Absolute Primary Radiometric Thermometry

Solar Irradiance Variability Monitor for the Galileo Solar Space Telescope Mission: Concept and Challenges

“…Once the system is in the vacuum environment, the voltage/current variations to keep the sensors thermally balanced equal the irradiance. The measured radiant flux (Φ) is written by Equation 1 regarding electrical heating (PE) and optical heating (PO) [71].…”

“…On the other hand, a larger cavity increases the time constant, determined by the thermal resistance and thermal capacity. Due to this limitation, the ESR's time constant will hardly be significantly reduced, especially for radiometers operating in a classical-complete cycle of measurement mode that could lead to excessive heating of the instrument [71]. Larger cavities in cryogenic radiometers work at approximately 4 K to decrease the specific heat [85].…”

“…Meanwhile, the thermo-optical sensor technology test allows for achieving significantly lower cadences. However, according to the theory, ESRs have some significant inherent limitations that still need to be overcome: 1) non-equivalence of the measured optical power and the electric Joule heat; 2) the relationship between the thermal time constant and the sensitivity of the sensor; 3) large thermal noises in the electrical heater and temperature sensor limiting the signal-to-noise ratio (SNR) [71]. The compact radiometer design is improved mainly by combining the alternatives for temperature measurement method, servo system replacement, and solar absorptance materials.…”