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.