Because of normal variations in the properties of materials used to construct radiation temperature sensors, new instruments must be individually calibrated in order to achieve even moderate levels of accuracy. Initial calibration is likely to be performed by the sensor manufacturer, but periodic recalibration--in-house or by a third-party laboratory or the original manufacturer--is necessary if any but the most qualitative measurements are expected.

Working much like a hot plate, this infared calibration source uses a high emissivity, specially textured surface to provide a convenient temperature reference.

  The ongoing accuracy of a non-contact temperature sensor will depend on the means by which the calibration is performed, how frequently it is recalibrated, as well as the drift rate of the overall system. Ensuring the absolute accuracy of non-contact temperature measurement devices is more difficult than with most direct contacting devices, such as thermocouples and resistance temperature detectors (RTDs). Limiting the absolute accuracy to 1% is difficult; even in the most sophisticated set-ups, better than 0.1% accuracy is seldom achieved. This arises, in part, from the difficulty in accurately determining the emissivity of real bodies. Repeatability or reproducibility is, however, more readily achievable than absolute accuracy, so don't pay more if consistency will do.
  If absolute accuracy is a concern, then traceability to standards such as those maintained by the National Institute of Standards & Technology (NIST) will also be important. Traceability, through working to secondary to primary standards is central to the quality standards compliance such as those defined by the ISO 9000 quality standard.

Figure 7-1: A Spherical blackbody Cavity

Why Calibrate?
There are generally three methods of calibrating industrial radiation thermometers. One method is to use a commercial blackbody simulator, an isothermally heated cavity with a relatively small aperture through which the radiation thermometer is sighted (Figure 7-1). As explained in the earlier chapter on "Theoretical Development," this type of configuration approaches blackbody performance and its emissivity approaches unity. A standard thermocouple or resistance temperature detector (RTD) inside the cavity is used as the temperature reference. At higher temperatures, calibrated tungsten filament lamps are commonly used as references. A final alternative is to used a reference pyrometer whose calibration is known to be accurate, adjusting the output of the instrument being calibrated until it matches.

A handheld IR thermometer is calibrated against a commercial blackbody source-the internal cavity is designed to closely approach a blackbody's unity emissivity

  In any case, the radiation source must completely fill the instrument's field of view in order to check the calibration output. If the field of view is not filled, the thermometer will read low. In some instruments, calibration against a blackbody reference standard may be internal--a chopper is used to alternate between exposing the detector to the blackbody source and the surface of interest. Effectively, this provides continuous recalibration and helps to eliminate errors due to drift.