We summarize recent progress in our infrared (IR) spectral radiance metrology effort. In support of customer blackbody characterization, a realization of the spectral radiance scale has been undertaken in the temperature range of 232 °C to 962 °C and spectral range of 2.5 μm to 20 μm. We discuss the scale realization process that includes the use of Sn, Zn, Al and Ag fixed-point blackbodies (BB), as well as the transfer of the spectral radiance scale to transfer standard BBs based on water, Cs and Na heat pipes. Further we discuss the procedures for customer source calibration with several examples of the spectral radiance and emissivity measurements of secondary standard BB sources. For one of the BBs, a substantial deviation of emissivity values from the manufacturer specifications was found. Further plans include expansion of the adopted methodology for temperatures down to 15 °C and building a dedicated facility for spectral characterization of IR radiation sources.
New transfer standard pyrometers, named "RT900" and "RT1550," operating at 900 nm and 1550 nm, respectively, have been designed, characterized, and calibrated with defined fixed points of the International Temperature Scale 1990 (ITS-90) at the National Institute of Standards and Technology (NIST). The pyrometers are designed for radiance temperature measurements in the range between the freezing temperatures of Sn (231.928 °C) and Ag (961.78 °C). These instruments also incorporate design elements optimized for compactness and portability that allow them to be used to interpolate, maintain and disseminate radiance temperature scales as well as for inter-laboratory comparisons. The calibration of the RT900 at different fixed points demonstrate agreement to within 25 mK. The size of source effect (SSE) correction for a source with a 40-mm diameter has been measured to be as low as 0.01 %.
Realization of a radiometric temperature scale for near ambient temperatures with accuracy at the 20 to 50 mK level is crucial for a number of demanding military and commercial applications. In support of such measurements, radiation sources with high stability and spatial uniformity must be developed as reference and working standards. Traditionally, the temperature scale, maintained at the National Institute of Standards and Technology (NIST), relies on water bath and oil bath blackbodies in this temperature range. Recently, a water heat pipe blackbody was used at NIST as a spectral radiance source in a spectral emissivity measurement facility. Now a new, more versatile high emissivity water heat pipe blackbody was designed and characterized to be used as a reference radiance source for the radiometric temperature scale realization between 50 °C and 250 °C. Furthermore, it will serve as a reference source for the infrared spectral radiance measurements between 2.5 μm and 20 μm. The calculated spectral emissivity of the painted copper alloy cavity was verified by reflectance measurements using a CO2 laser at 10.6 μm wavelength. The spatial thermal uniformity and stability of the blackbody were characterized. Two independent realizations of the radiometric temperature scale were compared in order to verify the accuracy of the scale. Radiance temperature, calculated from the cavity temperature measured with a calibrated PRT contact thermometer and from the emissivity of the cavity, was compared to the radiance temperature, directly measured with a reference pyrometer, which was calibrated with a set of fixed point blackbodies. The difference was found to be within measurement uncertainties.