High-Transmission Filters for Realizing Gray-Body Radiators
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Calibration of infrared radiation thermometers at non-unity emissivity settings is a poorly solved problem for establishment of traceability to meet user needs, for instruments with both fixed and variable emissivity setting functions. A variable-temperature gray-body radiator having a constant value of emissivity independent of both wavelength and temperature can be a perfect tool for the calibration purpose. In this paper, two types of high-transmittance optical neutral density filters, one utilizing a rotating-sector optical chopper, and another of a wire-mesh type, are shown to perform well with a precise transmittance between 90 % and 100 % in the wide infrared wavelength range. These optical filters in combination with a blackbody cavity traceable to ITS-90 can realize reliable gray-body radiation. These methods are applied successfully to several models of infrared thermometers operated in the emissivity correction mode.
KeywordsCalibration Emissivity setting Gray-body Infrared thermometer Optical filter
The authors would like to express their gratitude to Y. Kaneko and Y. Wang of NMIJ for their able support in filter fabrication and measurement work. Dr. Hamada of Tanaka Kikinzoku Kogyo is thanked for useful advice regarding fabrication of the platinum–rhodium thin wires. The authors also feel grateful to Mr. Kerekrit of Technology Promotion Association in Thailand for prompting them to look at the technical problem.
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