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Infrared Thermometry, the fastest-growing
segment of the temperature measurement industry, has several advantages over
other types of temperature measurement. These include convenience with
sub-second response, self containment, non-invasiveness and accuracy. All of
these benefits can be derived if the infrared thermometer is used properly.
Knowing where the infrared temperature sensor
is “looking” is one issue that makes it difficult to use infrared thermometers (IRT’s)
properly. Since the IR “beam” is invisible, the end user can’t “see” where the
instrument is pointing so it is very difficult to focus. This can cause
frustration and errors in resultant temperature readings.
Because the IR “beam” is invisible, the
position and physical characteristics of the target spot being measured -- such
as its location, shape, size, and surface texture -- cannot be ascertained by
the operator. Users of conventional IRT’s often realized that their results are
not repeatable and there is no apparent reason why they are not. Readings that
should be identical can often be divergent by 20% or more.
It is very important for the operator to know
where the target spot is located. Because, if any part of the infrared beam is
off the edge of the target surface, serious reading errors can occur. The
target must completely intercept the instrument’s field-of-view (FOV) to get an
accurate temperature reading. The shape of the FOV cross-section changes along
its length, being square near the instrument’s focal plane and circular afocally.
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 centerline of the FOV, the resulting spot shape is
rectangular or oval. The reading can still be quite accurate even though the
longitudinal dimension has increased considerably (+40% at a 45° tilt). This
accuracy is the same as long as all parts of the FOV are still within the target
confines.
When using a variable-focus infrared sensor,
the above considerations are relevant for each individual focus of that
instrument. The size of the FOV changes for each new focus chosen by the
operator and, hence, must be monitored.
Laser sights are often used by manufacturers of
IRT’s to attempt to solve the aiming problem. These sights are an insufficient,
paraxial, one-dimensional attempt to answer the problem. The light from these
IRT’s show only a single point on the target that is offset from the target area
being measured by parallax error. Therefore, they cannot focus on the exact
target spot size.
A slight shift in the position of the infrared
beam will cause the IRT to be “looking” at different targets. The beam may
extend over the edge of the intended target and average in background. Or,
other objects intruding into the IRT beam may be averaged into the reading.
The sighting system (laser) and the optics
(infrared field-of-view) use two entirely autonomous optical systems. Even the
centerlines are displaced by as much as 50 millimeters because of this.
This problem is exactly analogous to the
inadequacies of the old Kodak Brownie “range finder” cameras of the 1930’s,
which used a second “viewfinder”
optical system to try and
show where the primary optics were looking.
Dr. Ernst Leitz recognized this problem about this time and invented the TTL/SLR
(Through-the-Lens/Single Lens Reflex) camera with which the operator viewed the
scene through the same optics that were taking the picture.
The solution to the IRT’s parallax sighting
problem was borrowed from Dr. Leitz’ TTL/SLR Leicaflex® Camera – sight through
the same optics that the IR “beam” is using. This completely eliminates
parallax error at all working distances, focally and afocally.
In order to solve this Parallax Problem in an
infrared thermometer, a visible light is projected through the (achromatic) lens
system. The visible sighting light beam and the invisible infrared
field-of-view beam are co-incident everywhere, and so the visible light shows
all of the geometrical nuances of the infrared FOV.
A fascinating possibility enabled by an Everest
Interscience patented TTL/SLR Intra-Optical Sighting System (U.S. Patent No. 4,494,881) is measuring small objects
which were previously next to impossible to measure. With one inch of Parallax
Error, an operator couldn’t possibly measure objects that are as small as one
millimeter in diameter.
With Everest Interscience patented TTL/SLR
Intra-Optical Light Sighting System, the infrared thermometer is focused down to
a target spot size of one millimeter (0.040”). This enables non-contact
temperature measurement of a tiny overheated resistor
The TTL/SLR
Intra-Optical Light Sighting gives the ability to continuously vary the
working distance and target spot size to match targets as small as the head of a
pin or as large as a cotton field. This is a huge advantage. The target spot
(defined as the planar intercept of the instrument’s field-of-view with the
target surface) is always illuminated showing its exact size, shape and
distortions under any focal conditions.
A primary requirement for the Everest Interscience
Vario-Zooom™
(Patent No. 7,355,178) variable-focus infrared thermometers
are that the temperature reading (or calibration factor), does not vary as the objective
focal distance changes, for targets of constant temperature.
The variable focusing operation changes the
optical gain of the instrument by less than ±1 percent, making the accuracy
essentially independent of focus. The illuminated variable Field-of-View ranges
from 2° to 40°. The illuminated spot being measured on the target is variable
from 1.0 millimeter or less to 30 meters or more. Of course, the illumination
intensity fades out at wide angles or large spots, depending on the ambient
lighting conditions. The working distance is from 2 cm to 100 meters.
The LED light source has only 2000-1
of the power density of a laser, so it is perfectly safe, and does not need the
CAUTION! Labels mandated by the FDA for lasers, which can burn the retina if
mishandled.
For more information, please contact Everest Interscience.
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