Application Note

Emissivity
Of the kinds of energy—reflected,
transmitted and emitted—emanat-
ing from an object, only emitted
infrared energy indicates the
object’s surface temperature.
Transmitted and reflected energy
do not. When IR thermometers
measure surface temperatures,
they sense all three kinds of en-
ergy. Therefore, they have to be
adjusted to read emitted energy
only. The Fluke 62 Mini Infrared
Thermometer has a fixed, pre-set
emissivity of 0.95, which is the
emissivity value for most organic
materials as well as painted or
oxidized surfaces.
To accurately measure the
surface temperature of a shiny
object, cover the target surface
with masking tape or flat black
paint and allow enough time
for the tape or paint to reach
the temperature of the material
underneath.
Distance-to-spot ratio
The optical system of an infrared
thermometer collects the infrared
energy from a circular area or
spot and focuses it on the detec-
tor. The farther a target is from
the instrument the larger the spot
created on the target will be.
Optical resolution is defined by
the ratio of the distance from the
glass and, as noted, will be inac-
curate if used to measure shiny or
polished metal surfaces (stainless
steel, aluminum, etc.).
Users of IR thermometers also
must be alert to environmen-
tal conditions. Steam, dust and
smoke, for example, can prevent
accurate temperature readings
by obstructing a unit’s optics. A
dirty lens can also affect read-
ings. Lenses should be cleaned
with dry, clean plant air or a fluid
made specifically for cleaning
lenses. Also, changes in ambi-
ent temperature can influence
a thermometer’s performance. If
an IR unit is exposed to abrupt
temperature changes of 11 °C
(20 °F) or more, the user should
allow at least 20 minutes for the
unit to adjust to the new ambient
temperature.
instrument to the object com-
pared to the size of the spot (“dis-
tance-to-spot” or D:S ratio). For
the Fluke 62 Mini the distance-
to-spot ratio is 10:1. This means
that at a distance of 10 inches the
spot is about one inch in diam-
eter. The larger its ratio number
the better is the instrument’s
resolution.
Resolution is important because
it relates directly to getting good
readings by ensuring that the
target is larger than the spot size.
The smaller the target, the closer
one must be to it. When accuracy
is critical, the target should be at
least twice as large as the spot.
Other factors to consider
These instruments measure only
surface temperatures, not internal
temperatures. Furthermore, they
cannot take readings through
25mm @
250mm
50mm @
500mm
100mm @
1000mm
1" @
10"
2" @
20"
4" @
40"
D
S
D:S = 10:1
Non-contact thermometers are adjusted to consider only emitted energy.
Use D:S ratio for accurate measurements.
For the best readings, the measured target should be at least twice as large as the spot.
2 Fluke Corporation Frontline test tools for predictive maintenance