FLIR ThermaCAM P20 Operator's Manual page 112

■
τ = 0.88
■
T
= +20 °C (+68 °F)
refl
■
T
= +20 °C (+68 °F)
refl
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 situ-
ation 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 extrap-
olation. Imagine that we in a certain case measure U
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
we are actually performing extrapolation of the calibration curve when converting
4.5 volts into temperature.
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
results in U
obj
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 electronic 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, provided the calibration algorithm is based on ra-
diation physics, like the FLIR Systems AB algorithm. Of course there must be a
limit to such extrapolations.
100
= 4.5 / 0.75 / 0.92 – 0.5 = 6.0. This is a rather extreme extrapolation,
= 4.5 volts. The highest
tot
by means of Equation 4 then
obj
Publ. No. 1 557 536 Rev. a35 – ENGLISH (EN) – January 20, 2004
= U
,
obj tot