Decadal validation of the LASP TRF cryogenic radiometer by NIST, and establishment of a replacement room temperature standard

Abstract We present the results of a recent, extensive measurement campaign validating the traceability of the solar irradiance record and Earth radiation budget data. The campaign also established future traceability, thus ensuring confidence in the continuing climate-data record. The Total solar irradiance Radiometer Facility (TRF) at the Laboratory for Atmospheric and Space Physics (LASP) Boulder, uses a liquid helium cooled cryogenic radiometer as the reference standard for the validation of spaceflight total solar irradiance instrumentation. In 2008 the radiometer was directly compared to the National Institute of Standards and Technology (NIST) Primary Optical Watt Radiometer (POWR) at a wavelength of 532.12 nm. At total solar irradiance power levels, a correction factor of 1.000306 with an associated standard uncertainty () of 0.000098, was reported for the TRF radiometer scale when using external voltage measurement electronics, and not correcting for cavity heating non-equivalence or cavity absorptance. The TRF radiometer has recently been revalidated at LASP using a POWR calibrated silicon photodiode trap transfer standard named TT4. We report a correction factor of 0.999787, = 0.000285 to align the TRF radiometer scale with the current NIST POWR scale. A new room temperature reference standard radiometer NACR, was established. It measured 133 ppm higher than POWR using the same silicon transfer standard as above, and in a separate direct measurement, 168 ppm lower than the TRF radiometer at a shutter modulation of 200 seconds. The difference agrees within stated uncertainties. A correction of 0.999867, = 0.000305 will align the new radiometer scale with the NIST radiant power scale of POWR.