Comparison of solar spectral irradiance measurements using the average photon energy parameter

The average photon energy (APE) is commonly used as a means to classify solar irradiance based on the relative distribution of energy over the spectrum. Prior studies have shown a single APE can identify a unique spectral irradiance distribution with low standard deviation, but have not demonstrated this distribution will be the same across different geographic locations. This paper investigates this possibility by comparing the spectral distributions of global horizontal irradiance from two different locations, indexed by APE value. The APE is shown to be a strong predictor of spectral irradiance distribution for the two datasets after they are filtered to remove low irradiance data. Comparing the irradiance data from the two locations showed that the same APE value defines very similar distributions for global horizontal spectral irradiance recorded at the sites considered. This raises the possibility of using the APE value as a quality control for spectral irradiance measurements.