snow or rain and reliable measurement is no longer possible. This is mainly because
a heavy snowfall as well as heavy rain is impenetrable to infrared radiation and it is
rather the temperature of the snowflakes or raindrops that will be measured.
7.6.3 Distance to object
This image is taken from a helicopter 20 meters (66 ft.) away from this faulty connec-
tion. The distance was incorrectly set to 1 meter (3 ft.) and the temperature was
measured to +37.9°C (+100.2°F). The measurement value after changing the distance
to 20 meters (66 ft.), which was done afterwards, is shown in the image to the right,
where the corrected temperature is +38.8°C (+101.8°F). The difference is not too
crucial, but may take the fault into a higher class of seriousness. So the distance
setting must definitely not be neglected.
10714403;a3
Figure 7.18 LEFT: Incorrect distance setting; RIGHT: Correct distance setting
The images below show the temperature readings from a blackbody at +85°C
(+185°F) at increasing distances.
10714503;a3
Figure 7.19 Temperature readings from a blackbody at +85°C (+185°F) at increasing distances
7
36 Publ. No. 1558439 Rev. a156 – ENGLISH (EN) – February 28, 2006
7 – Introduction to thermographic inspections of electrical installations
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