Information

4 Fluke Corporation Infrared Temperature Calibration 101
resistance thermometer (PRT)
physically into the target. This
technique also neglects temper-
ature losses at the surface of the
infrared target. With this type of
calibration, the user may not be
aware that complicated emis-
sivity-related corrections are
required at each temperature in
order to achieve the accuracy
claimed by the manufacturer.
These corrections are based
on the difference between the
actual measured emissivity of
the target and the
emissivity setting of the ther-
mometer. A one percent error in
the emissivity at 500 °C would
result in a 3.5 °C error in the
calibration. On the other hand,
emissivity values offered by
manufacturers are usually
typical values only and are not
actually verified by calibration.
This can lead to a loss of
traceability and inconsistent
results over time and incon-
sistent results among different
calibrators.
To correct for temperature
errors in the measurement
surface, an infrared tempera-
ture calibrator should be cali-
brated with a radiometer, which
measures the amount of radiant
energy coming from the calibra-
tion target at each temperature.
A temperature display that has
had a radiometric calibra-
tion does not need additional
emissivity-related temperature,
Tech tip:
If the target is at a tempera-
ture below the dew point,
ice may form in crystal
patterns that will cause the
emissivity of the surface to
change, introducing errors
into the calibration. Purging
with dry gas is one method
of preventing the growth of
crystals which eventually may
form a sheet of ice that actu-
ally masks the temperature
of the target, creating even
larger errors.
The peak on the graph corresponds to the brightest wavelengths.
The peak moves to the left and the whole curve moves up as the temperature increases.
unless the emissivity setting of
the thermometer does not match
the calibrator. Look for a calibra-
tor that can compensate for the
emissivity settings of infrared
thermometers.
As shown in the Spectral
Radiance and Temperature
graph below, when temperature
causes an object to emit light,
the light comes out in many
different wavelengths. This is
called spectral radiance
(see graph below). If you could
line up all the waves from
shortest to longest, you would
see that the brightest waves
are somewhere in the middle.
If you then increased the
temperature of the object
emitting the light, you would
notice that the shorter waves
were getting brighter than
the longer waves. When the
object becomes hot enough,
even the very short waves from
0.400 µm to 0.700 µm start to
be bright enough that the radia-
tion emitted by the object is
visible to the human eye and
Spectral Radiance and Temperature
Wavelength (μm)
Spectral Radiance (W m-2 μm-1 sr-1)