Calibration of Radiation Thermometers up to 3000 ◦ C: Effective Emissivity of the Source

The growing demand of industry for traceable temperature measurements up to 3000 ◦ C encourages improvement of calibration techniques for industrial-type radiation thermometers in this temperature range. High-temperature fixed points can be used at such high temperatures, but due to the small diameter of apertures of their cavities(∼3mm),theyarenotadaptedforthelargefield-of-viewscommonlyfeatured by this kind of radiation thermometers. At LNE-Cnam, a Thermo Gauge furnace of 25.4 mm source aperture diameter is used as a comparison source to calibrate cus- tomers' instruments against a reference radiation thermometer calibrated according to the ITS-90 with the lowest uncertainties achievable in the Laboratory. But the furnace blackbody radiator exhibits a large temperature gradient that degrades its effective emissivity, and increases the calibration uncertainty due to the lack of information on the working spectral band of the industrial radiation thermometer. In order to estimate thecorrectionstoapply,thetemperaturedistribution(radialandon-axis)oftheThermo Gauge furnace blackbody radiator was characterized and the effective emissivity of the Thermo Gauge cavity was determined by three different methods. Because of this investigation, the corrections due to different fields of view and due to the different spectralbandsofthereferencepyrometerandthecustomer'spyrometerwereobtained and the uncertainties on these corrections were evaluated.