The Cryogenic Solar Absolute Radiometer (CSAR) will most probably become the new World Radiometric Reference (WRR) for Direct Normal Irradiance (DNI) measurements, and will replace the currently used World Standard Group (WSG) in the future. The Monitor to Measure the Integral Transmittance of Windows (MITRA) measures the attenuation of a window identical to the entrance window of CSAR to correct the irradiance values for window reflectance and absorptance losses. The solar irradiance measurements performed with the CSAR/MITRA system support previous findings of a ~0.3% difference between the International System of Units (SI) radiometric scale represented by CSAR and the currently used WRR scale.
Why Cryogenic?
At very low temperatures, the material (copper) used in the sensors (cavities, Figure 1) of the cryogenic radiometer becomes thermally “superconductive” (i.e. the thermal diffusivity becomes extremly high), which enables an idealised implementation of the electrical substitution principle. The cryogenic radiometer thus has a much lower measurement uncertainty compared to room temperature radiometers which suffer from imperfect electrical substitution of the absorbed radiative power.
Why MITRA?
Operating a terrestrial solar absolute radiometer at cryogenic temperatures in a vacuum requires an entrance window with a certain wavelength dependent reflectivity and absorptivity for the incident radiation. Therefore, the spectrally integrated window transmittance needs to be measured to correct the power reading of CSAR for these losses. Because the solar spectral irradiance and thus the transmittance change with air mass and atmospheric conditions, the Monitor to measure the spectrally Integrated TRAnsmittance of Windows (MITRA) measures the transmittance in parallel to the CSAR measurements.
The MITRA cavity is periodically covered by a window from the same production batch as that used for the CSAR instrument. Reflection and absorption losses of the window result in a slightly lower temperature of the cavity. Comparing the temperature rise of the obstructed with the unobstructed case defines the spectrally integrated window transmittance.
Results and Discussion
As shown in Figure 2, CSAR irradiance values are ~0.3% lower than those of the WRR. The CSAR-to-WRR ratio no longer depends on the window material used for the vacuum tank (sapphire or fused silica) after application of the respective window attenuation correction factor from MITRA.
Figure 1. Cross-section of CSAR. The cavities are heated by the absorbed solar radiation.
Figure 2. Top: Daily averages of WRR to SI comparisons performed at PMOD/WRC with CSAR/MITRA during IPC-XIII in October 2021. The mean relative difference between CSAR and WRR irradiance values (red dotted line in top panel) is -0.31 +/- 0.4 %. Bottom: Corresponding daily averages of the spectrally integrated window transmittance as measured with MITRA. The mean window transmittance (red dotted line in bottom panel) is 0.9291 +/- 0.0002.
General References
Fehlmann, A., (2012), Fourth World Radiometric Reference to SI radiometric scale comparison and implications for on-orbit measurements of the total solar irradiance, Metrologia, 49, S34.
Walter B, et al., (2014) Spectrally integrated window transmittance measurements for a cryogenic solar absolute radiometer, Metrologia, 51, 344–349, https://doi.org/10.1088/0026-1394/51/6/s344
Walter B. et al., (2017), Direct Solar Irradiance measurements with a Cryogenic Solar Absolute Radiometer, AIP Conference Proceedings, 1810, 080007.
Winkler, R. (2012), Cryogenic Solar Absolute Radiometer – A potential replacement for the World Radiometric Reference, PhD Thesis, University College London.