With the sun overhead, direct radiation that reaches the
ground passes straight through all of the atmosphere, all of the
air mass, overhead. We call this radiation "Air Mass 1 Direct"
(AM 1D) radiation and for standardization purposes use a sea
level reference site. The global radiation with the sun
overhead is similarly called "Air Mass 1 Global" (AM 1G)
radiation. Because it passes through no air mass, the
extraterrestrial spectrum is called the "Air Mass 0" spectrum.
The atmospheric path for any zenith angle is simply
described relative to the overhead air mass (Fig. 4). The actual
path length can correspond to air masses of less than 1 (high
altitude sites) to very high air mass values just before sunset.
Our Oriel Solar Simulators use filters to duplicate spectra
corresponding to air masses of 0, 1, 1.5 and 2, the values on
which most comparative test work is based.