Information

6 Fluke Corporation Infrared Temperature Calibration 101
the correct temperature. This is
because the reflected energy
is a relatively large proportion
of the signal received by the
thermometer when the target
temperature is below ambi-
ent temperature. This situation
is often referred to as a low
signal-to-noise ratio.
IR thermometers measure
a group of wavelengths called
a spectral band. IR ther-
mometers are spectral band
thermometers because they
measure spectral radiance,
the collective radiance of all
of the wavelengths inside a
particular spectral band, such as
8 µm to 14 µm.
Calculating temperature
from radiance
Finally, an accurate temperature
calculation requires the emissiv-
ity setting of the thermometer,
to match the real emissivity of
the objects being measured.
Alternatively, in the case of
a target with a radiometric
calibration, the thermometer’s
emissivity should match the
effective calibrated emissivity
of the target, so that a direct
comparison can be made
between the infrared tempera-
ture of the calibrator target
and the infrared thermometer
under test.
Conclusion
Infrared thermometers are used
in a variety of situations where
contact measurements are
impractical. Applications involv-
ing these useful devices are
often misunderstood, leading to
a reduction in the usefulness of
the resulting measurements. But
confidence in these measure-
ments increases with calibra-
tion. When the stakes are high,
or the application is important,
this additional confidence is
well worth the investment in
regular testing or calibration of
the IR thermometer.
All calibrations are not
created equal, and the right
equipment choice can make all
the difference. When choos-
ing a calibrator, look for one
with a large enough target to
accommodate the thermometer’s
peripheral vision. You probably
need the same size target the
manufacturer uses for its own
calibrations; thermometer manu-
facturers also recommend using
the same calibration distance
that they do to achieve labora-
tory levels of accuracy in a cali-
bration.
If your calibrator does not
have a radiometric calibration,
you need to know the emissivity
of its target so you can calcu-
late the appropriate corrections.
These calculations are difficult,
so getting the right calibra-
tion to begin with makes a big
difference. Similar calculations
must be made if the emissiv-
ity settings on the thermometer
do not match those of the cali-
brator, and that is why a well
designed calibrator that can
make those calculations for you
saves worry and time that you
can spend being productive.
Glossary
Absorption:
The process of converting
radiation energy that falls on
a surface into internal thermal
energy.
Blackbody:
A blackbody is an ideal surface
that emits and absorbs elec-
tromagnetic radiation with the
maximum amount of power
possible at a given tempera-
ture. Such a surface does not
allow radiation to reflect or pass
through it. In the laboratory a
blackbody is approximated by a
large cavity with a small open-
ing. Reflection is prevented
because any light entering the
hole would have to reflect off
the walls of the cavity multiple
times, causing it to be absorbed
before it could escape.
Dew point:
The dew point is the tempera-
ture to which air must be cooled,
at a given barometric pressure,
for water vapor to condense into
water.
Distance to spot ratio:
The distance-to-spot ratio (D:S)
is the ratio of the distance to
the object and the diameter of
the area containing a specified
percentage of the total energy
picked up by the infrared ther-
mometer. The D:S ratio is used
as a guide to determine the
appropriate distance for making
practical infrared temperature
measurements.
Electromagnetic radiation:
Energy emitted by a surface that
travels through space as a wave
with electric and magnetic
components. Examples include:
radio waves, microwaves, milli-
meter wave radiation, infrared
radiation, visible light, ultraviolet
radiation, X-rays and gamma
rays.
Emissivity:
The emissivity of a surface indi-
cates how efficiently it emits
radiation compared to a black-
body at the same temperature.
It is measured by the ratio of
energy radiated by the mate-
rial to the energy radiated by a
blackbody at the same tempera-
ture.
Field of view:
The field of view (FOV) is the
region in space containing a
specified amount of the radiant
energy collected by the optical
system of an infrared thermom-
eter. FOV is usually expressed
in angular degrees (e.g., 1 °).
The measurement surface must
completely fill the thermometer’s
field of view to ensure accurate
temperature measurements.
Graybody:
A surface that emits radiation
with constant emissivity over all
wavelengths and temperatures
is called a graybody. Although
graybodies do not exist in prac-
tice, they are a good approxima-
tion for many real surfaces.
Infrared thermometer:
A device that calculates the
temperature of an object by
measuring the infrared radia-
tion emitted by the object. It is