A tunable filter comparator for the spectral calibration of near-ambient temperature blackbodies

The calibration of infrared (IR) radiometers, thermal imagers and electro-optical systems relies on use of extended area blackbodies (BB) operating in the ambient environment. "Flat plate" designs, typically using a thermoelectric heat pump backed with an air- or liquid-cooled radiator, allow one to adequately meet the requirements of geometrical size and temperature span. The tradeoff comes in the form of limited temperature uniformity and lower emissivity that such an approach can provide given the limitations in achievable thermal conductivity of the plate and reflectance of the black paint, respectively. The availability of spectrally resolved radiance temperature data for infrared calibrators has become especially vital in the last few years with the widespread use of multi- and hyper-spectral electro-optical systems that enable better detection and identification of targets. In an effort to increase the measurement accuracy of IR spectral radiance of near-ambient BB calibrators, NIST has recently built a dedicated capability which is a part of its new AIRI (Advanced Infrared Radiometry and Imaging) facility. The Tunable Filter Comparator (TFC) is a key new element in this setup, allowing us to perform a precise comparison of the unit under test (UUT) with two reference blackbodies of known temperatures and emissivity. The report describes the major design features of the TFC comparator, the algorithm used for signal processing, and results of a performance evaluation of the TFC. The TFC development has enabled us to achieve BB radiance temperature comparisons with a standard deviation of 5 to 15 mK at temperatures of 15-150 C across the 3 to 5 mu m and 8 to 12 mu m atmospheric band ranges with a relative spectral resolution of 2 to 3 %.