Decadal Validation of the LASP TRF Radiometer by NIST and Establishment of a Replacement Room Temperature Standard

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 ± 0.000098 (k=1) 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 0.999970 ± 0.000294 (k=1) to align the TRF radiometer scale with the current NIST POWR scale.

A new room temperature reference standard radiometer NACR, was established. It measured 49 ppm lower than POWR using the same silicon transfer standard as above, and in a separate direct measurement, 15 ppm higher than the TRF radiometer at a shutter modulation of 200 secs. The difference is in agreement with stated uncertainties. A correction of 1.000049 ± 0.000313 (k=1) will align the new radiometer scale with the NIST radiant power scale of POWR.

All measurements were made in power mode, whereby the radiometer apertures were underfilled.

The presentation will discuss the experimental methodology and results of the comparisons.