It is obvious that measurement of low object temperatures are more critical than
measuring high temperatures since the ‘disturbing’ radiation sources are relatively
much stronger in the first case. Should also the object emittance be low, the situation
would be still more difficult.
We have finally 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, i.e.
U
obj
= U
tot
, we are actually
performing extrapolation of the calibration curve when converting 4.5 volts into tem-
perature.
Let us now assume that the object is not black, it has an emittance of 0.75, and the
transmittance is 0.92. We also assume that the two second terms of Equation 4 amount
to 0.5 volts together. Computation of
U
obj
by means of Equation 4 then results in
U
obj
= 4.5 / 0.75 / 0.92 – 0.5 = 6.0. This is a rather extreme extrapolation, particularly when
considering that the video amplifier might limit the output to 5 volts! Note, though,
that the application of the calibration curve is a theoretical procedure where no elec-
tronic or other limitations exist. We trust that if there had been no signal limitations in
the camera, and if it had been calibrated far beyond 5 volts, the resulting curve would
have been very much the same as our real curve extrapolated beyond 4.1 volts, pro-
vided the calibration algorithm is based on radiation physics, like the FLIR Systems
algorithm. Of course there must be a limit to such extrapolations.
21
168
Publ. No. 1557966 Rev. a155 – ENGLISH (EN) – February 6, 2006
21 – The measurement formula
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