How are the NIST spectral irradiance standards traceable to SI units?
At the NIST, the High-Accuracy Cryogenic Radiometer (HACR) serves as the nation’s primary standard for optical power measurements. The spectral irradiance scale is realized using filter radiometers calibrated for absolute spectral irradiance responsivity derived from the HACR. The calibrated filter radiometers are then used to determine the radiance temperature of a high- temperature (3000 K) blackbody (HTBB) with an expanded uncertainty of 0.86 K. A precision aperture is placed in front of the HTBB, and the distance from the HTBB aperture to the receiving aperture of the spectroradiometer is measured. The spectral irradiance responsivity of the spectroradiometer is assigned, and the spectroradiometer is then used to assign the spectral irradiances of the standard lamps. Thus the spectral irradiance at NIST is ultimately traceable to two SI units: the watt and the meter.
