Simple Model of the Atmospheric Radiative Transfer of Sunshine

The Simple Model of the Atmospheric Radiative Transfer of Sunshine (SMARTS) is a computer program designed to evaluate the surface solar irradiance components in the shortwave spectrum (spectral range 280 to 4000 nm) under cloudless conditions.

The program, written in FORTRAN, relies on simplifications of the equation of radiative transfer to allow extremely fast calculations of the surface irradiance.

[8] The latter standard represents the spectral distribution of global irradiance incident on a 37° tilted surface facing the sun at an air mass of 1.5.

This standard spectrum is mandated by IEC to evaluate the rating of photovoltaic (PV) solar cells in the absence of optical concentration.

[15][16][17] The reference spectra in ASTM G197 have been developed to evaluate the optical characteristics of fenestration devices when mounted vertically (windows) or on structures inclined at 20° from the horizontal (skylights on roofs).

The reference spectrum in ASTM G177 is limited to the global irradiance in the ultraviolet (280–400 nm), and corresponds to "high-UV" conditions frequently encountered in arid and elevated sites, such as in the southwest USA.

Besides the atmospheric conditions, another important input is the solar geometry, which can be defined by the sun position (zenith angle and azimuth), the air mass, or by specifying the date, time, and location.