Information

5 Fluke Corporation Infrared Temperature Calibration 101
the object appears to glow.
The emissivity at each wave-
length, called spectral emissiv-
ity, determines how bright each
wave will be. The thermometer
uses the total brightness of a
selected band of these waves to
determine the temperature.
Detecting radiance with
a thermometer
The radiated light energy from
the calibrator’s surface is trans-
mitted through the air until it
is detected by a thermometer.
Dust, smoke and glass surfaces
can impede the light and affect
the results of the measurement.
The detector has to collect the
light from the precise area of
the intended target to avoid
measuring unintended objects
in the background. The
distance-to-spot ratio (D:S)
assists the user in finding the
correct position of the
thermometer.
However, spot size is only a
percentage of the thermometer’s
entire field of view. Like
the human eye, an infrared
thermometer has some
peripheral vision. Peripheral
vision in IR thermometers,
sometimes called scatter, can
account for between 1 % and
35 % of the total measured
energy, depending on instru-
ment quality. While the spot
size may be adequate for practi-
cal field measurements, it is not
always adequate for the labora-
tory accuracy required during
a calibration. That is why cali-
brating most IR thermometers
requires a target
that is significantly larger than
the size suggested by the
distance-to-spot ratio of the
thermometer.
The infrared energy collected
by the thermometer will be
filtered by its optical system,
which is only sensitive to a
particular spectral band such
as 8 µm to 14 µm. Included in
the collected energy is reflected
radiation caused by the ambi-
ent temperature in the room.
Reflection is related to emis-
sivity in an interesting way.
The detectable infrared radia-
tion coming from the calibra-
tor is usually a combination of
emitted thermal radiation from
the measurement surface and
reflected energy from other
warm objects in the environ-
ment, such as windows, walls,
and people. For an opaque
object, emissivity plus reflectiv-
ity is equal to one.
Consequently, if the emis-
sivity of an opaque target is
zero, then none of the radiated
energy coming from that target
is caused by its own thermo-
dynamic temperature. All of
the light detected by the ther-
mometer is radiation reflected
from the target that originated
elsewhere, striking it from
elsewhere in the room. If the
target’s emissivity is 0.95, then
its reflectivity is 0.05. In other
words, the target absorbs 95 %
of the energy in the room and
reflects the other 5 %. It also
means that the energy emitted
by the target caused by temper-
ature is 95 % of what would be
expected from a perfect black-
body. IR thermometers attempt
to compensate for the energy
reflected by a target; however,
when a target becomes very
cold compared to the environ-
ment, or if the emissivity is very
low, then the reflected energy
may make it difficult to read
= IR thermometer field of view
Smaller target
Manufacturer
recommended
large target
IR thermometer
spot size
199.3
Peripheral vision in infrared thermometers makes large targets necessary for many calibrations.