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).

Parallax Error

Figure 1
Parallax Error

This problem is exactly analogous to the inadequacies of the old “range finder” cameras of the 1930’s, e.g. Kodak “Brownies,” 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. 

Most target venues are not homogeneous and a slight shift in the position of the infrared beam will cause it to be “looking” at different targets such as extending over the edge of the intended target and averaging in background or by other objects intruding into the IRT beam, to be averaged into the reading. 

Figure 2 illustrates the subtle but sometimes serious problem of using a laser-sighted infrared thermometer to scan an electrical circuit breaker panel to find overloaded breakers which heat up.    The IRT field of view, which actually makes the measurement, is 1.25” below the laser line, as shown in Figure 1.  So, in this illustration, the IRT is always measuring one breaker below the one indicated by the laser, so a truly hot breaker would be ignore and its cool neighbor would be incorrectly concluded to be overheated.

Infrared Measurement of Breaker with Parallax Error

Figure 2
Infrared Measurement of Breaker with Parallax Error