camera.
30.5 Relative humidity
The camera can also compensate for the fact that the transmittance is also dependent on
the relative humidity of the atmosphere. To do this set the relative humidity to the correct
value. For short distances and normal humidity the relative humidity can normally be left
at a default value of 50%.
30.6 Other parameters
In addition, some cameras and analysis programs from Flir Systems allow you to com-
pensate for the following parameters:
• Atmospheric temperature – i.e. the temperature of the atmosphere between the cam-
era and the target
• External optics temperature – i.e. the temperature of any external lenses or windows
used in front of the camera
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