The Irradiance package is a Swift translation of the spectrl2 C code from NREL's Bird Simple Spectral Model. It depends on SolarPos, which is a Swift implementation of NREL's Solar Position Algorithm for Solar Radiation Applications.
For most uses the main entity will be Irradiance, which calculates irradiance spectra for flat surfaces located/oriented on earth's surface.
The original NREL software is based on (and even listed in) Simple Solar Spectral Model for Direct and Diffuse Irradiance on Horizontal and Tilted Planes at the Earth's Surface for Cloudless Atmospheres. This paper is available [in HTML format].(https://rredc.nrel.gov/solar/pubs/spectral/model/spectral_model_index.html)
