IMPROVED SPECTRAL IRRADIANCE SCALE OFFERED BY NIST.

Researchers at NIST recently achieved realization of the NIST detector-based spectral irradiance scale using a high-temperature blackbody (HTBB) operated at 2950 K, with the temperature determined using absolute detectors calibrated for spectral irradiance responsivity. The detector-based spectral irradiances are assigned during a single transfer procedure using a spectroradiometer that compares the HTBB and the lamps' spectral irradiances.

For nearly 40 years, the U.S. spectral irradiance scales have relied upon radiometric transfers from a gold freezing-point blackbody (1337.33 K) to calibrate sources of spectral irradiance. Since the transfer standard of spectral irradiance, a 1 kW quartz halogen lamp, has a spectral distribution that approximates that of a blackbody radiator at 3000 K, this process of scale realization required five intermediate steps to increase the spectral irradiance output above that of the gold freezing-temperature blackbody.

The impact of the detector-based spectral irradiance scale is in reduced uncertainties in the spectral irradiance from 1 % to 0.5 % for the spectral region of 250 nm to 900 nm. In the short-wave infrared region (1000 nm to 2500 nm), the use of the detector-based scale reduces the uncertainties in the spectral irradiance by a factor of 5 % to 0.5 %. The spectral irradiance scale is transferred to customers lamp standards through the NIST Spectroradiometric Source Calibration Services. Industries and groups that rely on the improved spectral irradiance scale are aerospace, lighting, equipment manufacturers, other national measurement institutes, universities, and the precision measurement equipment laboratories (PMELs) for the Air Force, Navy, and Army.