Precipitable Water & Air Mass Spectral Shift Method
Lee et al, showed that the spectral shift of CdTe and c-Si are both dependent on precipitable water and absolute air mass. In CdTe PV modules, precipitable water is the primary driver of spectral shift, with air mass being of secondary importance. For c-Si PV modules, the opposite is true, and air mass is the primary variable effecting spectral shift.
Compute the spectral shift factor.
Accurate Measurement, Using Natural Sunlight, of Silicon Solar Cells by William M. Keogh and Andrew W. Blakers, http://cses.cecs.anu.edu.au/files/Natural_sunlight_PiPV.pdf
L. Nelson, M. Frichtl, A. Panchula, Changes in Cadmium Telluride Photovoltaic System Performance due to Spectrum by, IEEE Journal of Photovoltaics, Vol. 3, No. 1, January 2013.
M. Lee, L. Ngan, J. Sorenson and A. Panchula, Understanding Next Generation Cadmium Telluride Photovoltaic Performance due to Spectrum, 42nd IEEE Photovoltaic Specialists Conference, June 2015.
Lee, M, Panchula, A., Spectral Correction for Photovoltaic Module Performance Based on Air Mass and Precipitable Water, 43rd IEEE Photovoltaic Specialists Conference, June 2016