57
History and Theory of Infrared Technology
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
amplier 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 radiation physics, like the FLIR
Systems algorithm. Of course there must
be a limit to such extrapolations.
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