5.2: The global standard spectrum (AM1.5g)
Because the terrestrial spectrum varies, any reference spectrum has some arbitrariness. To make the standard spectrum 
representative to as many PV applications as possible, it is not derived from a particular measured spectrum, but it is calculated
from the reference AM0 spectrum under representative geometric and atmospheric conditions .
These conditions originate from an analysis of rather sunny sites of a past measuring network in the US , and are as follows:
- the air mass is 1.5 (where the sun is about 41° above the horizon, see the page
"The air mass (AM)";
- the cell is tilted by 37° from the horizon (so the sun shines at an angle of 11.2° from the cell's normal vector);
- the cell has no optical concentration system, so the cell faces a hemisphere including the surrounding ground (which is light sandy soil);
a spectrum that includes the blue sky and the surrounding ground is called a global spectrum, so the spectrum is called AM1.5g.
- the site is at sea level under standard pressure (1013.25 milibars);
- the atmospheric conditions are mostly from the U.S. standard atmosphere, which is representative for geographical mid latitudes; but
- as the CO2 level is rising (see here),
a higher CO2 level is taken (370 ppm of the year 2001 ), and
- the haziness – typical of sunny U.S. sites on a clear day – is fine-tuned (to a turbidity value of 0.084 at 500 nm) so that
- the total irradiance is 100 mW/cm2.
This spectrum is a good representative of the illumination conditions on a tilted PV flat plate array on a clear day without clouds near noon
in the geographical mid latitudes, and is neither too yellow nor too blue.
The standard spectrum is calculated from the AM0 spectrum using the software SMARTS , which we will use in the next chapter.
Figure: Spectral irradiance (left) and photon flux (right) of the AM1.5g spectrum.