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As can be seen in the graph, the effect of background temperature is more troublesome when you
measure lower temperatures than when you measure higher temperatures. The point of this discussion
is that in order to do good IR thermometry, background temperature must be controlled.
Effect of Angle on Emissivity
Another topic to be aware of relating to emissivity is that emissivity varies with angle. Typically,
emissivity is maximum when taking a measurement normal to an object and is zero when taking a
measurement parallel to an object. An example of this effect when using the Product is shown in
Figure 4. As can be seen in the graph, any measurements made within 15° of normal will cause an
error less than 0.001 in emissivity. This number can be evaluated against the graph in Figure
obtain uncertainty due to angular deviation from normal.
0.960
0.950
0.940
0.930
0.920
0.910
0.900
0.890
0
Figure 4. Effect of Angular Emissivity Calibration Geometry
Along with emissivity, calibration geometry is one of the most misunderstood topics related to the
calibration and use of IR thermometers. Among the topics that are misunderstood are center of spot,
size of spot, scatter, size of source and distance to target.
Center of Spot
Many IR thermometers include lasers as a guide to show the user where to point them. These act as
approximating guides only. In reality, IR thermometers see areas much bigger than that small laser dot.
Also, an IR thermometer's center of spot may not be exactly at the point where the laser is pointed. To
understand the impact of these two factors on a calibration, testing should be done to determine the
center of the spot. This information may be provided by the manufacturer as well.
Basic Infrared Thermometry Theory - Relating to the use of the Product
Directional Emissivity
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Angle (°)
Precision Infrared Calibrator
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