9 – Infrared Primer
A6600/A6650 User’s Manual
55
This is the general measurement formula used in all the FLIR Systems thermographic equipment. The
voltages of the formula are:
Uobj
= Calculated camera output voltage for a blackbody of temperature Tobj i.e. a voltage that can
be directly converted into true requested object temperature.
Utot
= Measured camera output voltage for the actual case.
Urefl
= Theoretical camera output voltage for a blackbody of temperature Trefl according to the
calibration.
Uatm
= Theoretical camera output voltage for a blackbody of temperature Tatm according to the
calibration.
The operator has to supply a number of parameter values for the calculation:
The object emittance
ε,
The relative humidity,
Tatm
Object distance (Dobj)
The (effective) temperature of the object surroundings, or the reflected ambient temperature Trefl, and
The temperature of the atmosphere Tatm
This task could sometimes be a heavy burden for the operator since there are normally no easy ways
to find 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 though be of interest to get a feeling for this problem already here by looking into
some different 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 of which parameters.
The figures below illustrates the relative magnitudes of the three radiation contributions for three
different object temperatures, two emittances, and two spectral ranges: SW and LW. Remaining
parameters have the following fixed values:
τ = 0.88
Trefl = +20 °C (+68 °F)
Trefl = +20 °C (+68 °F)
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
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
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