23 The measurement formula
As already mentioned, when viewing an object, the camera receives radiation not
only from the object itself. It also collects radiation from the surroundings reflected
via the object surface. Both these radiation contributions become attenuated to some
extent by the atmosphere in the measurement path. To this comes a third radiation
contribution from the atmosphere itself.
This description of the measurement situation, as illustrated in the figure below, is so
far a fairly true description of the real conditions. What has been neglected could for
instance be sun light scattering in the atmosphere or stray radiation from intense ra-
diation sources outside the field of view. Such disturbances are difficult to quantify,
however, in most cases they are fortunately small enough to be neglected. In case
they are not negligible, the measurement configuration is likely to be such that the
risk for disturbance is obvious, at least to a trained operator. It is then his responsibil-
ity to modify the measurement situation to avoid the disturbance e.g. by changing
the viewing direction, shielding off intense radiation sources etc.
Accepting the description above, we can use the figure below to derive a formula for
the calculation of the object temperature from the calibrated camera output.
10400503;a1
Figure 23.1 A schematic representation of the general thermographic measurement situation.1: Surround-
ings; 2: Object; 3: Atmosphere; 4: Camera
Assume that the received radiation power W from a blackbody source of temperature
T
source
on short distance generates a camera output signal U
source
that is proportional
to the power input (power linear camera). We can then write (Equation 1):
146 Publ. No. T559382 Rev. a358 – ENGLISH (EN) – June 23, 2009
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