John,
I contacted a Professional astronomer and good friend Professor Steve Shore who has provided the following information which we hope will answer your question.
There is also another document which I can’t upload so will send you an e mail.
http://rredc.nrel.gov/solar/spectra/am1.5/ASTMG173/ASTMG173.html
https://arxiv.org/pdf/1405.4780.pdf
http://adsabs.harvard.edu/abs/2010A&A…523A..57V
Regards,
Jack
Huggins Spectroscopic Observatory UK