56
Appendix C
This task can sometimes be a heavy
burden for the operator since there are
normally no easy ways to nd accurate
values of emittance and atmospheric
transmittance for the actual case. The
two temperatures are normally less of a
problem provided the surroundings do
not contain large and intense radiation
sources.
A natural question in this connection is:
How important is it to know the right
values of these parameters? It could
be of interest to get a feeling for this
problem by looking into some dierent
measurement cases and compare the
relative magnitudes of the three radiation
terms. This will give indications about
when it is important to use correct values
for which parameters.
Figures 15 and 16 illustrate the relative
magnitudes of the three radiation
contributions for three dierent object
temperatures, two emittances, and two
spectral ranges: SW and LW. Remaining
parameters have the following xed
values:
τ = 0.88•
T•
re
= +20°C (+68°F)
T•
atm
= +20°C (+68°F)
It is obvious that measurement of low
object temperatures is more critical
than measuring high temperatures since
the “disturbing” radiation sources are
relatively much stronger in the rst case.
Should the object emittance be low, the
situation would be more dicult.
We have nally to answer a question
about the importance of being allowed
to use the calibration curve above the
highest calibration point, what we call
extrapolation. Imagine that we in a
certain case measure U
tot
= 4.5 volts. The
highest calibration point for the camera
was in the order of 4.1 volts, a value
unknown to the operator. Thus, even if
the object happened to be a blackbody
(U
obj
= U
tot
), we are actually performing
extrapolation of the calibration
curve when converting 4.5 volts into
temperature.
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