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4180, 4181
Technical Guide
Size of Spot, Size of Source and Scatter
IR thermometer manufacturers frequently indicate a distance-to-spot-size ratio. It is computed at
specific distances. The spot size is the diameter that contains a given amount of energy detected by
the IR thermometer. This is typically 90 % to 95 % of the energy. This information is also just an
approximating guide. The energy outside this detection area is referred to as scatter. This means that a
surface area much larger than the given spot size is needed when calibrating IR thermometers. Spot
size is often referred to as size of source effect.
Two solutions to size of source effect problems are available through the calibration of the IR
thermometer, both related to calibration geometry including distance and target size. One method is to
calibrate the instrument under the same conditions under which it is used. That is, you point it at the
same size of target at a known surface temperature as what it is used to measure, from the same
distance, and under similar ambient conditions.
The other method is to duplicate the calibration geometry used by the manufacturer of that device. This
information should be available from the manufacturer. Under either method, you calibrate in a certain
intended amount of scatter into your measurement.
The target size needed to calibrate a given IR thermometer is dependent on the IR thermometer's
optical system. Many IR thermometers come with a spot size diagram. These diagrams can be
misleading. As mentioned above, all of the energy the IR thermometer detects is within the given spot
diagram. Generally you will need at least 2 to 3 times the spot size diameter to use for calibration.
The Product provides a 150 mm (6 inch) diameter flat plate. This gives the user a large temperature
controlled surface to calibrate IR thermometers. 150 mm (6 inches) is a large enough diameter to
accommodate most handheld IR thermometers.
Distance to Target
The third topic of concern with calibration geometry is distance between the calibration target and the
IR thermometer being calibrated. It is important not to have the IR thermometer too close to the target.
This will cause the IR thermometer's optics to heat excessively which will cause false readings. It is
also important to be not too far away. This will cause the target to not fill the IR thermometer's spot size
and will cause a false reading.
Knowing the Correct Geometry
Information on these 3 factors, center of spot, size of source, and distance to target, should be
provided by the IR thermometer manufacturer. If this information cannot be obtained, ASTM provides a
guideline (ASTM Standard E1256-95) to determine some of these parameters.
Traceability
Fluke Calibration's traceability for the Product calibration comes through a radiometric transfer
standard. The Product is calibrated with an 8 μ m to 14 μ m highly-accurate IR thermometer
(radiometer). This IR thermometer is calibrated with blackbody cavity baths. The temperature of these
baths is measured by PRTs which have a calibration traceable to NIST. A diagram of this traceability is
shown in Figure 5.
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