Extension of the NIST spectral power-responsivity calibration service to 2500 nm

The National Institute of Standards and Technology (NIST) is working to extend the upper wavelength limit of the spectral power-responsivity calibration service from 1800 nm to 2500 nm. This extension is based on extended-InGaAs (EIGA) transfer-and working-standard radiometers and low-NEP pyroelectric transfer-standard detectors. The scale is traceable to the electrical-substitution cryogenic radiometer through the transfer-standard EIGA radiometers. The total uncertainty of the extended-responsivity reference scale (realized on four EIGA working-standard radiometers) is 1% (k = 2). The transfer of the reference scale from one of the four working standards to a test EIGA radiometer in direct-current (dc) measurement mode is described. The uncertainty of the transferred scale is 1.6% (k = 2) between 1200 nm and 2300 nm. The transfer was performed on the NIST Spectral Comparator Facility (SCF) without any modifications to the SCF. The EIGA radiometers have been characterized for spatial-response uniformity, temperature-dependent spectral responsivity, linearity, noise, long-term and short-term temporal stability. The preliminary results show that EIGA radiometers can be used over the spectral range between 900 nm and 2500 nm in dc mode and could eventually replace the regular InGaAs detectors in the SCF measurements. The total uncertainty budget for these measurements is discussed.