AM1GH solar spectral irradiance

AM1GH solar spectral irradiance is a clear sky Air Mass 1 Global Horizontal (hence, "AM1GH") irradiance evaluated under the atmospheric conditions specified in ASTM Standard G197. Its simulation differs from that of the G197 global tilt irradiance in three ways.

1. It characterizes a horizontal surface when the sun is at zenith (air mass 1), rather than a sun-facing 20° tilt surface when the solar zenith angle is 48.3° (air mass 1.5). The latter geometry was intended to represent the surface of a sun-facing pitched roof.
2. It employs version 2.9.5, rather than 2.9.2, of the Simple Model for Atmospheric Transmission of Sunshine (SMARTS). Version 2.9.5 is current as of November 2008; version 2.9.2 was released in year 2002.
3. At the recommendation of the model's author, it uses the extraterrestrial spectrum "Gueymard 2004," rather than "Gueymard 2002 (synthetic)."

Here are the inputs and outputs of this calculation.

• SMARTS 2.9.5 ".inp" input file (calculation parameters supplied by user)
• SMARTS 2.9.5 ".out" output file (restatement of calculation parameters)
• SMARTS 2.9.5 ".ext" output file (extraterrestrial, direct normal, diffuse horizontal, global horizontal and direct horizontal spectral irradiances from 280 to 4000 nm at a 5 nm interval; W m^-2 nm^-1).

Note that while for completeness we have output 5 different solar spectral irradiances, we require only one field—the global horizontal irradiance.

The SMARTS model can calculate solar spectral irradiance from 280 to 4000 nm, which we denote the "extended" solar spectrum. However, since

1. over 99% of the extended AM1GH solar irradiance arrives between wavelengths of 300 to 2500 nm, with virtually none at wavelengths less than 300 nm;
2. the range of a solar spectrophotometer is typically 250 to 2500 nm;
3. the dome of a pyranometer typically blocks wavelengths longer than 2800 nm; and
4. the Solar Spectrum Reflectometer does not respond to wavelengths longer than 2000 nm,

it is usually more sensible to work with an AM1GH solar spectral irradiance that spans only 300 to 2500 nm. We recommend using this "standard" AM1GH solar spectral irradiance (300 - 2500 nm), which is simply the 300 to 2500 nm subset of the extended AM1GH solar spectral irradiance (280 - 4000 nm).

The extended AM1GH solar spectral irradiance (280 - 4000 nm) is available for comparison here.

The applications of the AM1GH solar spectral irradiance are detailed in these two articles:

1. R. Levinson, H. Akbari and P. Berdahl. 2010. Measuring solar reflectance—Part I: defining a metric that accurately predicts solar heat gain. Solar Energy 84, 1717-1744.
2. R. Levinson, H. Akbari and P. Berdahl. 2010. Measuring solar reflectance—Part II: review of practical methods. Solar Energy 84, 1745-1759.
   

Ronnen Levinson, Ph.D.
Staff Scientist, Heat Island Group
Lawrence Berkeley National Laboratory
e-mail: RML27@cornell.edu