Information

3 Fluke Corporation Infrared Temperature Calibration 101
Emissivity,
blackbodies
and graybodies
Emissivity can be any value
between and including zero
and one. Zero emissivity means
that no matter what an object’s
temperature, no light will be
radiated. An emissivity of one
means that the surface will
radiate perfectly at all wave-
lengths. Scientists give these
special perfectly radiating
objects the name “blackbody.”
Objects with emissivity very
close to one are also usually
called blackbodies. A flat plate
calibrator with an emissivity
around 0.95 is referred to as a
graybody if the emissivity is
uniform across all wavelengths.
Emissivity
The radiant energy coming from
an opaque target is a combina-
tion of emitted radiance caused
by the target’s temperature and
reflected radiance coming from
elsewhere in the environment.
Transmission through the
object is another source of radi-
ant energy that must be consid-
ered when objects are not
opaque. The amount of light
emitted at a particular tempera-
ture is determined by the emis-
sivity of the surface. Emissivity
is the ratio of the radiant energy
emitted by a surface to that
emitted by a blackbody at the
same temperature. Emissivity is
greatly affected by the type of
material and surface finish of
an object. Metals with smooth
surface finishes tend to have
low emissivity and high reflec-
tivity while long narrow holes
have relatively high emissivity
and very little reflectivity. The
sum of emissivity, reflectiv-
ity, and transmission is always
equal to one.
Infrared
temperature cali-
brators must be
designed to have
a known emis-
sivity, which must
remain constant
over the full operating tempera-
ture. Unfortunately, emissivity
is neglected in the calibration
of most IR calibrators. These
calibrators are themselves cali-
brated by inserting a contact
thermometer such as a platinum
Effect of a 1 % increase of emissivity on apparent temp
TBG=23 °C, ε=0.95, λ=8 μm to 14 μm
-1.500
-1.000
-0.500
0.000
0.500
1.000
1.500
2.000
2.500
3.000
3.500
4.000
-50 0 50 100 150 200 250 300 350 400 450 500
Target Temperature (°C)
Apparent Temperature Change (°C)
Temperature errors equivalent to a one percent error in emissivity. A 1 % error in emissivity could lead to a 3 °C error in temperature at 450 °C.
The relationship between emissivity, reflectivity and transmission.
reflectivity + emissivity + transmissivity = 1