The measured average temperatures are, from left to right, +85.3°C
(+185.5°F),+85.3°C (+185.5°F), +84.8°C (+184.6°F), +84.8°C (+184.6°F), +84.8°C
(+184.6°F) and +84.3°C (+183.7°F) from a blackbody at +85°C (+185°F). The ther-
mograms are taken with a 12° lens. The distances are 1, 2, 3, 4, 5 and 10 meters (3,
7, 10, 13, 16 and 33 ft.). The correction for the distance has been meticulously set
and works, because the object is big enough for correct measurement.
17.6.4 Object size
The second series of images below shows the same but with the normal 24° lens.
Here, the measured average temperatures of the blackbody at +85°C (+185°F) are:
+84.2°C (+183.6°F), +83.7°C (+182.7°F), +83.3°C (+181.9°F), +83.3°C (+181.9°F),
+83.4°C (+181.1°F) and +78.4°C (+173.1°F).
The last value, (+78.4°C (+173.1°F)), is the maximum temperature as it was not
possible to place a circle inside the now very small blackbody image. Obviously, it
is not possible to measure correct values if the object is too small. Distance was
properly set to 10 meters (33 ft.).
10714603;a3
Figure 17.20 Temperature readings from a blackbody at +85°C (+185°F) at increasing distances (24°
lens)
The reason for this effect is that there is a smallest object size, which gives correct
temperature measurement. This smallest size is indicated to the user in all FLIR Sys-
tems cameras. The image below shows what you see in the viewfinder of camera
model 695. The spot meter has an opening in its middle, more easily seen in the detail
to the right. The size of the object has to be bigger than that opening or some radiation
from its closest neighbors, which are much colder, will come into the measurement
Publ. No. T559382 Rev. a358 – ENGLISH (EN) – June 23, 2009 115
17 – Introduction to thermographic inspections of electrical installations
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