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Infrared Thermometry has become the
fastest-growing segment of the temperature measurement industry. This is
largely due to its convenience – sub-second response, self containment,
non-invasiveness and accuracy when properly used.
Accurate aiming of the instruments’ invisible
infrared “beam” is an ongoing issue that detracts from its usefulness as a
thermometer, but not necessarily as a heat sensor.
Many users are frustrated by the fact that they
can’t “see” where the instrument is pointing, because the I.R. “beam” is
invisible. Therefore, the position and physical characteristics of the target
spot being measured, such as its location, shape, size, surface texture, etc.,
cannot be ascertained by the operator. It’s like playing baseball in the dark!
Users of conventional Infrared Thermometers (IRT)
often realized that their results are not repeatable for any apparent reason.
Readings that should be identical can often be divergent by 20% or more.
It is very important for the operator to know
where all parts of the target spot are located, because, if any part of the
infrared beam is off the edge of the target surface, serious reading errors will
ensue. An accurate temperature reading can be made only if the target
completely intercepts the instrument’s field-of-view (FOV) anywhere
along its length. The shape of the FOV cross-section changes along its length,
however. It is square near the instrument’s focal plane and circular afocally.
Also, this is true only if the target’s surface is perpendicular to the
centerline of the FOV.
If the target’s surface is tilted at an angle
to the FOV centerline, the resulting spot shape is rectangular or oval. The
reading is still quite accurate, but the longitudinal dimension has increased
considerably (+40% at a 45° tilt). It is critical to know if all parts of the
FOV are still within the target confines.
All of the above considerations are relevant
for each individual focus of a variable-focus instrument. The size of the FOV
changes for each new focus chosen by the operator; hence, it must be monitored.
Laser sights are an insufficient, paraxial,
one-dimensional attempt to answer the problem, showing only a single point on
the target that is offset from the target area being measured by parallax error
and cannot be focused. This is like playing baseball in the dark with
flashlights!
“Parallax Error” is illustrated in Figure 1.
The sighting system (laser) and the operational optics (infrared field-of-view)
use two entirely autonomous optics systems. Even the optical centerlines are
displaced by as much as 50 millimeters (the “Parallax Error” of Figure 1).
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