This is the general measurement formula used in all the FLIR Systems AB thermo-
graphic equipment. The voltages of the formula are:
Figure 19.2 Voltages
Calculated camera output voltage for a blackbody of temperature
T
obj
i.e. a voltage that can be directly converted into true request-
ed object temperature.
U
obj
Measured camera output voltage for the actual case.
U
tot
Theoretical camera output voltage for a blackbody of temperature
T
refl
according to the calibration.
U
refl
Theoretical camera output voltage for a blackbody of temperature
T
atm
according to the calibration.
U
atm
The operator has to supply a number of parameter values for the calculation:
■
the object emittance ε,
■
the relative humidity,
■
T
atm
■
object distance (D
obj
)
■
the (effective) temperature of the object surroundings, or the reflected ambient
temperature T
refl
, and
■
the temperature of the atmosphere T
atm
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 indi-
cations about when it is important to use correct values of which parameters.
The figures below illustrates the relative magnitudes of the three radiation contri-
butions for three different object temperatures, two emittances, and two spectral
ranges: SW and LW. Remaining parameters have the following fixed values:
Publ. No. 1 557 536 Rev. a35 – ENGLISH (EN) – January 20, 2004 99
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