V. Air Mass

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By the time sunlight reaches the earth’s surface, it has an intensity that is dependent on the path it has taken through the atmosphere. The optical air mass is the ratio of an actual path length of sunlight to the minimum path length (when the sun is directly overhead). When the sun is at an angle θ from the horizon and s is the length of a shadow cast by a vertical structure with height h, the air mass is given by

Therefore the AM0 radiation (see “Solar Radiation Outside Our Atmosphere”) is only practically used for sunlight capture outside our atmosphere; we never receive AM0 radiation once the light has reached the surface of the earth. AM1 radiation is given when the sun is directly overhead, whereas the radiation condition AM1.5 is fulfilled when the sun is at an angle of 41.8° above the horizon (or 48.2° from directly above). AM1.5 radiation corresponds to a mean irradiance of about 900 W/m2, but has been standardized to 1000 W/m² or 1 kW/m². Much information about solar cells, such as efficiency, will be expressed in terms of this standard, because AM1.5 represents the ideal spectrum of sunlight received at temperate latitudes, at which much solar research is performed¹. In summary,

Air mass is the measurement of how direct the sunlight’s path is to a location on the earth at a given time of day.
AM0 is the solar constant, which is a spectrum of light characteristic of radiation outside our atmosphere. We never receive this because of atmospheric effects (see “Atmospheric Effects on Solar Intensity”).
AM1.5 is the standard for measurement and comparison of solar efficiency in temperature latitudes, and is about 1 kW/m².

As illustrated above, AM0 radiation is always attenuated by the atmosphere; thus, even when directly overhead, the sun's radiation still has AM1 by the time it reaches the surface.

Source: eyesolarlux.com. <http://www.eyesolarlux.com/Solar-sim...ion-energy.htm>

References

1. Nelson, Jenny. The Physics of Solar Cells. London: Imperial College Press, 2003.