Standard Spectra
Solar radiation reaching the earth's surface varies
significantly with location, atmospheric conditions (including
cloud cover, aerosol content, and ozone layer condition),
time of day, earth/sun distance, and solar rotation and
activity. Since the solar spectra depend on so many variables,
standard spectra have been developed to provide a basis for
theoretical evaluation of the effects of solar radiation and as a
basis for simulator design. These standard spectra start from a
simplified (i.e. lower resolution) version of the measured
extraterrestrial spectra, and use sophisticated models for the
effects of the atmosphere to calculate terrestrial spectra.
The most widely used standard spectra are those
published by The Committee Internationale d'Eclaraige (CIE),
the world authority on radiometeric and photometric
nomenclature and standards. The American Society for
Testing and Materials (ASTM) publish three spectra, AM 0 AM
1.5 Direct and AM 1.5 Global for a 37° tilted surface. The
conditions for the AM 1.5 spectra were chosen by ASTM
"because they are representative of average conditions in the
48 contiguous states of the United States."
Fig. 5 shows typical differences in standard direct and
global spectra. These curves are from the data in ASTM
Standards, E 891 and E 892 for AM 1.5, a turbidity of 0.27
and a tilt of 37° facing the sun and a ground albedo of 0.2.