Uniqueness verification of blue fraction as a parameter of spectral irradiance quantification

The spectral distribution of global irradiance is measured in the optimal tilt (45° south) plane of array for the calender year 2020 in Grimstad, Norway. In this work the spectral distribution is quantified in terms of the Blue Fraction (BF). The uniqueness of BF has been tested and reported using the same methodology used for uniqueness verification of average photon energy (APE). The results show that the coefficient of correlation between the APE and BF was 0.98, meaning a possible bijective relation between these two parameters whereby both indicate a similar spectrum. The percentage contribution of integrated irradiance (Rc) was calculated for 25 nm wavelength bins for the BF region 350-650 nm. Spectra are grouped by their BF value in steps of 0.02, each group containing measurements falling within ±0.01 of the central value. The corresponding standard deviation (SD) varied from 0.05% to 0.21% for the spectra denoted by BF values from 0.44 to 0.60. The coefficient of variation (CV) was found as high as 12% for the wavelengths below 450 nm while it remained relatively low from 450-650 nm, meaning a low dispersion of Rc values around the mean value. The results indicate that the uniquenss of BF can be statistically confirmed for the measured wavelength range.